Manual Measuring and Mapping from Stereomodels

In a single image, the location of a point is given by its position x, y. Its corresponding 2D object space coordinate X, Y can be determined by some method of georeferencing as described above. when measuring from stereo images that have been relatively and absolutely oriented in a ground coordinate system, the stereoscopic parallax of a point is measured additionally for deriving its height Z. All stereoscopic measurement devices—analog parallax bars for stereoscopes, analytical stereoplotters, digital photo-grammetric workstations—make use of the principle of the floating mark or cursor. Small marks etched onto a glass bar or superimposed as dots or crosses of light are displayed over the left and right images independently. By adjusting their position so that they fuse into a single mark at the exact location of the point to be measured, the x-parallax and thus the height of the point can be determined.

The floating mark is perceived by the operator as resting exactly on the terrain surface, when the left and right marks are placed on corresponding points, i.e. over the same feature in the two images (Fig. 3-14). Most digital photo-grammetric workstations now offer the possibility of automatically adjusting the floating cursor height when the mouse is moved over the terrain. This is accomplished by the same digital image correlation techniques that also are used for automatic tie point generation and DEM extraction (see below). This technique saves the operator continually adjusting the x-parallax to changing terrain height; however, its success does significantly depend on local image contrast, texture and pattern.

Apart from individual measurements of 3D point positions, distances, areas, volumes, angles, and slopes, digital photogrammetry systems with stereoviewing facilities allow the direct stereoscopic collection of georectified 3D GIS vector data as point, line, or polygon features. These can then be used in GIS software for further editing, analysis, and visualization as well as combination with orthophoto maps. Such softcopy stereoplotting techniques are basically the fully computerized implementation of traditional ste-reoplotting methods, where the movements of the floating mark in the analog stereomodel were mechanically transmitted to a tracing pencil on a sheet of paper (mechanical stereoplotters) or converted into coordinates recorded by a linked CAD system (analytical stereoplotters).

Stereo Aerial Photography Images
FIGURE 3-14 Floating mark (stereo cursor) positioned at the edge of an erosion channel in a digital stereomodel (subset). Image width ~4.3 m, height of channel wall ~2.3 m. Kite aerial photograph by IM and JBR at the Bardenas Reales, Province of Navarra, Spain, February 2009.
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100 Photography Tips

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