implies either a reduction in the information contents of the geo
data, or requires an increase of the amount of field completion.
The reduction in interpretability could be counteracted by using
larger photoscales than we were used to - as long as the implied
cost increase of the total mapping process does not overtake the
expected efficiency gain of using digital instead of analytical
photogrammetry.
To find proper trade offs for the choosable parameters in a
photogrammetric geo-data production line the influence of the
image scale and the scanning pixel size on the interpretability of
the digital images should be known.
One of the production lines, where digital photogrammetry
allows already a far going automation, is image mapping
(orthophotography). This gives the possibility to produce image
maps quickly and cheaply, and many developing countries see
there a chance to get a complete coverage at scale 1:50,000.
Here the interpretability of the ortho images plays an important
role for the question how much annotation is required to make
the image maps a reasonable substitute for the line maps. Exten-
sive annotation however is expensive and time consuming.
We carried out a series of interpretational tests to particularly
investigate the impact of photoscale and scan resolution. The
tests are related to 1:50,000 topographic map specifications and
limited to wide-angle photography. We studied interpretability
for stereo observation using a digital photogrammetric work-
station (Traster T10 of Matra) as well as mono observation with
unaided eye of hard copies at scale 1:50,000 of digital ortho-
photos (also produced by the T10).
2. MATERIALS AND METHODS
2.1 Images and test site
We wanted to use good quality images of two different photo-
scales in the usual range for 1:50,000 mapping (1:25,000 to
1:80,000), preferably from the same site, and the same period.
From a site in Southern France we had first generation copies
(diapositives) of B/W aerial photographs at scales 1:30,000 and
1:60,000 with a difference of only three years and decided to
use them for this test. An area of 6km by 7km was selected,
containing varied terrain: flat and hilly agricultural parts up to
rugged mountainous forested parts. It does not contain urban or
industrial areas. It is contained in a single model of the 1:60,000
images, but from the 1:30,000 photography 4 models are needed
to cover it.
2.2 Scanning
To make sure, that differences in scan pixel size have a signifi-
cant influence on the interpretability, at least the largest pixel
size had to give a lower resolution than the original images. To
guarantee this, a "worst case" estimation for the resolution of the
aerial images was done.
With 6546 forward overlap and 3596 sidelap the maximum radial
distance to be used is appr. 110 mm. The camera calibration re-
port shows 40 Ip/mm tangential and 49 1p/mm radial resolution.
With aerial film the resolution is probably 20% less than with
the film used in the calibration, thus only 32 Ip/mm (tangential),
Usually the resolution is determined using high contrast targets
(100 : 1), but for interpretability the resolution at low contrast
(1.6 : 1), which may be up to 50% less, is more relevant. To be
on the safe side we used for the "most pessimistic" estimate a
value of 16 lp/mm, thus all used parts of the original diaposi-
tives should have a better resolution than 16 Ip/mm. Scanning
with 32 pixels per mm can thus not preserve the resolution fully,
We could therefor be sure to find a significant difference in
resolution between scans with pixel sizes of 60 um (appr. 17
pixels per mm), of 30 um (appr. 33 pixels per mm) and of
15 um (appr. 67 pixels/mm).
Two images (one model) 1:60,000 and six images (four models)
1:30,000 were scanned at GeoRas (Intergraph) with a Zeiss PS1
scanner, using a scanning pixel size of 15 um. We asked Geo-
Ras not to cut the tails from the histogram. Pixel sizes of 30 um
and 60 um were obtained by pixel aggregation from the 15 um
images. Considering the principle of the scanner this can be
assumed to be a good simulation of actual scans with 30 um and
60 um pixel sizes.
2.3 Orientation
Geometric accuracy was not part of the study. Only orientation
errors, which would make it difficult to relate digitized features
and features in the reference data had to be avoided. The
available ground control was far from ideal for our images, but
this was no problem. Even large errors, if made consistently in
all orientations including the reference data, would have been
tolerable.
2.4 Orthophoto production
Automatic DTM generation and ortho-image production of the
Matra Traster T10 was used as much as possible with the
default parameters. The digital orthophotos were exported to
another workstation, enhanced (3x3 Laplace + original image)
and then negatives were produced by an Optronics filmwriter
and photographic processing. Finally paper prints were made as
photographic contact copies. Five types of orthophotos were
produced according to table 1.
Input (digital image) Output (orthophoto)
original pixel size on ortho- pixel size on
scale photo
original | ground scale orthoph. | ground
1:60,000 | 60 um | 3.6 m | 1:50,000 | 100 pm | 5m
1:60,000 | 30 pm | 1.8m | 1:50,000 | 100 pm | 5m
1:60,000 | 30 pm | 1.8 m | 1:50,000 | 50 um | 25m
1:60,000 | 15 um | 09 m | 1:50,000 | 20 um 1m
1:30,000 { 60 um | 1.8 m | 1:50,000 | 50 um | 25m
Table 1: Types of Orthophotos
From the 1:30,000 photography four separate orthophotos were
produced. To avoid any influence of the mosaicing on the test,
306
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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