|
|
|
Mapping from Space
|
Multi-Sensor image interpretation applications in Sabah
Results And Discussion
- SAR Imagery
The SAR imagery was useful for providing general information regarding roads, drainage, some settlements and forestry activities. In certain cases, ancillary data sources were useful for identifying roads, especially where the presence or absence of roads became questionable. However, the features of Interest had to run in a direction parallel to the aircraft flightlines to be detected. This is a dcirect result of the radar pattern where by shadowing effects of features highlight cleared areas, such as roads, by changing contrast. If the feature was perpemndicular to the signal, it would not generally be visible because there would be no shadowing effects and therefore minimal contrast change.
Road networkks included both major routes and secondary access roadss. Major rroutes were verified where 1:50,000 mapcoverage was available and by using the 1:5000,000 geological map. In the area selected, for its potential geological closure, roads were mapped under neglecte category rather than differentiating between primary and secondary routes. This was done to avoid a visual overload of information since rthe structural geology was of primary importance. Figure 1 illustrates an example of an interpreted area, with Table 1 representing the associated interpretation key.
Figure 1- SAR Stereopair of a cultural interpertation
Table 1 Cultural Interpretation Key
Drainage networks included major rivers and associated trirbutes. Tertiary drainage was only mapped if it was well defined and visible. As expected, drainage was often situated along lineaments and fault trends although this was not always observed. . The extent of potential boat passage could not be interpreted due to limitations in the scale and limited ground truth data.
Areas interpreted as a settlement or structures relating to some form of industry (i.e. forestry) were denoted under the heading of settlement. In all cases, these aqreas were interpreted based on their characteristic relative brightness,s a direct result of high reflectance.
The use of radar data alone sas limited in the cultural interpretation process even though the texturally variable signatres of the image were importrant to the interpretative process. The areas providing providing significant textural contrast allowed for more accurate interpretations to be made while areas of limited contrast made interpretation difficult without the benefit of alternative sources of information. An example of this was observed in clear out areas near topographically level tidal flats. In these areas road networks were often difficult to differentiate due to the lack of contrast, resulting from the absence of shadowing.
Further, in mountainous area, there was a loss of information resulting from radar shadowing. To compensate, these areas were interpreted and determined using the 1:50,000 topographic map coverage or the 1:500,000 Sabah geologic map (Ketiga, 1985). Where background information was not available , a "best estimate" was made in sitations of less than 2 km. Areas greater than 2 km were left uninterpreted simply due to the lack of available data. In most cases, the questionable areas occuredalong secondary roads.
Although the radar imagery provided excellent topographical representation, in many areas it did not provide adequate data for the interpretation of road networks, drainage and settlements. As a result, a SPOT image was acquired for the area so that it could be merged with the radar imagery in order to assesswhether the results could be improved.
|
|
|
|
|
|
|
