202 P.L. Brown et al. Agricultural and Forest Meteorology 100 2000 199–212
The aim of this study was to develop a simple, re- peatable method for measuring crown diameter and
projected area using vertical photographs. This aim evolved out of the need to characterize the crown ar-
chitecture of Queensland maple, for we were partic- ularly interested in obtaining accurate measurements
on crowns with irregular or asymmetrical shapes.
2. Methodology
The photographic method of crown measurement was developed by calibrating images taken of objects
with known dimensions at known distances from the camera. The method was then applied in a 66-year-old
plantation of Queensland maple. This plantation is at Gadgarra State Forest, 17
◦
17
′
S 145
◦
43
′
E, and 640 m above sea level on the Atherton Tablelands in
north Queensland. Measurements using the photo- graphic method were compared with a conventional
ground-based method using a tape Philip, 1994. Both methods were applied to the same maple crowns.
Crown diameters were measured on 147 maple trees in an unthinned, 66-year-old plantation. The measured
stand was then thinned in order to release the best potential crop trees. This thinning operation isolated
and facilitated the definition of the crowns used in this study. Also, all the understorey plants were removed
in order to obtain a clear view of the canopy structure.
Estimation of projected crown dimensions by tape involved two measurements of crown diameter, on
North–South and East–West axes as located by com- pass Philip, 1994. Most crowns were asymmetrical
to varying degrees, so measurements were made with- out reference to the position of the stem. Crown ex-
tremities were located by looking vertically from the ground and crown diameter was calculated as the mean
of the two axis lengths.
The photographic method involved: 1. Calibration of image estimates using standards;
2. Refining methods to capture crown images; 3. Processing images using image analysis software;
4. Calculating crown dimensions from processed im-
ages in ‘Hemiphot’ ter Steege, 1994, a software package designed for hemispherical image analy-
sis. A Canon hemispherical lens 7.5 mm was used
throughout this study. Its use was not driven by an inherent superiority in its ability to record the visible
extent of crowns for all spherical lenses create radial distortions, particularly in the periphery of the field
and these limit the accurate estimation of area in im- age components. The best lens for a given task is the
longest focal length that can capture the entire dimen- sions of the largest size crowns under investigation,
as this minimizes radial distortions. Seldom is a sin- gle crown so large that it requires a hemispherical lens
for complete capture. However, in studies that require additional information on both a target crown and its
spatial relationships with its nearest neighbors, a hemi- spherical lens with its large field of view may have
advantages. A hemispherical lens was chosen for this study because hemispherical images were required for
stand canopy analysis in the ‘Hemiphot’, image anal- ysis package ter Steege, 1994.
2.1. Calibration of image estimates using standards To enable horizontal projected area calculations to
be derived from a spherical image, allowance had to be made for the effects of both perspective and radial
distortion. Perspective affects all images regardless of the lens focal length, but radial distortion occurs only
with a spherical lens. The effects of radial distortion are largest near the lens boundary and least near the
lens axis and they increase as the focal length of the lens decreases. Calibrations for radial distortion have
been noted for a variety of hemispherical lenses Her- bert, 1987; Clark and Follin, 1988, and the process
should be completed for each lens.
Templates for the measurement of distance on pho- tographic images were made from a set of six calibra-
tion images specific for a Canon 7.5 mm lens. To pre- pare these calibration images nine pegs were placed
2 m apart along a straight line in a relatively flat and open field. The centre peg was used as the beginning
of a line perpendicular to the first line, along which six positions were marked off at 5 m intervals between
15 and 40 m.
Six exposures were taken of the row of pegs, one at each 5 m interval between the 15 and 40 m point along
the perpendicular line. The lens was aligned so that the top of the centre peg was at the image centre. Resulting
negatives were scanned at 50 × 50 screen pixel scale resolution 2000 dpi dots per inch then imported
P.L. Brown et al. Agricultural and Forest Meteorology 100 2000 199–212 203
Fig. 2. Image manipulation and the effect of thinning on crown visibility. Image a: Full hemispherical view taken prior to thin-
ning. Image b: 50 × 50 pixel subset of the centre of image a note the difficulty in defining the full extent of individual crowns.
Image c: Same location as image a taken after thinning. Image d: 50 × 50 pixel subset of the centre of image c. Image e:
Merging the area-defining template on image d. Image f: Final merged image, ready for cover calculation in Hemiphot.
into the Adobe Illustrator drawing package Adobe, 1997. Four concentric rings were drawn around the
centre peg giving the appearance of a shooting target or ‘bulls-eye’ see Fig. 2e.
The limits to calibration dimensions were estab- lished from those encountered in the measured plan-
tation, where tree heights ranged from 15 to 40 m and crown diameters up to 10 m. The row of pegs in images
represented a horizontal plane in the forest canopy and the photo point the actual lens position near the forest
floor. Due to the effects of radial distortion, the fixed distances between pegs appeared in images to be sym-
metrically and progressively reduced away from the centre peg. Due to perspective, the further the line of
pegs was from the camera, the smaller the resulting sets of rings in templates.
2.2. Capturing crown images Throughout this study 400 ASA, Ilford, HP5 film
was used in a Canon T60 body with the Canon 7.5 mm fish-eye lens 1 : 5.6. Procedures for tak-
ing vertical photographs in forest environments are well documented Becker et al., 1989; Barrie et
al., 1990; Chen et al., 1991; Schaffer et al., 1991 www.gemlab.ukans.eduhp. Photographs of the
crowns were taken at a time when the contrast be- tween sky and vegetation components was maximal.
This required even illumination in the sky background and no direct sunlight being reflected by target vegeta-
tion. These conditions usually occurred at sunrise and sunset or on days with a dense and even cloud cover.
Windy periods were avoided because of crown sway.
Consistency in the speed of exposure relative to the prevailing light conditions is important. Chen et al.
1991 showed that variation in shutter speed for the same image taken at the same time causes large vari-
ation in canopy cover estimates. In this study, prior to each exposure, a light reading was taken with a
Minolta Auto Meter IV F. After some testing, it was found the best contrast was achieved by increasing
the shutter speed underexposing three conventional levels above that recommended by the meter. Shut-
ter speeds calculated this way are not independent of the site cover Chen et al., 1991. However, indepen-
dent readings were not available due to an absence of nearby canopy openings of sufficient size.
2.3. Selection of photo-points in the forest Ground points for photographs were selected using
the following three criteria: 1. Photo-points must be vertically below the target
crown; 2. Photo-points allowed the isolation of the target
crowns area from its neighbors;
204 P.L. Brown et al. Agricultural and Forest Meteorology 100 2000 199–212
3. The number of photo-points for each crown was a function of its size horizontal extent. Some
large crowns were photographed from four points, while the smallest crowns were photographed
from one point.
The lens was orientated as close as possible to ver- tical using a tripod with a geared head and finely cal-
ibrated bubble level on the camera body. The recent innovation of a self-leveling mount Oberbauer et al.,
1993; Rich et al., 1993 marketed by Delta T Devices, Cambridge UK would make this process faster.
2.4. Image processing and analysis Exposed negatives were scanned twice using a
Kodak Professional RFS 2035 Film Scanner. Critical factors in scanning include resolution, contrast and
brightness. The first image was scaled to take in the entire hemisphere at a resolution of 1000 dpi and
used to calculate the image centre. The second image, which took in a 50 × 50 pixel screen resolution sub-
set of the image centre, focused on the target crown and its nearest neighbors. It contained only those
components in the inner 30
◦
solid angle surround- ing the lens axis and was digitized at a resolution of
2000 dpi Fig. 2. The same screen scale and resolu- tion was used throughout all scanning procedures and
a contrast value of 20, the maximum for the Kodak Film Scanner, was used throughout the study.
Brightness settings varied between 5 and 10, de- pending on the visibility of texture within vegeta-
tion components. This variation in brightness settings caused less than 2 variation in cover readings. The
texture of vegetation was an unnecessary detail in this study that could confound results. Therefore, gray
tones representing vegetation were kept as close to black as possible.
All images including calibration templates were saved in a PCX format and converted to a 400 × 400
screen pixel scale in ‘Adobe Photoshop’ Adobe Sys- tems., 1996. This was the only common format and
maximum size acceptable in ‘Hemiphot’ ter Steege, 1994. Using the above settings in a more recent model
of scanner, the Nikon Coolscan II LS-20E, images were obtained with a crisper appearance, but with sim-
ilar cover readings 1 variation.
When using a spherical lens, knowledge of the proximity of crown components to the centre of the
image or lens axis is necessary to determine the accu- racy of area measurements. Heavy shade from dense
vegetation on the image periphery made it impossible to clearly define the edge of the full hemispherical
images of crowns. Inverting the image sky-black, vegetation-white improved edge resolution, but it
was still not always possible to clearly define the min- imum of three boundary points, necessary to locate
the image centre.
The image boundary is constant and set by the lens focal length. A circle at this specific diameter was cre-
ated and used as a template to fit as closely as possi- ble on the diffuse boundary of the full hemispherical
images of the crown. The inverted crown image was overlain with a circle template in Adobe Photoshop
Adobe Systems., 1996.
2.5. Manipulation of crown images Digitized crown images were recorded in 256
gray-tones. Converting all sky components to white and all vegetation components to black increased the
accuracy of final cover estimates in Hemiphot by sharpening sky-vegetation boundaries. The Adobe
Photoshop program Adobe Systems., 1996 was used to perform black and white conversions at a variable
threshold level.
A histogram was created within Adobe Photoshop Adobe Systems., 1996, representing the pixel dis-
tribution of all 256 gray-tones, from light to dark, within the image. Within this histogram the distribu-
tion of pixels representing vegetation components and sky could be separated and the accuracy of the photo-
graphic methodology in maximizing contrast between sky and vegetation could be determined.
2.6. Isolation of a target crown Each image targeted a specific crown and this crown
lay at or close to the image centre. So that cover val- ues represented only the crown of interest in each im-
age target crowns were isolated from their neighbors, by erasing all neighboring crowns and trunks from
images in Adobe Photoshop Adobe Systems., 1996. The trunk connected to the target crown was erased
up to the point where it first overlapped the foliage of the target crown. Using this point of first foliage
P.L. Brown et al. Agricultural and Forest Meteorology 100 2000 199–212 205
overlap, the trunk was erased perpendicular to its axial direction.
2.7. Merging crown images with an calibration templates
The height of the calibration template distance from photo point to the perpendicular line of pegs
corresponded to the height of the crown mid-point, or the mean of live crown base and top height Philip,
1994. The accuracy of crown projected area estimates is directly proportional to the total tree heightcrown
depth ratio. The larger the distance between the mea- surement plane and the actual locations of maximum
horizontal extension the greater the error resulting from inadequate calibration for perspective in images.
For example, conifer stands with deep narrow crowns are unsuited to its application, due to the high proba-
bility that the applied plane of horizontal measurement would be a relatively large vertical distance from the
actual locations of maximum horizontal extension.
Acceptable values of the tree heightcrown depth ratio exceed 2.5 and all trees in the present study sat-
isfied this criterion. The modified image of a single, isolated and
trunk-less crown was merged with the appropriate calibration template in Adobe Photoshop Adobe
Systems., 1996. In these merged images, the crown image formed the bottom layer and the appropriate
calibration template the upper layer. Before merg- ing, the calibration template was reduced to a 50
transparency using the opaque tool so that both the target crown and the templates concentric circles
were visible in the overlaid image Fig. 2. Circles and sections of the upper layer overlapping the crown
were removed using the eraser tool resulting in im- ages where crowns were enclosed by a single circle.
These layers were merged and saved in PCX format for analysis in Hemiphot Fig. 2.
Two methods were tested for positioning the cali- bration template on the modified crown image:
1. Centering the calibration template on the centre of the photographic image theoretically the most
accurate placement; 2. Centering the area template on the approximate
centre of the target crown without reference to the image centre.
Photographic points on the ground were selected to minimize the linear difference between the cen-
tre of the target crown and centre of the image. Centering the calibration template on the crown
saved considerable processing time and caused no more than 3 difference from the estimates de-
rived from photographic-centered templates. This acceptable reduction in accuracy was attributed to
the fact that all target crowns were located within 20
◦
of the lens axis, a zone of limited radial distortion.
2.8. Hemiphot analysis Each concentric ring of the calibration template rep-
resented an increment in radius of 2 m from its cen- tre. Given the limited depth of the tree crowns usu-
ally less than 5 m and their height above the lens be- tween 15 and 25 m, the height of each crown mid-
point generally corresponded with the height of its lateral extremities. As a result the circular calibration
template fitted to each target crown depicted an imag- inary yet plausible horizontal surface corresponding
to the height of the target crown’s maximum lateral extension.
Percentage crown cover was calculated with a cir- cular calculation template annex in the Hemiphot
program ter Steege, 1994. The user sets both the size and position on the image of this circular calcula-
tion template. The merged crown image was imported into Hemiphot and the circular calculation template
was overlaid onto the merged image circle of known diameter, which encompassed the target crown. The
percentage cover of the target crown within this merged image circle could then be derived and con-
verted into an estimate of crown projected area. Final estimates of crown projected area were made from
one to four, but mostly two different images of the target crown. Crown diameter was calculated by the
diameter of a circle equivalent of the crown-projected area.
Given that an absolute measurement of crown diam- eter was not possible for the trees under investigation,
the above Hemiphot-based procedure was considered to be the most precise estimate and was used as a stan-
dard against which a number of simpler modifications for this photographic technique could be compared.
206 P.L. Brown et al. Agricultural and Forest Meteorology 100 2000 199–212
2.9. Linear image estimates A rapid estimate of crown size on photographic
images can be obtained by measuring linear dimen- sions at fixed orientations. All images were orientated
with the north point at the top and the crown di- ameters on the cardinal axes were measured using
the pixel-locating tool in Adobe Illustrator Adobe, 1997. Linear pixel measurements were converted
into distance by calibration with the pixel diameter of the encompassing circle, which was known for each
tree. This technique was applied in the following two ways:
1. A rigid rule where the cut-off point depicting the crown’s extremities in each cardinal direction was
at the point of greatest lateral extension, no matter how small or isolated the associated cluster of
leaves may be;
2. A flexible rule whereby account was taken of small and isolated lateral extremities and a mean
crown extremity was estimated by eye. It was observed that most crowns were made up
of clusters of discrete units Fig. 1 and as a re- sult it was expected that the rigid approach, that
took no account of gaps in cover within the crowns area, would result in overestimates of crown pro-
jected area. The second approach was developed as a rapid, subjective method for increasing the accuracy
of these linear image estimates. The outcomes of both of these approaches have been compared in the
results.
To further analyze the differences between results from the tape and photographic techniques, crowns
were divided into two groups according to their degree of asymmetry, viz.:
1. Symmetric crowns were made up of one-two dis- crete units and were often lightly skewed to the
cardinal axes 69 of the total sample. A single or two unit crown was determined as symmetric if
a superimposed circle that encompassed the entire crown with the minimum diameter, had more than
70 vegetation cover within it. In encompassing circles with less than 70 cover, the cover distri-
bution must be even throughout all quadrants. The total amount of cover contained in each quadrant
was estimated and if any two of these four values varied more than 50 the crown was deemed as
asymmetric; 2. Asymmetric crowns with either three or more,
discrete units or one to two heavily skewed or highly irregular crown units according to the
above rule. A crown unit was considered discrete when the distance between adjacent clusters was
greater than the diameter of clusters. Crowns deemed as asymmetric made up 31 of the total
sample.
This description of crown symmetry differs some- what from Umeki 1995 who places more importance
on the spatial relationship between crown and trunk. Tape and linear image estimates were compared
with Hemiphot estimates through t-tests that utilized regression coefficients two sample t-tests and a
relative variable derived from tape or rule method es- timateHemiphot estimate one sample t-tests. T-tests
on regression coefficients were calculated with sig- nificance levels adjusted according to Bonferoni’s
rule.
3. Results and discussion
