3.3 Comparisons between the visual and the mathematical
method
The coefficient a in equation 6 was determined on the basis of
the visual method. Table 6 shows the differences of spatial
resolution of both methods for 1:3 000 and 1:60 000
photographs. Transportable targets for scale 1:60 000 were on
the field (fi) and near the forest (fo) and their direction cannot
be expressed in terms of tangential or radial.
Type 1:3 000 1:60 000
visual mathem visual mathem
image | 51 | 64 | 51 | 64 | 9 16 | 9 16
Perm | tan 71 121 1.71 | 56 |.48 1 48 ] 49 | 48
radial 1 71 1 62 1 711 71 | 481 401 48 | 48
Tran. | tan/i. | 79 1 71 | 791 79 | 5371 481 67.1 87
rad/fo 171 171 | 791] 71 1 48 1 40] 67. ] 48
Table 6. Spatial resolution (l/mm) determined visually and
mathematically from images number 9, 16, 51 and 64.
No difference between the methods was detectable.
3.4 AWAR-values
The AWAR-values were determined from the visual method
measurements. The image was divided into circular zones. The
mean values for tangential and radial spatial resolution were
calculated in each zone. Spatial resolution values were
determined graphically from 4 or 9 radius values depending on
the method. Extrapolation was used when needed. Detailed
description of the calculation method is given by (Hakkarainen
et al., 1992). AWAR-values were calculated using equation (1)
and they are presented in table 7. The two methods gave almost
the same results. There is a little difference between permanent
and transportable targets. This is possibly due to the range of
spatial resolution and also to the ratio of widths of bars of the
permanent targets.
Target type Ap. 4, time 1/410s | Ap. 5.6, time 1/410s
4rs 9 r's 4rs 9 r's
Permanent 40 36 48 47
Transportable 34 32 52 52
Table 7. AWAR-values for 1:60 000 scale photographs.
4. DISCUSSION
Preliminary measurements and calculations show that the usage
of transportable test-bar targets and the mathematical method
can be developed to a routine. It needs the standardization of
the measurement procedure and the determination of the
constant a values. The constant a has to be determined for
targets having groups of equal width bars and for targets, which
have continuously changing bar width. It seems that for large
scale photography traditional or standard targets can be used
because the size of the target still remains moderate whereas for
small scale aerial imagery targets having continuously changing
bars are needed.
The spatial resolution measured from five overlapping images,
if 80% forward overlap is used, is strictly speaking the spatial
resolution of those images. A photogrammetric block may
consist of hundreds of aerial images and the area can be very
large. The properties of the atmosphere can be different in
various parts of the block. In practise the results of the
104
transportable test-bar target method to control the quality of
aerial imagery have to be generalized over the whole
photogrammetric block. If more observations are needed the
targets can be photographed again at the end of the task.
References
Hakkarainen, J., Schroeder, M., 1992. In flight resolving power
tests of aerial cameras. Wissenschaftliche Arbeiten der
Fachrichtung Vermessungswesen der Universitit Hannover. Nr.
177.
Kuittinen, R., Ahokas, E., Hógholen, A., Laaksonen, J., 1994.
Test field for aerial photography. The Photogrammetric Journal
of Finland, Vol 14, No 1., pp. 53-62.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B1. Vienna 1996
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