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March 2003 |
Dem accuracy in analytical and digital photogrammetry
Automatic Generation of DEM
Automatic generation of surface models was made successful with digital photogrammetric techniques. To do this, the software automatically finds common or conjugate points on both the images of stereo pairs with the knowledge of position and orientation of two images. The 3D co-ordinates in object space can then be extracted from these. The image matching was done by area based matching, feature based matching or symbolic matching.
Area based matching is associating with matching of gray levels of small patches of area from both the digital images using least square or correlation techniques.
Feature based matching identifies the edges of buildings, roads; it's shape sign and strength of edges from both images.
Symbolic matching refers to methods, which compare symbolic descriptions of images and measure the similarity by a cost function.
The accuracy of the DEM generated by automatic image matching is controlled by many factors of which the size of the template selected for automatic matching is the basic factor, along with the terrain type, flying height, X & Y parallax and image noise level.. Except the template size and terrain type all other parameters are constant for a stereo pair. Other parameters are software dependent. Along with these, five parameters need to be input by the user for the automatic image matching and DEM generation. They are patch width, path height, column space, row space and terrain type. Briefly they are touched below-
Patch width: Number of pixels within each row of the epipolar image on which image matching will be taking place
Patch Height: Number of pixels within each column of epipolar for which image matching should be done.
Column Space: The number of columns between each patch being measured.
Row Space:
The number of rows between each patch being matched.
Determination of Patch width/Height
Patch width and height value depends on the scanned resolution of the image, scale of photography and DEM interval. For this photograph of scale 1 : 12 000, scanned with a resolution of 15 microns, the ground distance of one pixel is
15 * 12 000 = 180 mm or 0.18 m
If the patch width is set to 25 pixels then a match point on every 4.5m on ground (25 * 0.18) can be got. For a DEM interval of 10m, it is more than sufficient. But considering different parameters and terrain conditions the accuracy in terms of quality should be optimised. Normally the above four parameters were given the same value.
Fig 1: Comparison of ground DEM generated from analytical and digital photogrammetric systems
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