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3.9 Topographic Mapping of Asela in Ethiopia - a pilot
study
A topographic map sheet at a scale of 1:50,000 was produced in
1988 over a map sheet in central Ethiopia (Westin et al, 1988).
The area had very high relief, with elevations varying between
1700 and 3800 m. SPOT panchromatic stereo images were used
to derive the DEM and contour lines with 20 metre intervals.
Orthophotos from both panchromatic and multispectral SPOT
images were also produced. Two more map sheets were later
produced for the Ethiopian Mapping Authority (EMA) in an
area newly mapped with conventional technology based on
aerial photographs. These two sheets were produced as a blind
test. In Fig. 6 a part of the contours extracted from SPOT is
shown and in Fig. 7 the corresponding part produced with
conventional technology is shown.
The results were considered very satisfactory. The planimetric
accuracy of orthophotos extracted from SPOT is fully sufficient
for 1:50,000 mapping. Compared to aerial photographs the
image resolution of SPOT limits the possibilities of mapping
details - and thus increases the need for field completion. The
height accuracy was better than 10 meters r.m.s., compared to a
few ground check points. The similarity in appearance between
the contours extracted from SPOT and from aerial photographs
was good. This was possible because of the short time
differences, only a few days between the scenes in the pairs, and
the good base-to-height ratios.
3.10 Topographic Map Revision in Bhutan
For map revision purposes Bhutan was in the end of the 1980's
covered with totally 79 satellite image maps at a scale of
1:50,000 from panchromatic SPOT data (Brook, 1991). Where
height contours already existed, the data was orthocorrected,
while satellite image maps had to be produced without height
correction in large parts of the country. The existing 1:50,000
topographic map series was outdated, especially the
infrastructural changes had been major. By combining the
satellite image maps with the feature separations of the existing
height contours, the road network from visual interpretation of
the satellite data (including field work) and the place names, an
intermediate mapping series was produced.
3.11 Topographic Mapping of Cebu and Bohol in the
Philippines
In co-operation with the National Mapping and Resource
Information Authority (NAMRIA), SSC produced 27 1:50,000
topographic map sheets of the islands of Cebu and Bohol and
some neighbouring smaller islands based on existing old
topographic maps and SPOT images. The map sheet delineation
and the sheet size were changed from 10 x 15 geodetic minutes
to 15 x 15 geodetic minutes. Panchromatic orthoimages were
produced for all map sheets. For all map sheets except one, the
contours from the existing maps were digitised and transferred
into the new system. Due to cloud cover it was possible to
obtain stereo data for only one map sheet, covering the island of
711
Siguiohor. Also in this pair the panchromatic stereo pair had to
be complemented with one multispectral SPOT image due to a
cloud covered mountain top.
Extracting height information from SPOT stereo images did not
give any major advantages compared to digitising existing
contours. The reasons were all related to cloudiness such as
partial cloud cover, varying b/h-ratios and large differences in
acquisition times. For revision of area and line objects SPOT
was excellent, except where large side-looking angles made
identification of roads difficult in areas with high vegetation
(coconut palmtrees). For most point objects, such as single
houses, the 10-meter resolution of SPOT was not sufficient.
3.12 Topographic Mapping of an area in Malaysia
Satellite Orthophoto Maps based on SPOT Panchromatic scenes
were produced for 14 map sheets at 1:50 000 in co-operation
with the Department of Survey and Mapping (DSSM) of
Malaysia and used for revision of existing topographic maps.
For 5 more map sheets DEMs and 20 meter contours were also
produced. No formal accuracy evaluation was made although
the SPOT produced DEMs were merged with DEMs extracted
from existing topographic maps without specific border
problems. Thirteen land cover classes, related to topographic
maps were interpreted. Changes concerning roads, powerlines
and built-up areas were detected. To extract changes of all map
details used in Malaysia was not possible with SPOT data.
3.13 Topographic Base Mapping in the Baltic States
In the Baltic States ongoing projects are producing a new series
of topographic maps based on orthocorrected SPOT Satellite
Image Maps (Klang, 1995) . They are carried out in co-
operation with the national mapping authorities in Estonia,
Latvia and Lithuania respectively.
From existing maps, produced in the old Soviet cartographic
system, existing cartographic information., including height
contours, was digitised. The information was transformed to the
new Baltic Geodetic datum. DEMs were extracted from the
contours. Depending on the quality of the existing data versus
the SPOT images three methods for production of databases are
used; (i) when the need for revision is fairly limited the existing
data base is updated using the SPOT image maps; (ii) the
information is directly acquired by interpretation of the SPOT
image maps, sometimes supported by existing maps; (iii) when
information is impossible to acquire from satellite data the
digitised data is used directly, typical examples being contours
and administrative boundaries. Infrastructural elements and six
area classes are delineated according to a priority scheme. The
final map is produced as an orthophotomap with line
information (Fig 8).
The project has given broad basic experience in how satellite
images - and their excellent geometry - can be used for
correction and revision of existing maps, conversion and
production of new maps.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
