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TRANSPORTABLE TEST-BAR TARGETS AND MICRODENSITOMETER MEASUREMENTS - A METHOD TO
CONTROL THE QUALITY OF AERIAL IMAGERY
R. Kuittinen, Finnish Geodetic Institute,
Geodeetinrinne 2, 02430 Masala, Finland
E. Ahokas, Finnish Geodetic Institute,
Geodeetinrinne 2, 02430 Masala, Finland
P. Jürvelin, University of Jyváskylà, Department of Physics,
PL 35, 40351 Jyváskylà, Finland
ISPRS Commission I, Working Group 1
KEY WORDS: Quality, Control, Targets, Imagery, Measurement
ABSTRACT
Transportable polyester plastic test bar targets were made and used to test their usefulness for determining the spatial resolution of
aerial photography. The results show that it is possible to construct and measure by microdensitometer light transportable targets to
test the spatial resolution of aerial photography.
1. INTRODUCTION
High quality is usually important for the user of aerial
photographs regardless of whether they are used by the
photographer or other users. When processing a
photogrammetric task the quality of the camera, film and
navigation devices are usually known and the task is carried out
using controlled methods. Thus the quality of the photographs
can be estimated in advance which is the basis for a good result.
In practice, however, some unpredictable factors like
atmospheric characteristics, defects in the camera, irregular
movements of the platform, rapid changes in terrain radiances
or problems in the film development process can significantly
lower the quality of the images. To be able to monitor the
quality of aerial photographs, objective testing methods are
needed.
The main quality criteria of aerial photography are spatial
resolution, geometric accuracy and the correctness of grey tones
or the colours in the image. A lot of factors affect these criteria,
(Hakkarainen et al., 1992). The spatial resolution of the image
has been selected here as the only quality criterion, because the
most important of these factors like poor transmittance of the
atmosphere, camera, film or development process reduce the
spatial resolution and thus the quality of the image.
The only objective method to test the spatial resolution of films
or images is to use standardized test fields with known figures,
contrasts and reflectances. These permanent fields are very few
in number and usually used only for testing the quality of
imaging systems. The fields cannot be used to control the
quality of operational aerial photography which are carried out
at different places and under different circumstances. To obtain
an objective method to test the spatial resolution of aerial
photography wherever wanted, transportable test targets are
needed.
In summer 1995 the Finnish Geodetic Institute carried out an
experiment to test the usefulness of transportable test targets.
The idea of the targets is that they are so small and light that
they can be placed in the terrain at the same time as the
targeting of control points is made. The results of the
experiment are presented in this paper.
99
2. METHOD
2.1 Targets
To be easy to transport the test target must be light in weight so
that it can be transported to the area which will be
photographed. The lightness of the targets were reached in two
ways. The targets were made of strong polyester plastic which is
light compared with wood or metal plates. The size of the target
was made small by using continuously changing bar widths.
This means that the width of the bars and the backgrounds
decreases according to a selected constant. This reduces the
length of the target by a factor of four or at least three. The bars
were painted on the plastic using suitable paints (two grey
tones). The reflectances and the contrasts of the painted bars
and the background correspond approximately to green grass
and bare soil. Thus the targets offer good possibilities to
evaluate the spatial resolution of the images when rural areas
are photographed. The size of the target depends on the scale of
the photography and will be made as small as possible. In 1995
test targets for 1:3 000 and 1:60 000 scale aerial photography
were made as Table 1 shows. Table 2 shows the characteristics
of permanent targets of the test field for 1:3 000 (C) and 1:60
000 (D) scale photography. These permanent targets were used
as a reference. Figure 1 shows a B-type test target in the field.
A B
size of target (m^) 2.5x1.5 5.0x10.5
width of bars (cm) 3.0-12.0 53.0-212
length of bars (m) 2,5 5.0
range of spatial res. (I/mm) 25-100 28-113
weight (kg) 4 50
number of bars and back- 1 1
grounds having the same width
ratio of the width between the 95 4/5
adjacent bars
Table 1. The characteristics of the transportable test-bar targets
for 1:3 000 (A) and 1:60 000 (B).
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
