Directory UMM :Data Elmu:jurnal:A:Advances In Water Resources:Vol23.Issue1.1999:
Advances in Water Resources 23 (1999) 41±48
Digital terrain modeling of small stream channels with a total-station
theodolite
Richard F. Keim
b
a,*
,
Arne E. Skaugset a, Douglas S. Bateman
b
a
Oregon State University, Forest Engineering Department, 215 Peavy Hall, Corvallis, OR 97331, USA
Oregon State University, Forest Science Department, 026 Forest Sciences Laboratory, Corvallis, OR 97331 USA
Received 7 May 1998; received in revised form 12 December 1998; accepted 3 February 1999
Abstract
A Digital Terrain Model (DTM) is an alternative to traditional measures of stream channel morphology that allows for extraction of many dierent types of data. This paper describes a method of creating high-resolution Digital Terrain Models of stream
channels using an electronic, digital, total-station theodolite and standard methods of land surveying, and also includes considerations unique to hydrological application. Included is a detailed description of one application in Oregon, and also suggestions of
how to apply the method in other research of morphology. Ó 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Digital terrain model; Mapping; Channel morphology
1. Introduction
Quanti®cation of channel morphology is a requirement for a variety of subdisciplines within surface
hydrology and geomorphology. Morphological characterizations taken from data collection in the ®eld commonly are limited in scope, are not useful for multiple
objectives, and often suer from low precision or poor
accuracy. For example, wildland hydrologists have used
many dierent methods to quantify the shape of small,
headwater streams, including cross sections (e.g. Refs.
[14,16,18]), longitudinal pro®les (e.g. Refs. [10,15,19]),
or qualitative delineation of aquatic habitat units such
as pools and ries (Refs. [1,10]). Most of these methods
collect two-dimensional data, although the processes
and conditions they seek to describe are three-dimensional. Qualitative data, which some methods collect,
are insucient to quantitatively link hydrological
processes in the channel to morphology (Ref. [8]).
In research of channel morphology, the most broadly
useful information is a three-dimensional model of the
stream channel. One such model is a Digital Terrain
Model (DTM), which is a digital representation of a
surface designed to show topographic dierence. The
*
Corresponding author. Tel.: 001-541-737-0515; fax: 001-541-7374316; e-mail: [email protected]
digital format is usable in many software packages, including geographic information systems (GIS), so multiple quantitative analyses are possible and data may be
extrapolated for multiple research objectives. Precision
and accuracy of DTMs are limited only by the amount
of eort expended during collection of data, so can be as
high as required for nearly any application.
Digital terrain models provide a quantitative basis for
analyses of many aspects of channel morphology, including sediment transport, local scour and ®ll, and
bank stability. A time series of DTMs allows changes in
the channel to be measured precisely without prior
designation of exact sample points. A single DTM serves
as a base for research of microtopographic eects on
velocity, turbulence, and scour. Also, companion data
may be easily incorporated to a DTM using a GIS.
These may be the object of research that must be considered in the context of the DTM, or they may simply
enhance the usefulness of the DTM. As an example, Ref.
[22] and Keim et al. (in preparation) describe movement
of coarse debris in channels using a chronosequence of
DTMs.
We have developed a method to collect data for the
creation of DTMs by making topographic surveys of
stream channels using a total-station theodolite (``total
station''). A total station collects data electronically and
incorporates an electronic distance-measuring device
that eliminates the need for manual measurements of
0309-1708/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 3 0 9 - 1 7 0 8 ( 9 9 ) 0 0 0 0 7 - X
42
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
distance. While others have made maps of streams,
using taped distances from a baseline (Ref. [3]), dumpy
level and range ®nder (Ref. [9]), or survey by stadia with
a theodolite (Refs. [4,11,12]), these methods result in
either an imprecise DTM or a less complete model, and
are subject to a large amount of error from bias by
personnel and are very susceptible to mistakes during
collection of data (Ref. [6]). Using a total station eliminates many of these problems, and is also faster than
other mapping methods capable of producing comparable data.
A DTM may be generated from source data by a
variety of methods, but for traditional land surveys, the
most common is creation of a Triangular Irregular
Network (TIN) (Ref. [2]). A TIN is, in general, a continuous network of triangles connecting points of data
within a set. In this application, the vertices of the triangles in the TIN are de®ned by critical points on the
ground that describe breaks in slope (Ref. [5]). The data
in a topographic survey consist of three-dimensional
coordinates of the critical points that will later become
vertices of the triangles in the TIN.
Despite the substantial requirements of equipment
and time, a DTM and topographic map made with a
total station is a useful research tool for intensively
measuring local channel features and processes. It can
also be used to assess the accuracy and precision of
other methods that measure channel morphology. In
this paper, we present the methods we used to create
DTMs and topographic maps of small, headwater
stream channels using a total station. We also recommend ways to use the method for hydrogeomorphic
research.
2. Topographic surveying of mountain streams
We selected reaches of three small (second and third
order from USGS 7.5 min maps), headwater streams in
the Oregon Coast Range for our research of channel
morphology. The approach was to delineate control and
experimental reaches, apply a treatment to the experimental reaches, and then study eects of treatment with
a chronosequence of DTMs generated using data collected with a total station. We were speci®cally interested in quantifying changes in physical aquatic habitat
for salmonid ®sh, but could have used the data for many
other uses.
the entire experimental reach. Reaches were 460±700 m
long, requiring seven to ten control points each. The
control points were 2.5 cm ´ 5 cm (1 in ´ 2 in) wooden
stakes (``hubs'') or 1.25-cm (0.5 in) diameter steel bars
driven into the ground.
We established a local coordinate system for each
survey, assigning an arbitrary elevation and north and
east coordinates for the ®rst control point (hereafter:
point of beginning [POB]). A second control point was
set due north of the POB at the longest practicable
distance from the POB for use as a reference backsight.
Once we had established coordinates for the POB and
backsight, we traversed through the control points to
establish their coordinates and elevations.
2.2. Topographic mapping procedures in the channel
Using a Leica T1010 electronic digital theodolite
equipped with a Leica DI1001 electronic distance measuring device (Leica AG, Heerbrugg, Switzerland), we
took radial side ``shots'', composed of a horizontal angle, vertical angle, and slope distance, from each control
point to points in and adjacent to the stream. With this
information, the coordinates of each point were determined relative to the POB. Along with coordinates, each
point was given a description and a unique point identi®er. The points were selected as local minima and
maxima that would best describe the ground surface
when used to create a TIN.
We strati®ed the density of side shots by location
relative to the idealized cross section illustrated in
Fig. 1, so that the highest density of shots would be
within the area of greatest interest. Between the toes of
slope (the area of maximum interest), we took shots at a
density sucient to describe features larger than 15 cm
(0.5 ft) in size. A greater number of shots would increase
the resolution of the resulting DTM, but would require
more time. We took side shots outside the channel only
at gross topographic breaks in slope to roughly depict
the riparian area outside the channel on the resulting
maps.
To describe the banks, we took shots along the top of
both banks and at the toe of each slope. We rarely took
2.1. Layout of topographic survey
After the reaches for the experiment had been selected, we established control points for the topographic
survey along each reach, outside the active channel. We
situated the control points about 50±100 m apart, so
that the views from them collectively aorded a view of
Fig. 1. Diagram of idealized standard stream cross-section used in our
research. We imposed breaklines at top of bank and toe of slope, but
not for the thalweg.
43
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
shots between the top of the bank and the toe of slope
on each bank. Where breaks in slope occurred so that
large features lay between the top of the bank and the
toe, we moved the toe of slope shot upslope to describe
the feature and used thalweg shots to describe the true
toe. Exceptions were when unusual features such as
uprooted trees occurred between the top and toe of the
slope; these were captured with supplementary shots.
We used ``breaklines'', which force the terrain-modeling algorithm to triangulate between points such that
triangle axes lie along breaks in slope, to connect shots
along the banks. The top of each bank and the toe of
each slope, corresponding to the idealized cross-section
in Fig. 1, were each described by a breakline because
they are natural boundaries in our streams. The channel
did not always conform to our idealized cross section,
but we continued all breaklines through non-standard
reaches so we could use them as boundaries between
areas of dierent sampling intensities. In practice, we
usually chose locations of breaklines based on both the
breaks in slope and areas of ¯uvial activity. For exam-
ple, breaklines de®ning the top of bank were always
outside of any obvious recent changes in morphology,
even if the most distinct break in slope was nearer the
thalweg.
Overhanging banks present a special problem in topographic mapping (Fig. 2), because there is no standard method for representing an empty space below the
surface of the ground on a topographic map (Ref. [2]). It
is nearly impossible to take shots beneath most overhanging banks. The most common option is to place the
rod at an estimated, oset position. We used several
methods to try to solve this problem, and each method
had tradeos in accuracy (Table 1). We settled on a
method (option 4 from Table 1) that ignored overhangs
that consisted primarily of roots and vegetation. Undercuts were also ignored if, in our judgment, including
them would have resulted in a signi®cant error in measurement of volume of the bank. To describe banks with
ignored undercuts, we estimated oset positions of the
rod for shots below the overhangs. Most of the overhangs we ignored consisted primarily of vegetation, and
the undercuts we ignored were minor compared to
stream volume.
2.3. Non-topographic features
Fig. 2. Potential errors in measurement due to overhanging banks.
Breaks in slope that cannot be represented without an oset shot are
indicated by X.
Some features of the stream channel and the adjacent
¯ood plain were included in the survey but not the
DTM. We included the location of large woody debris in
our surveys (Fig. 3). We also took shots at the estimated
limits of aquatic habitat units (Ref. [1]) to help describe
the relationship between morphology and habitat.
Table 1
Options of protocols for measurement of overhanging banks (refer to Fig. 2)
Protocol
Advantages
Disadvantages
Recommendations
1. Allow breaklines to follow
true toe slope and top of bank
Most accurate measurement
of both stream bottom and bank
Use when the higher accuracy is
important enough to warrant
increased complexity in analysis
of data
2. Ignore overhangs; treat
as a vertical bank
Accurate measurement of stream
bottom; precise
3. Ignore undercuts; treat
as a vertical bank
Accurate measurement of top of
bank; precise; no estimations
of oset position of rod necessary
4. Ignore selected undercuts
or measure some undercuts
dierently than others; based on
forming agents and morphology
Compromise between accuracy
and precision; most eective way
to distinguish between overhangs
of vegetation and those of soil
and rock
Results in breakline that cross
horizontally - must be manually
edited for accurate model; must
estimate oset positions of the
rod for shots below overhangs;
still ignores other vertical breaks
in slope within overhangs
Volume of bank underestimated;
must estimate oset position of
the rod for shots below overhangs
(prone to error)
Measurement of stream bottom
inaccurate; volume of stream
underestimated; overestimate
volume of bank
Criteria for selection of which
undercuts to include is dicult
to de®ne, therefore this is an
imprecise method; potential for
bias from observers; must estimate
some oset positions of the rod
for shots below overhangs (prone
to error)
Use when measurements of the
bank are less important than measurements of the stream bottom
Use when measurements of the
bank are more important than
measurements of the stream bottom
Use for multiple objectives. This
is the most versatile method
44
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
that were relevant to our objectives occurred on an annual or semi-annual frequency. For example, Fig. 4
shows a small pool developing under a piece of large
woody debris, caused by local scour. This scour does not
occur during summer low-¯ow, but the eects of higher
¯ows from the previous winter persist and constitute the
physical aquatic habitat for much of the following year.
3. Recommendations
3.1. Survey setup
Fig. 3. Perspective view, upstream to downstream, of a digital terrain
model of a 5-m wide stream channel. Lines are either contours or lines
of constant longitude. The central portion of this view is also shown in
Fig. 4e.
2.4. Terrain modeling and data analysis
We used Leica's LISCAD Plus Surveying and Engineering Environment software (Leica AG and LIStech,
Boronia, Vic., Australia) and Surfer (Golden Software,
Golden, CO) to process the data from surveys, and to
create a DTM and a topographic map of the channels
(Figs. 3 and 4). To create a DTM, this software forms a
TIN by triangulating between points and interpolating
elevations linearly along the axes of the triangles. The
breaklines are important in this process, since the algorithm will not triangulate across a breakline. Once
triangulation is complete, the software draws contour
lines to connect extrapolated points of equal elevation.
Once the DTMs were complete, we were able to use
LISCAD and other software to calculate depths or
volumes of pools, sinuosity of the channel, ratios of
width to depth, and other measures of morphology.
Repeat topographic surveys of the same stream reaches
enabled us to quantify such processes as local scour and
®ll and make reach-level assessments of aggradation,
degradation, and stability of banks.
2.5. Repetition of surveys
We repeated the topographic survey of each stream
channel once per year at periods of low ¯ow (Fig. 4);
some changes in channel morphology occurred much
more frequently than once annually, but the changes
Before establishing a network of control points, researchers should decide whether to orient the network of
control points to true north, property boundaries, or sea
level. True north can be important if, for example, solar
angles or topographic shading of the stream are to be
included in the research. Orientation to boundaries
usually is important only when boundaries of ownership
are involved. If orientation is necessary, it can be accomplished by a solar shot, star shot, section survey, or
by using a Global Positioning System (GPS) device. It
may be important to establish elevation relative to mean
sea level if, for example, tides or ¯ood levels are integral
to research objectives. In establishing elevation, greatest
accuracy can be achieved by referencing the network of
control points to a benchmark of known elevation,
though a rough estimate from a topographic map may
suce. A survey-grade GPS unit can be used to establish
elevation, but inexpensive mapping-grade units may not
be accurate enough for ®ne work.
There should be enough control points to allow unobstructed views of the entire channel. However, there
should not be more than necessary, because increasing
the number of control points increases the potential for
errors (Ref. [6]). We recommend closing the traverse if
possible (i.e. including the ®rst control point in the
network as the last as a check). Even if the traverse is
not closed, frequently checking the backsight will reduce
systematic error by operators. Leaving the traverse open
can lead to many problems in reduction of data and
deduction of errors, which are common when the work
is done by people with limited surveying experience.
It is important to place control points where they will
not be disturbed by maintenance of roads, vandalism,
high stream ¯ow, or channel meanders. Recording at
least three ties (measurements to established secondary
marks such as nails in trees or secondary monuments in
the ground) to control points reduces the chance that a
control point will be lost, or if lost can be re-established.
In our experience, steel bars are preferable to wooden
stakes as control points because they are more durable
and easier to use when setting up the instrument.
Control points should be established either where
vegetative growth will not block the view in subsequent
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
45
Fig. 4. Chronosequence of digital terrain models of the same stream channel.
surveys or where vegetation can be easily removed. Not
all control points must be permanent, however. If
needed, additional control points may be set temporarily
for convenience in an individual survey.
3.2. Intensity of surveys
The choice of appropriate temporal and spatial intensity for topographic surveys should be based on objectives of the research, available resources,
characteristics of the channel, and the processes of in-
terest (Ref. [13]). Inappropriate intensity can lead to
data that are not useful for the objectives of the
research.
Choosing an appropriate temporal scale for repetition of surveys requires knowledge of the stream and
processes being examined. The appropriate scale might
range from decades to minutes or even less. For example, we surveyed once year to measure characteristics
that change primarily during storms in the winter and
remain relatively stable during the summer. Ref. [8]
surveyed a channel up to three times per day to measure
46
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
scour and ®ll in a glacial-outwash stream where morphology changed signi®cantly hourly. Ref. [7] suggested
that an initial sampling to capture multiple temporal
scales of change allows researchers to make more informed decisions about selecting an appropriate temporal scale. For example, conducting a pilot study of
weekly, then monthly, then yearly surveys would allow
better understanding of the temporal scale of the processes of interest if this is not already known.
The processes being studied help de®ne the appropriate density of shots by de®ning the size of changes in
the channel that are to be measured. It is important to
de®ne spatial resolution by the sizes of the eects or by
some threshold of size above which the changes are
important, because incorrectly matching spatial scale or
measurement to processes can lead to erroneous or
spurious interpretations (Ref. [21]). For example, we did
not attempt to measure small changes in morphology
outside of the most ¯uvially active parts of the channel,
because we assumed that such changes would be largely
due to non-¯uvial forces such as animals or vegetation.
Even within the channel, we limited interpretation of
channel changes to changes in morphology of P 15 cm,
because we made no attempt to measure features smaller
than this.
Besides the processes of interest, the appropriate
density of shots is de®ned by the size of particles in
the stream channel (Ref. [8]). As the size of substrate
increases, the practically achievable resolution of surveys decreases; it would be necessary to map individual particles for an accurate model if the desired
resolution is ®ner than the dominant substrate in the
channel.
Strati®cation of density of shots allows greater resolution where it is most useful. Reduced density can be
useful when there are predictable regions of less ¯uvial
activity or of larger substrate. Fig. 4 is an example of the
eect of stratifying spatial intensity of surveying. Outside the banks, low density of data is useful as little more
than a suggestion of the landform. The banks themselves are more precisely described, but there has been
no eort to describe ®ne changes. Only between the
banks are data dense enough to discern changes as small
as 15 cm, which was the minimum spatial scale that was
considered in this application.
Selection of temporal and spatial intensity should be
linked. Since smaller features are more ephemeral, the
appropriate density of shots increases as frequency of
surveys increases. Practical considerations limit surveys
as well. Our maximum speed was 240 shots per hour,
equal to that reported in Ref. [8], though due to dense
vegetation and time spent traveling to our research sites,
some 300-m experimental reaches required up to six
days to survey. Researchers planning large, detailed
surveys should be aware that they require large investments in time.
3.3. Methods of topographic mapping
Using breaklines when creating DTMs increases accuracy considerably (Ref. [2]). Examples of where
breaklines are particularly useful are at still logs, linear
features in bedrock, ridges in gravel bars, and areas of
maximum depth in trench pools. For example, we used
breaklines to help de®ne the banks, and imposing additional breaklines in the bottom of the channel would
have increased accuracy. Breaklines should be de®ned
concurrently with collection of data, since they are impossible to impose during analysis of data without very
speci®c descriptions of shots. Another option is to impose breaklines during proo®ng of topographic maps in
the ®eld.
One way to make application of breaklines easier and
more consistent in the ®eld is to match them to repeated
features of channels. Often, these repeated features
demarcate areas of dierent channel-forming processes
that are important when describing morphology. For
example, the cross-sectional representations that Rosgen
[17] used to classify channel morphology could easily be
adopted as standard cross-sections for a topographic
survey.
When deciding upon a method for measuring undercut banks, it is important to remember that it is not
possible to measure all con®gurations of overhanging
banks using any of the methods listed in Table 1. Spaces
with complex shapes and attributes often exist where
undercut banks combine with roots or underground
channels. The spatial and temporal variability of these
features makes accurate measurements of the bank dif®cult. Choosing a method to measure undercut banks
involves deciding which characteristics of the bank
should be measured most accurately, and should be
based on which features are most important for objectives of a particular study.
Surveyors should identify and reduce error (Table 2).
When inexperienced crews are involved, it is important
to concentrate on reducing undetectable errors. Researchers who are unfamiliar with equipment can reduce
random errors and blunders by attending training, or by
hiring professional surveyors, though this can be expensive.
3.4. Equipment and software for analysis of data
Almost any total station is appropriate for topographic surveys of streams. The precision of the instrument must be highest for surveys of highest intensity,
though topographic mapping is generally a technique of
low precision when compared to other tasks such as
boundary surveys. For the spatial intensities we used, an
instrument accurate to the nearest 20 s of arc would
have been sucient. Most total stations are accurate to
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
47
Table 2
Errors associated with the method (modi®ed from [8])
Error type
Examples of speci®c errors
Detectability
Random (precision)
Incorrect entry of manually recorded
control network information
Detectable if the control network is designed to provide data
redundancy or if the traverse is closed
Measurements associated with radial
observations
Undetectable since there are no repeat observations, but
minimized through the use of electronic data collection
Systematic (accuracy)
Incorrect measurement of instrument,
rod, or traverse target height
Undetectable unless procedures are heavily standardized or
errors are gross and interpretable
Systematic and random
Measurements associated with the control
network
Detectable if observations are repeated during the control survey
or if traverse is closed
Incorrect use of the instrument or rod: Out
of level, incorrect backsight procedure,
incorrect foresight procedure
Gross errors usually detectable, but ®ne errors undetectable
Non-vertical survey rod
Minimized through use of bubble level attached to survey rod
the nearest ®ve seconds or one second, so are suitable
for almost any desired density of shots.
Many dierent software packages will reduce raw
survey data, though software is usually designed speci®cally for each data collector. Total stations that collect data internally are usually useful only with software
for reduction that was designed for use with that instrument. Instruments that send data to external collectors allow the used more choices of software for
reduction of raw data, but can be less convenient to use
in the ®eld.
Software for creating DTMs is sometimes packaged
with the survey data reduction software, but there are
other packages besides those speci®cally designed for
applications in surveying. Software used for creation of
DTMs uses a variety of algorithms (Ref. [2]), and correct interpolation between points is dependent on a set
of data that were collected for that speci®c algorithm.
For example, using an algorithm that creates a TIN
assumes that data were collected such that there are no
breaks in slope between any shots, that shots were taken
at local minima and maxima of elevation, and that the
shots were spaced in a pattern that takes advantage of
the algorithm's method of maximizing the eciency of
the triangulation (Ref. [5]). By contrast, algorithms
based on kriging expect data in somewhat regularly
spaced intervals, and local minima and maxima are inferred instead of measured (Ref. [20]). When designing a
survey, it is essential that researchers understand which
method of interpolation will be used in the analysis.
Inaccurate DTMs may result if created by an algorithm
designed for a dierent type data than was collected.
Once data from the ®eld have been reduced, the data
are three-dimensional coordinates that can be used by
many programs designed for analysis of DTMs. The
structure of the data enables a large number of strategies
of analysis and applications related to stream channel
morphology. This versatility represents the major
strength of an exhaustive set of data.
Acknowledgements
The authors thank Liz Dent, John Donahue, Jim
Schroeder, and Vanessa Stone for helping develop the
method in the ®eld. The Coastal Oregon Productivity
Enhancement (COPE) program of the Oregon State
University College of Forestry funded this research.
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Digital terrain modeling of small stream channels with a total-station
theodolite
Richard F. Keim
b
a,*
,
Arne E. Skaugset a, Douglas S. Bateman
b
a
Oregon State University, Forest Engineering Department, 215 Peavy Hall, Corvallis, OR 97331, USA
Oregon State University, Forest Science Department, 026 Forest Sciences Laboratory, Corvallis, OR 97331 USA
Received 7 May 1998; received in revised form 12 December 1998; accepted 3 February 1999
Abstract
A Digital Terrain Model (DTM) is an alternative to traditional measures of stream channel morphology that allows for extraction of many dierent types of data. This paper describes a method of creating high-resolution Digital Terrain Models of stream
channels using an electronic, digital, total-station theodolite and standard methods of land surveying, and also includes considerations unique to hydrological application. Included is a detailed description of one application in Oregon, and also suggestions of
how to apply the method in other research of morphology. Ó 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Digital terrain model; Mapping; Channel morphology
1. Introduction
Quanti®cation of channel morphology is a requirement for a variety of subdisciplines within surface
hydrology and geomorphology. Morphological characterizations taken from data collection in the ®eld commonly are limited in scope, are not useful for multiple
objectives, and often suer from low precision or poor
accuracy. For example, wildland hydrologists have used
many dierent methods to quantify the shape of small,
headwater streams, including cross sections (e.g. Refs.
[14,16,18]), longitudinal pro®les (e.g. Refs. [10,15,19]),
or qualitative delineation of aquatic habitat units such
as pools and ries (Refs. [1,10]). Most of these methods
collect two-dimensional data, although the processes
and conditions they seek to describe are three-dimensional. Qualitative data, which some methods collect,
are insucient to quantitatively link hydrological
processes in the channel to morphology (Ref. [8]).
In research of channel morphology, the most broadly
useful information is a three-dimensional model of the
stream channel. One such model is a Digital Terrain
Model (DTM), which is a digital representation of a
surface designed to show topographic dierence. The
*
Corresponding author. Tel.: 001-541-737-0515; fax: 001-541-7374316; e-mail: [email protected]
digital format is usable in many software packages, including geographic information systems (GIS), so multiple quantitative analyses are possible and data may be
extrapolated for multiple research objectives. Precision
and accuracy of DTMs are limited only by the amount
of eort expended during collection of data, so can be as
high as required for nearly any application.
Digital terrain models provide a quantitative basis for
analyses of many aspects of channel morphology, including sediment transport, local scour and ®ll, and
bank stability. A time series of DTMs allows changes in
the channel to be measured precisely without prior
designation of exact sample points. A single DTM serves
as a base for research of microtopographic eects on
velocity, turbulence, and scour. Also, companion data
may be easily incorporated to a DTM using a GIS.
These may be the object of research that must be considered in the context of the DTM, or they may simply
enhance the usefulness of the DTM. As an example, Ref.
[22] and Keim et al. (in preparation) describe movement
of coarse debris in channels using a chronosequence of
DTMs.
We have developed a method to collect data for the
creation of DTMs by making topographic surveys of
stream channels using a total-station theodolite (``total
station''). A total station collects data electronically and
incorporates an electronic distance-measuring device
that eliminates the need for manual measurements of
0309-1708/99/$ ± see front matter Ó 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 3 0 9 - 1 7 0 8 ( 9 9 ) 0 0 0 0 7 - X
42
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
distance. While others have made maps of streams,
using taped distances from a baseline (Ref. [3]), dumpy
level and range ®nder (Ref. [9]), or survey by stadia with
a theodolite (Refs. [4,11,12]), these methods result in
either an imprecise DTM or a less complete model, and
are subject to a large amount of error from bias by
personnel and are very susceptible to mistakes during
collection of data (Ref. [6]). Using a total station eliminates many of these problems, and is also faster than
other mapping methods capable of producing comparable data.
A DTM may be generated from source data by a
variety of methods, but for traditional land surveys, the
most common is creation of a Triangular Irregular
Network (TIN) (Ref. [2]). A TIN is, in general, a continuous network of triangles connecting points of data
within a set. In this application, the vertices of the triangles in the TIN are de®ned by critical points on the
ground that describe breaks in slope (Ref. [5]). The data
in a topographic survey consist of three-dimensional
coordinates of the critical points that will later become
vertices of the triangles in the TIN.
Despite the substantial requirements of equipment
and time, a DTM and topographic map made with a
total station is a useful research tool for intensively
measuring local channel features and processes. It can
also be used to assess the accuracy and precision of
other methods that measure channel morphology. In
this paper, we present the methods we used to create
DTMs and topographic maps of small, headwater
stream channels using a total station. We also recommend ways to use the method for hydrogeomorphic
research.
2. Topographic surveying of mountain streams
We selected reaches of three small (second and third
order from USGS 7.5 min maps), headwater streams in
the Oregon Coast Range for our research of channel
morphology. The approach was to delineate control and
experimental reaches, apply a treatment to the experimental reaches, and then study eects of treatment with
a chronosequence of DTMs generated using data collected with a total station. We were speci®cally interested in quantifying changes in physical aquatic habitat
for salmonid ®sh, but could have used the data for many
other uses.
the entire experimental reach. Reaches were 460±700 m
long, requiring seven to ten control points each. The
control points were 2.5 cm ´ 5 cm (1 in ´ 2 in) wooden
stakes (``hubs'') or 1.25-cm (0.5 in) diameter steel bars
driven into the ground.
We established a local coordinate system for each
survey, assigning an arbitrary elevation and north and
east coordinates for the ®rst control point (hereafter:
point of beginning [POB]). A second control point was
set due north of the POB at the longest practicable
distance from the POB for use as a reference backsight.
Once we had established coordinates for the POB and
backsight, we traversed through the control points to
establish their coordinates and elevations.
2.2. Topographic mapping procedures in the channel
Using a Leica T1010 electronic digital theodolite
equipped with a Leica DI1001 electronic distance measuring device (Leica AG, Heerbrugg, Switzerland), we
took radial side ``shots'', composed of a horizontal angle, vertical angle, and slope distance, from each control
point to points in and adjacent to the stream. With this
information, the coordinates of each point were determined relative to the POB. Along with coordinates, each
point was given a description and a unique point identi®er. The points were selected as local minima and
maxima that would best describe the ground surface
when used to create a TIN.
We strati®ed the density of side shots by location
relative to the idealized cross section illustrated in
Fig. 1, so that the highest density of shots would be
within the area of greatest interest. Between the toes of
slope (the area of maximum interest), we took shots at a
density sucient to describe features larger than 15 cm
(0.5 ft) in size. A greater number of shots would increase
the resolution of the resulting DTM, but would require
more time. We took side shots outside the channel only
at gross topographic breaks in slope to roughly depict
the riparian area outside the channel on the resulting
maps.
To describe the banks, we took shots along the top of
both banks and at the toe of each slope. We rarely took
2.1. Layout of topographic survey
After the reaches for the experiment had been selected, we established control points for the topographic
survey along each reach, outside the active channel. We
situated the control points about 50±100 m apart, so
that the views from them collectively aorded a view of
Fig. 1. Diagram of idealized standard stream cross-section used in our
research. We imposed breaklines at top of bank and toe of slope, but
not for the thalweg.
43
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
shots between the top of the bank and the toe of slope
on each bank. Where breaks in slope occurred so that
large features lay between the top of the bank and the
toe, we moved the toe of slope shot upslope to describe
the feature and used thalweg shots to describe the true
toe. Exceptions were when unusual features such as
uprooted trees occurred between the top and toe of the
slope; these were captured with supplementary shots.
We used ``breaklines'', which force the terrain-modeling algorithm to triangulate between points such that
triangle axes lie along breaks in slope, to connect shots
along the banks. The top of each bank and the toe of
each slope, corresponding to the idealized cross-section
in Fig. 1, were each described by a breakline because
they are natural boundaries in our streams. The channel
did not always conform to our idealized cross section,
but we continued all breaklines through non-standard
reaches so we could use them as boundaries between
areas of dierent sampling intensities. In practice, we
usually chose locations of breaklines based on both the
breaks in slope and areas of ¯uvial activity. For exam-
ple, breaklines de®ning the top of bank were always
outside of any obvious recent changes in morphology,
even if the most distinct break in slope was nearer the
thalweg.
Overhanging banks present a special problem in topographic mapping (Fig. 2), because there is no standard method for representing an empty space below the
surface of the ground on a topographic map (Ref. [2]). It
is nearly impossible to take shots beneath most overhanging banks. The most common option is to place the
rod at an estimated, oset position. We used several
methods to try to solve this problem, and each method
had tradeos in accuracy (Table 1). We settled on a
method (option 4 from Table 1) that ignored overhangs
that consisted primarily of roots and vegetation. Undercuts were also ignored if, in our judgment, including
them would have resulted in a signi®cant error in measurement of volume of the bank. To describe banks with
ignored undercuts, we estimated oset positions of the
rod for shots below the overhangs. Most of the overhangs we ignored consisted primarily of vegetation, and
the undercuts we ignored were minor compared to
stream volume.
2.3. Non-topographic features
Fig. 2. Potential errors in measurement due to overhanging banks.
Breaks in slope that cannot be represented without an oset shot are
indicated by X.
Some features of the stream channel and the adjacent
¯ood plain were included in the survey but not the
DTM. We included the location of large woody debris in
our surveys (Fig. 3). We also took shots at the estimated
limits of aquatic habitat units (Ref. [1]) to help describe
the relationship between morphology and habitat.
Table 1
Options of protocols for measurement of overhanging banks (refer to Fig. 2)
Protocol
Advantages
Disadvantages
Recommendations
1. Allow breaklines to follow
true toe slope and top of bank
Most accurate measurement
of both stream bottom and bank
Use when the higher accuracy is
important enough to warrant
increased complexity in analysis
of data
2. Ignore overhangs; treat
as a vertical bank
Accurate measurement of stream
bottom; precise
3. Ignore undercuts; treat
as a vertical bank
Accurate measurement of top of
bank; precise; no estimations
of oset position of rod necessary
4. Ignore selected undercuts
or measure some undercuts
dierently than others; based on
forming agents and morphology
Compromise between accuracy
and precision; most eective way
to distinguish between overhangs
of vegetation and those of soil
and rock
Results in breakline that cross
horizontally - must be manually
edited for accurate model; must
estimate oset positions of the
rod for shots below overhangs;
still ignores other vertical breaks
in slope within overhangs
Volume of bank underestimated;
must estimate oset position of
the rod for shots below overhangs
(prone to error)
Measurement of stream bottom
inaccurate; volume of stream
underestimated; overestimate
volume of bank
Criteria for selection of which
undercuts to include is dicult
to de®ne, therefore this is an
imprecise method; potential for
bias from observers; must estimate
some oset positions of the rod
for shots below overhangs (prone
to error)
Use when measurements of the
bank are less important than measurements of the stream bottom
Use when measurements of the
bank are more important than
measurements of the stream bottom
Use for multiple objectives. This
is the most versatile method
44
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
that were relevant to our objectives occurred on an annual or semi-annual frequency. For example, Fig. 4
shows a small pool developing under a piece of large
woody debris, caused by local scour. This scour does not
occur during summer low-¯ow, but the eects of higher
¯ows from the previous winter persist and constitute the
physical aquatic habitat for much of the following year.
3. Recommendations
3.1. Survey setup
Fig. 3. Perspective view, upstream to downstream, of a digital terrain
model of a 5-m wide stream channel. Lines are either contours or lines
of constant longitude. The central portion of this view is also shown in
Fig. 4e.
2.4. Terrain modeling and data analysis
We used Leica's LISCAD Plus Surveying and Engineering Environment software (Leica AG and LIStech,
Boronia, Vic., Australia) and Surfer (Golden Software,
Golden, CO) to process the data from surveys, and to
create a DTM and a topographic map of the channels
(Figs. 3 and 4). To create a DTM, this software forms a
TIN by triangulating between points and interpolating
elevations linearly along the axes of the triangles. The
breaklines are important in this process, since the algorithm will not triangulate across a breakline. Once
triangulation is complete, the software draws contour
lines to connect extrapolated points of equal elevation.
Once the DTMs were complete, we were able to use
LISCAD and other software to calculate depths or
volumes of pools, sinuosity of the channel, ratios of
width to depth, and other measures of morphology.
Repeat topographic surveys of the same stream reaches
enabled us to quantify such processes as local scour and
®ll and make reach-level assessments of aggradation,
degradation, and stability of banks.
2.5. Repetition of surveys
We repeated the topographic survey of each stream
channel once per year at periods of low ¯ow (Fig. 4);
some changes in channel morphology occurred much
more frequently than once annually, but the changes
Before establishing a network of control points, researchers should decide whether to orient the network of
control points to true north, property boundaries, or sea
level. True north can be important if, for example, solar
angles or topographic shading of the stream are to be
included in the research. Orientation to boundaries
usually is important only when boundaries of ownership
are involved. If orientation is necessary, it can be accomplished by a solar shot, star shot, section survey, or
by using a Global Positioning System (GPS) device. It
may be important to establish elevation relative to mean
sea level if, for example, tides or ¯ood levels are integral
to research objectives. In establishing elevation, greatest
accuracy can be achieved by referencing the network of
control points to a benchmark of known elevation,
though a rough estimate from a topographic map may
suce. A survey-grade GPS unit can be used to establish
elevation, but inexpensive mapping-grade units may not
be accurate enough for ®ne work.
There should be enough control points to allow unobstructed views of the entire channel. However, there
should not be more than necessary, because increasing
the number of control points increases the potential for
errors (Ref. [6]). We recommend closing the traverse if
possible (i.e. including the ®rst control point in the
network as the last as a check). Even if the traverse is
not closed, frequently checking the backsight will reduce
systematic error by operators. Leaving the traverse open
can lead to many problems in reduction of data and
deduction of errors, which are common when the work
is done by people with limited surveying experience.
It is important to place control points where they will
not be disturbed by maintenance of roads, vandalism,
high stream ¯ow, or channel meanders. Recording at
least three ties (measurements to established secondary
marks such as nails in trees or secondary monuments in
the ground) to control points reduces the chance that a
control point will be lost, or if lost can be re-established.
In our experience, steel bars are preferable to wooden
stakes as control points because they are more durable
and easier to use when setting up the instrument.
Control points should be established either where
vegetative growth will not block the view in subsequent
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
45
Fig. 4. Chronosequence of digital terrain models of the same stream channel.
surveys or where vegetation can be easily removed. Not
all control points must be permanent, however. If
needed, additional control points may be set temporarily
for convenience in an individual survey.
3.2. Intensity of surveys
The choice of appropriate temporal and spatial intensity for topographic surveys should be based on objectives of the research, available resources,
characteristics of the channel, and the processes of in-
terest (Ref. [13]). Inappropriate intensity can lead to
data that are not useful for the objectives of the
research.
Choosing an appropriate temporal scale for repetition of surveys requires knowledge of the stream and
processes being examined. The appropriate scale might
range from decades to minutes or even less. For example, we surveyed once year to measure characteristics
that change primarily during storms in the winter and
remain relatively stable during the summer. Ref. [8]
surveyed a channel up to three times per day to measure
46
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
scour and ®ll in a glacial-outwash stream where morphology changed signi®cantly hourly. Ref. [7] suggested
that an initial sampling to capture multiple temporal
scales of change allows researchers to make more informed decisions about selecting an appropriate temporal scale. For example, conducting a pilot study of
weekly, then monthly, then yearly surveys would allow
better understanding of the temporal scale of the processes of interest if this is not already known.
The processes being studied help de®ne the appropriate density of shots by de®ning the size of changes in
the channel that are to be measured. It is important to
de®ne spatial resolution by the sizes of the eects or by
some threshold of size above which the changes are
important, because incorrectly matching spatial scale or
measurement to processes can lead to erroneous or
spurious interpretations (Ref. [21]). For example, we did
not attempt to measure small changes in morphology
outside of the most ¯uvially active parts of the channel,
because we assumed that such changes would be largely
due to non-¯uvial forces such as animals or vegetation.
Even within the channel, we limited interpretation of
channel changes to changes in morphology of P 15 cm,
because we made no attempt to measure features smaller
than this.
Besides the processes of interest, the appropriate
density of shots is de®ned by the size of particles in
the stream channel (Ref. [8]). As the size of substrate
increases, the practically achievable resolution of surveys decreases; it would be necessary to map individual particles for an accurate model if the desired
resolution is ®ner than the dominant substrate in the
channel.
Strati®cation of density of shots allows greater resolution where it is most useful. Reduced density can be
useful when there are predictable regions of less ¯uvial
activity or of larger substrate. Fig. 4 is an example of the
eect of stratifying spatial intensity of surveying. Outside the banks, low density of data is useful as little more
than a suggestion of the landform. The banks themselves are more precisely described, but there has been
no eort to describe ®ne changes. Only between the
banks are data dense enough to discern changes as small
as 15 cm, which was the minimum spatial scale that was
considered in this application.
Selection of temporal and spatial intensity should be
linked. Since smaller features are more ephemeral, the
appropriate density of shots increases as frequency of
surveys increases. Practical considerations limit surveys
as well. Our maximum speed was 240 shots per hour,
equal to that reported in Ref. [8], though due to dense
vegetation and time spent traveling to our research sites,
some 300-m experimental reaches required up to six
days to survey. Researchers planning large, detailed
surveys should be aware that they require large investments in time.
3.3. Methods of topographic mapping
Using breaklines when creating DTMs increases accuracy considerably (Ref. [2]). Examples of where
breaklines are particularly useful are at still logs, linear
features in bedrock, ridges in gravel bars, and areas of
maximum depth in trench pools. For example, we used
breaklines to help de®ne the banks, and imposing additional breaklines in the bottom of the channel would
have increased accuracy. Breaklines should be de®ned
concurrently with collection of data, since they are impossible to impose during analysis of data without very
speci®c descriptions of shots. Another option is to impose breaklines during proo®ng of topographic maps in
the ®eld.
One way to make application of breaklines easier and
more consistent in the ®eld is to match them to repeated
features of channels. Often, these repeated features
demarcate areas of dierent channel-forming processes
that are important when describing morphology. For
example, the cross-sectional representations that Rosgen
[17] used to classify channel morphology could easily be
adopted as standard cross-sections for a topographic
survey.
When deciding upon a method for measuring undercut banks, it is important to remember that it is not
possible to measure all con®gurations of overhanging
banks using any of the methods listed in Table 1. Spaces
with complex shapes and attributes often exist where
undercut banks combine with roots or underground
channels. The spatial and temporal variability of these
features makes accurate measurements of the bank dif®cult. Choosing a method to measure undercut banks
involves deciding which characteristics of the bank
should be measured most accurately, and should be
based on which features are most important for objectives of a particular study.
Surveyors should identify and reduce error (Table 2).
When inexperienced crews are involved, it is important
to concentrate on reducing undetectable errors. Researchers who are unfamiliar with equipment can reduce
random errors and blunders by attending training, or by
hiring professional surveyors, though this can be expensive.
3.4. Equipment and software for analysis of data
Almost any total station is appropriate for topographic surveys of streams. The precision of the instrument must be highest for surveys of highest intensity,
though topographic mapping is generally a technique of
low precision when compared to other tasks such as
boundary surveys. For the spatial intensities we used, an
instrument accurate to the nearest 20 s of arc would
have been sucient. Most total stations are accurate to
R.F. Keim et al. / Advances in Water Resources 23 (1999) 41±48
47
Table 2
Errors associated with the method (modi®ed from [8])
Error type
Examples of speci®c errors
Detectability
Random (precision)
Incorrect entry of manually recorded
control network information
Detectable if the control network is designed to provide data
redundancy or if the traverse is closed
Measurements associated with radial
observations
Undetectable since there are no repeat observations, but
minimized through the use of electronic data collection
Systematic (accuracy)
Incorrect measurement of instrument,
rod, or traverse target height
Undetectable unless procedures are heavily standardized or
errors are gross and interpretable
Systematic and random
Measurements associated with the control
network
Detectable if observations are repeated during the control survey
or if traverse is closed
Incorrect use of the instrument or rod: Out
of level, incorrect backsight procedure,
incorrect foresight procedure
Gross errors usually detectable, but ®ne errors undetectable
Non-vertical survey rod
Minimized through use of bubble level attached to survey rod
the nearest ®ve seconds or one second, so are suitable
for almost any desired density of shots.
Many dierent software packages will reduce raw
survey data, though software is usually designed speci®cally for each data collector. Total stations that collect data internally are usually useful only with software
for reduction that was designed for use with that instrument. Instruments that send data to external collectors allow the used more choices of software for
reduction of raw data, but can be less convenient to use
in the ®eld.
Software for creating DTMs is sometimes packaged
with the survey data reduction software, but there are
other packages besides those speci®cally designed for
applications in surveying. Software used for creation of
DTMs uses a variety of algorithms (Ref. [2]), and correct interpolation between points is dependent on a set
of data that were collected for that speci®c algorithm.
For example, using an algorithm that creates a TIN
assumes that data were collected such that there are no
breaks in slope between any shots, that shots were taken
at local minima and maxima of elevation, and that the
shots were spaced in a pattern that takes advantage of
the algorithm's method of maximizing the eciency of
the triangulation (Ref. [5]). By contrast, algorithms
based on kriging expect data in somewhat regularly
spaced intervals, and local minima and maxima are inferred instead of measured (Ref. [20]). When designing a
survey, it is essential that researchers understand which
method of interpolation will be used in the analysis.
Inaccurate DTMs may result if created by an algorithm
designed for a dierent type data than was collected.
Once data from the ®eld have been reduced, the data
are three-dimensional coordinates that can be used by
many programs designed for analysis of DTMs. The
structure of the data enables a large number of strategies
of analysis and applications related to stream channel
morphology. This versatility represents the major
strength of an exhaustive set of data.
Acknowledgements
The authors thank Liz Dent, John Donahue, Jim
Schroeder, and Vanessa Stone for helping develop the
method in the ®eld. The Coastal Oregon Productivity
Enhancement (COPE) program of the Oregon State
University College of Forestry funded this research.
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