7.1.1 Airborne versus Spaceborne Platforms

This section will compare aerial photographs, airborne SAR images, SPOT, ERS-1, and RADARSAT image pairs. Aerial photography has traditionally been used for stereo viewing. Various airborne SAR systems such as the CCRS CV-580 and Intermap's Star 1 & 2 systems have collected images for use in stereo. ERS-1 imagery can be viewed in stereo from an opposite side configuration. SPOT and RADARSAT satellite systems are easily adapted for stereo usage. The stereo pairs will be compared in terms of geometry, as it is the first distinguishing element between different imaging platforms.

This image pair is very easy to view in stereo. Vertical exaggeration is pronounced and accentuates subtle differences in relief. In the stereomodel, ridges appear to be deeper and folds appear higher than they are in reality. The elevation difference of the terrain pictured in this image is 150 to 200 metres. The topography is the result of a very long erosion process, which has worn down mountains but has left valleys intact.

The stereomodel resulting from this image pair is similar to one provided by aerial photographs. This is due to the fact that SPOT is an optical system. Change in elevation in this area, from the surface of Lake Okanagan to the top of Isintok Mountain (bottom right of both images) is approximately 2000 m. Slopes are steep. Slopes leading down to the valley bottom through which the highway and railway run are in the range of 50 per cent. However, there is some vertical exaggeration in this stereo pair. Slopes appear steeper than they are in reality. For example, Mt. Acland appears to rise almost vertically. Vertical exaggeration is due to the large intersection angle formed by this stereo pair.

This image pair is difficult to view in stereo. The terrain is characterized as being one of rolling topography. The elevation ranges from 300 to 500 m and slopes are generally no greater than 10°. Throughout the image area foreshortening and shadow effects are reduced, due to the shallow slopes. In some specific areas these double bounce reflection effects occur along road or forest borders and interfere with stereo viewing locally. The opposite side configuration causes these effects to be on opposite sides on the images and adds to the difficulty of effective stereo viewing. Vertical exaggeration is not evident in this image pair.

These images were acquired by the CCRS CV-580 in nadir mode. A full stereomodel can be viewed with this image pair, but can be confusing at first glance. The viewer is advised to focus on the flatter areas first and then to view the mountains. Otherwise, the stereo overlap will appear to slope steeply towards the near edges of the stereo image pair. There is some vertical exaggeration. The volcano in the top half of the images appears to be greater in height than its actual 2000 m.

Vertical exaggeration is less pronounced than in the RADARSAT image pair. The aerial photos show a more realistic image of Bathurst Island's topography. However, they are at such a large scale that it would take many image pairs to cover the area of one RADARSAT stereo pair.