7.1.2 Optical and VIR sensors versus radar sensors

The stereo pairs will now be compared in terms of the qualitative information they give. This is expressed by the image radiometry, which is a measure of the power of the return signals. In radar systems, the power of the return signals is dependent on the type of surface the transmitted signal encounters. As outlined in previous sections of this tutorial, ground moisture, the presence of vegetation, and terrain topography are all factors affecting the return signal strength. On SAR images, the range of greys represents the strength of return signals. The brightest greys (white) represent the strongest return signals, while the darkest greys (black) represent the weakest, or no return signal. In aerial photographs and SPOT and Landsat VIR images, the power of reflected visible light is represented by a range of greys for each channel.

On the images there is a very strong contrast between land, water, and different types of rock units. Bodies of water and landforms are clearly delineated. Different geological formations can be easily identified. Sensitivity to roughness is indicated by rougher terrain returning brighter signals than smoother terrain. Incidence angle effects such as foreshortening, layover and shadowing are not evident due to the nature of the land surface. Seasonal vegetation differences cannot be identified since these images were taken within days of each other.

These two images are very similar radiometrically. The only pronounced contrast on the images is between cleared and forested areas. Cleared areas include the road network, cut lines, hydro lines and areas with a slope too steep to support tree growth. Vegetation seasonality cannot be easily distinguished due to the black and white nature of the images.

There is a great deal of variation in radiometry between the two images. This difference is largely due to the images being collected from ascending and descending passes which means the images were taken during the day and at night. Since there is generally less wind at night, water/land boundaries may appear different than an image that was taken during the day. Both images contain speckle. Some double bounce reflection effects are visible causing bright returns in some areas. These bright areas interfere locally with stereo-viewing. Soil moisture can be distinguished. Lower lying wetlands are evident. Vegetation seasonality cannot be identified since the images were taken within six days of each other.

The contrast between different land features ranges from medium to strong on these images. Sensitivity to roughness is shown by different return signals from different ground cover types. Forested areas are rougher than agricultural fields. Both are easily distinguished. Forested areas are brighter than agricultural areas. There is some brightness due to foreshortening. Speckle is minimal.

There is a medium contrast between water, land and rock units. Rougher terrain is visible in an optical sense since the scale of the image pair is large. Graininess is not evident. Soil moisture and vegetation seasonality cannot be easily distinguished due to the nature of black and white photography.