Características del distrito de Pilcuyo

An optical camera records an image in the same manner as our eyes, therefore, if two cameras are located with the same relative geometry, a satisfactory stereo model could be obtained. Unfortunately, for radar imagery, an image is recorded in a completely different manner. The radar imagery is presented by a range projection, unlike normal photography by a central projection [Rosenfield, 1967], indicating a

different geometric structure. For example, the parallax resulting from a topographic feature will be completely different from that of the airphotoes by the camera. To explore this difference, various aspects of the stereoscopic condition should be discussed first. Initial work on radar stereoscopy was carried out by LaPrade. In his paper [LaPrade, 1972], determined the optimum flight configurations for airborne radar by keeping the parallax constant for all images. LaPrade also was the first one to propose the two most common stereo configurations for side-looking radar and defined these as same-side or opposite-side. There are still other arrangements such as the cross-wise which is with smaller angular separations between look directions. It is not possible to achieve the stereo with a single flight line, for the projection circles of the two images will not intersect in a distinct point.

For stereoscopic research, one important factor should be considered namely vertical exaggeration. The vertical exaggeration expresses the scale difference of the vertical scale that is greater than the horizontal scale. It is of great concern to interpreters, who must take this into account when estimating the heights of objects, rates of slopes etc.. In [LaPrade, 1972], the author stated that the vertical exaggeration is irrelevant to stereoscope type and is only dependent on the convergence angle to the eye by stereo pair separation. The vertical exaggeration factor is determined by the value of the image base to height ratio (Bn/Hn) with respect to the stereo viewing ratio (Bs/Hs). The Bn and Hn are the air base and flying height respectively as shown in the Fig. 3.2 for the camera stereo model, and this ratio determines the possibility of creating stereo models in the object space. The and Bs are the eye base and the distance between the height and stereo model which is related to the ability for stereo viewing. If the vertical exaggeration factor is equal to 1, then there is no vertical exaggeration of the stereo model. From experimental work, LaPrade concluded that the optimum exaggeration factor should be 5 for the radar imagery. This exaggeration factor and the parallax could be a function of the height of a feature [Pisaruck et al., 1984], who utilise the regression method to derive their proportional relationship.

Apart from the vertical exaggeration, the stereo viewing of SAR imagery should also be considered. In [Leberl. 1979], the author summarised four factors that would affect radar stereo viewing:

(1) The stereo arrangement (2) The look angles off-nadir (3) The stereo intersection angles (4) The ruggedness of the terrain

Chapter 3. Stereoscopy Using SAR Imagery

camera stereo base

focal length

Pi

Flying Height

hy: object height Pr : object parallax

Pc : camera parallax = P^tj + I^t]

Fig. 3.2: Definition in object space for the vertical exaggeration of camera stereo model (after [LaPrade et al., 1980])

The stereo viewing is inversely related to the geometric conditions of intersection. A good geometric condition should have larger intersection angles. However, this will cause significant differences in the image contents such as tone and texture, which in turn will result in poor stereo viewing ability. An obvious example is that for the opposite side stereo pair, when the illumination direction changes, the resulting image appearance would look very different and thus hampers the viewing abilities greatly. At the same time, opposite side stereo pairs have larger intersection angles leading to greater parallax, resulting in a greater exaggeration factor and consequently giving rise to better intersection accuracy. In contrast, for good visual perception, the smaller intersection angles would be preferred which result in little difference in image tone and texture. Unfortunately, it will cause smaller parallax which is disadvantageous to the intersection. This theory, however, was under scrutiny in [Leberl et a l ., 1985], in which the author claimed that it is not suitable for all terrain features, as it excludes the error propagation of the base width and parallax. It was therefore one of our objectives to validate this theory by establishing the relationship between intersection angle and the accuracy of intersection and this is shown in Chapter 8.

Considering the look angles, the view ability is better for the same side when the look angles are greater [Leberl et ah, 1982], and as the look angles become smaller, greater parallax will be produced creating greater vertical exaggeration. In [Kaupp et ah, 1983], this statement was validated by using 18 different combinations of incidence angle on various terrain models. It was shown that the parallax to height ratio was the greatest for small incidence angle of the individual stereo pair, while resulting in the largest intersection angle. This was evident from observing three stereo pairs of (60720°, 75°/45°, 70°/40°), where the parallax to height to ratios were 2.17, 0.732, and 0.828 respectively. When the incidence angle became smaller, the relief displacement or image appearance would be adversely influenced to a great extent. The incidence angle was therefore said to be not less than 40° in general [Leberl, 1979]. This requirement, however, does not consider the situation of terrain ruggedness. One should notice that for the high relief terrain, the look angle should be even larger compared to any flat terrain.

In addition to the intersection angles, the viewing ability could also be influenced by several image characteristics such as layover and shadow introduced previously. Layover often leads to confusion for the observers when the image pair is viewed stereoscopically. The problems of stereoscopic viewing on some terrain features by shadow can be overcome by observing from two different directions such as the hill or mountain ridges. However, this is not applicable to the bottom of valleys, and this is the reason why rugged terrain could not be previewed stereoscopically by the opposite stereo pair [Trevett, 1986].