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those pieces were printed separately and individually oriented
for the digitizing. For the orthophoto with 20 um pixel size the
negative was produced with 25 um pixels and photographic re-
duction was used instead of contact printing.
2.5 Data collection
Stereo data collections were done by three operators on the
Digital Photogrammetric Workstation Traster-T10 from Matra
using Demeter as database software. A "coarse to fine" scheme
was used such, that remembering the previous interpretation
could not bias the new one. When an operator had to interpret
an image part again, which he had interpreted before, the new
interpretation was with a much higher resolution.
scale of pixel size on operator
original
image image ground west east
1:60,000 60 um 3.6 m A B
1:60,000 30 um 1.8 m B C
1:30,000 60 um 1.8 m C A
1:30,000 30 um 0.9 m A B
1:60,000 15 um 0.9 m C A
1:30,000 15 um 0.45 m B C
1:60,000 original photographs another operator
1:30,000 original photographs C A
Table 2: Scheme (sequence) for stereo digitizing
To allow some comparison between different operators, the area
was split in two halves, and each operator did only either the
eastern or the western half for a photo scale / pixel size combi-
nation. Table 2 shows the scheme for the stereo digitizing, in-
cluding the collection of the reference data.
The operators were asked to vary image contrast, brightness and
Zoom to be able to interpret the features as good as possible.
For the mono interpretations there was only a single operator
available, and only small differences were expected, so here
biases from "memory" could not be excluded. Digitizing tablets
With ordinary cursor (no optical magnification) and Microstation
software were used. Proper viewing conditions like glare free
illumination and a constant viewing distance of 25 cm were dif-
ficult to achieve, but the major problem for the interpretation
was the small area, which was visible around the cursor of the
digitizing tablet. The operator stated, that he could clearly
identify features on the orthophoto, but often "lost" them when
trying to set the cursor there "with a finger on the button".
consistency: all interpretations were done "ignorance based".
Available "other information" like existing topographic maps or
higher resolution images were not to be used. The operators
Were familiar with this terrain type, they had done map comple-
tion exercises not far from the site at some time before. To
achieve a reasonable consistency they carried out an interpre-
307
tation training in the neighboring area prior to the actual data
collection.
features: only critical features were selected from the specifica-
tions for the French 1:50,000 map. Features which are probably
"always identified" or "never visible" are not considered.
Features assumed critical and occurring in the test area were:
- point features: small isolated houses, bridges, reservoirs,
towers, monuments.
- line features: foot paths, single lane roads, single track rail-
ways, ditches, high tension power lines.
- area features: vineyards, orchards, cemeteries.
All houses up to 120 m* area had to be digitized. To be on the
safe side, slightly larger ones had to be digitized too, but not
considered in the analysis.
The same interpretation key was used for the stereo- and for the
mono-interpretations.
2.6 Reference data
These were digitized from the original diapositives using the
Analytical Plotter Zeiss Planicomp C120 and Kork software for
the data collection.
The same interpretation key was used as for the test data, only
houses were digitized as area features, and the size limit was
bigger. This was necessary to limit the test to houses up to
120 m? without treating a digitized house which is a little bigger
as an error.
In addition to the necessary setting of proper viewing conditions
(eye base, focussing, image rotation and squint) operators were
asked to and to vary the illumination the magnification to get
the interpretations as good as possible.
2.7 Analysis Method
Data digitized from the orthophotos or on the Digital Photo-
grammetric Workstation Matra Traster T10 were compared with
digitizations from the original diapositives on the Analytical
Plotter Zeiss Planicomp C120. Thus the reference data for the
two image scales had to be different. This was necessary, be-
cause there were obviously significant changes in the three years
between the flights.
All data were transferred to ArcInfo, creating separate coverages
for each feature in each digitization. After cleaning of the data
(e.g. all houses > 120 m? had to be identified and eliminated
from the analysis) matches between the test data and the refe-
rence data were identified and counted for point features or the
lengths added up for line features respectively. For area features
intersections of the polygons were used.
For different sets of test data (6 stereo digitizations from 2
image scales, each with 3 pixel sizes - with and without sub-
division in two parts for different operators - and 5 orthophoto
types) separate "confusion matrices" for point features, line fea-
tures and area features respectively were constructed. In the
mono interpretations point features could not be identified, so
these confusion matrices were not set up. Table 3 gives an
example of a confusion matrix for 4 features (1 to 4).
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
