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March 2003 |
Dem accuracy in analytical and digital photogrammetry
The template size first needs to be optimized, so that the automatic image matching process is made accurate. The quality of auto matching is compared visually and the DEM thus generated by the Digital Photogrammetric workstation (DPW) is compared with the surface DEM generated with analytical stereo plotters.
To create a perfect ground DEM, the automatically matched surface points need to be edited so that all the points are made to be exactly on the ground. The DEM thus extracted will give the actual ground co-ordinates and is termed as ground DEM.
Analytical Photogrammetry
The diapositives of stereo pair for Hants county, Canada were used for the photogrammetric process in planicomp P3. The basic operation of IO, RO, AO were completed after inputting the camera parameters and ground control points. The residuals of IO, RO, AO were accepted when it was below the tolerance. A uniform grid of ground points with an interval of 25 m were created.
A perfect ground model was first generated. Around 23,000 points were identified in the overlapping area for the regular grid of 25 m. The entire model had a variable landuse patterns ranging from dense coniferous forest, residential and commercial areas, roads, railways and water bodies. Collection of ground points in the coniferous forest regions was extremely difficult, as the ground was invisible. The ground (mass) points underneath the forest were collected fully based on the terrain visulaisation with less zoom factor. The break lines (points measured along the elevated portion) and skeletal lines (points measured along the stream/drain) over the entire study area were then collected. The skeletal lines and break lines were used along with the mass points for a more accurate terrain interpolation.
Then full surface DEM points were collected for portion of the area. Surface points were collected by making the floating mark to exactly coincide over the top of the surface features such as building roof top, tree top. This had been done very accurately as it was visualised clearly and matched in stereo vision. The points thus collected were then processed further for generation of contours and DEM interpolation.
Fig 2: Comparison of Surface DEM generated from analytical and digital photogrammetric systems
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