3. THE STUDY 
3.1 Method 
SPOT images, on one hand, and aerial photos, on the 
other, were interpreted for map revision purposes. The 
results were compared using field controlled data. 
3.2 Test area 
The topographic sheet Osthammar 12I SV at scale 
1:50 000 located in the central part of Sweden was cho- 
sen for the tests. The study was carried out during the 
production of the new map in this area, for which 
reason field data was available. The northwest and 
southwest quadrants of the map, each covering 12 500 
meters x 12 500 meters, were used in the study. 
The area consists mainly of homogeneous forested areas 
with a rather large number of clearings, and also of 
cultivated areas, marsh land and a few minor urban 
areas. 
3.3 SPOT images 
SPOT data, both PAN and XS, over the test area was 
registered in May, 1990. The near-nadir images were 
precision corrected using map control points to fit the 
Swedish national grid and sampled to a pixelsize of 5 
meters. For each map quadrant high quality diapositi- 
ves at scale 1:67 000 were produced by a filmrecorder. 
From these, film enlargements at scale 1:25 000 were 
finally made. It is essential for the quality of the final 
images that the photographic enlargement factor is, 
such as in this case, held low. 
The SPOT images were produced at SSC Satellitbild in 
Kiruna using standard production methods. Digital data 
was also available. 
In the study, the interpretation of the films at scale 
1:25 000 was carried out using a light table. 
3.4 New aerial photos 
The high altitude b/w aerial photos (9 200 meters, 
negative scale 1:60 000) used in the study, as a compa- 
rison to the satellite imagery, were taken in May, 1989. 
Up-to-date very high altitude photos (13 200 m) were 
not available over the area. 
In the study, orthophotos enlarged to scale 1:25 000 
were used for the interpretation along with stereointer- 
pretation at scale 1:60 000. 
3.5 Field controlled new map 
The new topographic map over the test area was used 
for the evaluation of interpretation results from the 
SPOT images and the aerial photos used in the study. 
The new map is in other words regarded as the true 
representation of the area. 
The new map is based on interpretation of aerial photos 
from May, 1989, altitude 9 200 meters, and field control 
during the summer of 1990. Photointerpretation for the 
new map was done using ortophotos at scale 1:25 000. 
Stereointerpretation was also carried out, at scale 
1:60 000 or at the enlarged scale 1:25 000. 
In the new map, there are two classes of forested area: 
forest and clearings. Clearings are those stands with a 
tree height of less than 1.5 meters. 
3.6 Interpretation of SPOT and aerial images in study 
Initially, the interpretors involved in the interpretations 
were introduced to satellite remote sensing and in- 
terpretation. A third quadrant of the map was used in 
this step. The three interpretors from the Land Survey 
engaged in this study, all have a very long experience 
of aerial photo interpretation, but less experience with 
satellite imagery, especially multispectral ones. 
The new map, although digitally stored, is based on 
visual image interpretation. Conventional visual 
interpretation methods has therefore been used also for 
this study, thereby focusing on differences in the quality 
of the image materials rather than the interpretation 
methods used. However, interactive digital interpreta- 
tion of image data on a display of an image processing 
system was also carried out. 
SPOT and aerial photos were interpreted by the three 
NLS interpretors for the two map quadrants covering 
the test area, see table 1. 
Table 1. SPOT and aerial images covering two map 
quadrants were interpreted according to the model 
shown below. Two interpretors were involved to ensure 
independent interpretations in different image materials 
covering the same area, see # 2, 3, 4 & 5. A third 
interpretor was engaged in order to quantify any diffe- 
rences between the interpretors, see # 1 and 2. Interpre- 
tation # 6 and 7 (SPOT XS) was made a month after the 
others, thus minimizing the influence from # 3 and 5 
(SPOT PAN) respectively. 
  
  
  
  
  
  
  
  
  
  
  
  
  
Interpre- | Interpretor | Image Map 
tation # material quadrant 
Visual interpretation. 
1 C aerial photo nw 
2 A aerial photo nw 
3 B SPOT PAN nw 
4 B aerial photo sw 
5 A SPOT PAN sw 
6 B SPOT XS nw 
7 A SPOT XS sw 
Interactive digital interpretation. 
8 NLS, SSC | SPOT XS part of sw 
  
  
  
  
  
  
As for any map revision operation, old maps and aerial 
photos were available during the interpretations in this 
study. 
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