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calibration. In any case, when system calibration is applied to 
the photogrammetric mapping process, there has to be an 
accurately measured test field, but there also have to be various 
point classes so that there are targeted points, points on 
various terrain surfaces, and details of man-made features. The 
need for this kind of mapping test field became obvious during 
experimental work in Kaukajárvi area, Tampere, Finland 
(Niskanen, 1990, and Salmenperá, 1990). 
Calibrated stereomodel of Rusko 
There exists a photogrammetric test field for the purposes of 
system calibration in the Rusko area, Tampere, Finland, near 
Tampere University of Technology (Rantaniemi, 1993, and 
Salmenperá, 1993). It has been measured and photographed in 
1992. There are 61 control points with a standard deviations of 
2 mm and approximately 1600 test points with a standard 
deviation of 5 mm for the coordinates and the heights. The test 
points are located on terrain surfaces or on man-made features. 
The area has been photographed with Wild RC20 (c = 153.19 
mm) on Kodak MS2448 and Agfa AvChr 200 films. The flying 
height is 500 m, which gives the scale 1:3300. This scale is 
usually converted to a mapping scale of 1:500 in urban areas 
in Finland. Forward motion compensation has also been used, 
which is necessary to produce high-quality images for large- 
scale mapping. The images are available in digital form, too. 
As the test field exists as these images, it is called the 
calibrated stereomodel of Rusko. 
The images form two stereomodels. One of them is used for 
research and system calibration and it has approximately 1200 
points. The other one is used for training purposes and it has 
400 test points. The resolving power of the camera/film 
combinations varies from 25 to 30 lines/mm as evaluated from 
test figures. The geometric accuracy of the stereomodels has 
been tested by bundle block adjustment, where the image 
coordinates of all the targeted control points are adjusted with 
known ground coordinates. The resulting standard deviation of 
the unit weight equals 4.5 pm, which corresponds to 15 mm on 
the ground. 
TEST RESULTS 
Testing of stereo-operators 
The stereo-operator has to identify, classify and select points 
which belong to model surfaces, lines and objects, and then he 
or she has to point at them. Operators seem to have their 
personal ways of doing this, which results in systematic errors. 
Especially heights and unsymmetric objects show these kinds 
of systematic errors. The calibrated stereomodel can be used 
to test their capabilities to perform these tasks and to classify 
them. 
The operator starts the test by internal and external orientation 
of the stereopair. Then he or she goes over to computer- 
controll so that the measuring program brings the measuring 
marks near the test points. In height measurement, the 
operator has to set and record the height only, but when all 
the coordinates have to be measured, the given position is set 
randomly aside and the operator has to set the measuring 
marks in all dimensions. Before the end, part of the 
measurements are repeated. After one day's work the 
487 
measurements are processed and the operator's performance is 
evaluated. In this stage long-term statistical means are used as 
reference (table 1). 
Identification of terrain points 
Good definition of the points to be measured is the basic 
condition for achieving high and uniform accuracy in a 
photogrammetric survey. To define a point in a satisfactory 
manner, ground objects or features must have symmetrical 
form, suitable size and sufficient contrast with the surrounding 
area (Blachut, Chrzanowski and Saastamoinen, 1979). Very 
few natural or man-made ground features meet these 
requirements. Relevant factors are also the scale of the 
photography, the resolving power of the camera/film 
combination and image motion compensation. Furthermore, 
colour photos usually give better interpretability than black- 
and-white photos, and details can be recognized more easily if 
they cast shadows. On the other hand shadows may cover 
details. And, to make the matter still more complicated, the 
definition of terrain points to be measured is a matter of 
subjective judgement with the stereo-operator. 
Another side of this question is the role of quality control. 
According to Finnish instructions, photogrammetrically 
measured coordinates have to be checked against field 
surveyed coordinates, using a random sample (The Finnish 
Society of Photogrammetry and Remote Sensing, 1993). 
Because of identification problems, the instructions define the 
concept of the range of interpretability. If the deviations in the 
photogrammetrical coordinates from the field surveyed 
coordinates are less than the range of interpretability , the 
deviation is set to zero, or taken as errorless. 
Dimensional stability of photographs 
Uneven heating of the photographs can be a problem when 
using some analytical stereoplotters (Salmenperä and 
Miettinen, 1993). For example fluorescent tubes have high 
surface temperatures, they illuminate only part of the 
photographs and the area of intense heating moves as the work 
proceeds. This gives rise to local deformations. The way the 
photos are placed on the picture carriers below the cover 
glasses may strengthen these deformations. Then the pressure 
of the cover glass may at certain spots hinder the deformations. 
The resulting deformations are then both local and time- 
dependent and may exceed tens of micrometres (figure 1). In 
addition, the deformations at the locations of the fiducial 
marks are generally not representative of the deformations 
throughout the photograph. This means that corrections for film 
distortion cannot be computed reliably from the measurement 
of the fiducial marks unless special actions are taken. It is 
possible to reduce these deformations. One way to do this is to 
use a light source which is placed outside the plotter. Another 
means is to let the temperature of the photos reach the working 
temperature before the cover glasses are tightened. In 
addition, one could produce cover glasses which would be 
slightly lifted from the picture carriers, while still keeping the 
photographs flat. In the future the expanding use of digital 
images will improve the photogrammetric survey process in 
this respect. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996