selecting such features and the number of measure- ments per feature, human interpretation always plays
a role. The modelling strategy relates to the function Ž
used for interpolation and to the data format e.g., .
grid or TIN . At present, a study is being carried out to assess the accuracy of photogrammetric DSMs by
using tachymetric measurements as reference.
3. Study area and data
The laser data were acquired by the contracted Ž
survey company Eurosense P.O. Box 4923, 4803 .
EX Breda, The Netherlands . This company was responsible for the flight procedure, the calibration
of the laser system, and post-processing of the laser measurements. The latter included conversion of the
coordinates of the points to the Dutch national refer- ence system, and filtering of blunders and of mea-
surements made on trees and houses. Filtering was carried out by means of automatic procedures devel-
oped at the company, followed by visual control and manual editing.
The laser system consists of a laser scanner, the Saab TopEye, a Trimble 4000 SSi GPS receiver and
Ž a Honeywell H423 INS technical details of the laser
. system can be found in Janssen, 1997 . The expected
planimetric and height accuracy varies between 10 Ž
. and 30 cm www.combitech.sersurvey . To obtain a
dense distribution of measurements, the system was mounted on a Eurocopter AS 350 helicopter that
flew at a mean flying height of 240 m. The flight took place in March 1996, during which period the
amount of leaves on trees is minimal, facilitating acquisition of data at the ground level. By using a
pulse frequency around 5 kHz and a scanning angle of 10
8 a swath width of approximately 80 m was obtained. This, together with a mean flying speed of
15 m s
y1
, yielded on the average 4 points per m
2
. These points are distributed on the ground in a
Z-shaped form. The data acquired with the help of the laser
system refer to the X, Y, and Z coordinates of points
2
Ž .
in a 10 km area in the region of Groningen Fig. 1 . The area has flat relief and parts are covered with
trees and man-made objects such as buildings and roads.
To study the feasibility of extracting the geo- graphical information in Table 1 from laser data, the
unfiltered laser measurements need to be used. To facilitate the extraction of the terrain relief, the fil-
tered data, delivered by the company in a grid format of 1 m = 1 m, is used instead. These two datasets
will be referred to as laser-DSM and laser-DTM.
To inspect the laser data for systematic errors and to assess the accuracy, both in planimetry and altime-
Ž Ž
. Ž
Fig. 1. Laser-DSM and reference measurements photogrammetric DSM region with black overlay and tachymetric measurements two ..
small grey parallelograms above ‘‘tachymetric measurements’’ .
try, three sets of reference measurements are used. Two of these datasets are tachymetric measurements
Ž .
Fig. 1 carried out in March 1996. The third set is a DTM of the area and the coordinates of the roof
Ž .
corners of buildings Fig. 1 . This DSM has been generated by means of photogrammetric measure-
ments on true colour aerial photographs at a scale of 1:4000 also acquired in March 1996. The expected
planimetric and height accuracies of the measured features are 6 and 7 cm, respectively.
4. Information content of the laser data
