dtm dem generation
TRANSCRIPT
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DTM/DEM Generation
SUG541
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Introduction → Map/GIS LayersSpatial information layers require a base comprising a DTM/DEM & imagery
The ortho-image layer: a critical element, which is draped over a DTM
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Introduction → DEM & ortho-imagery
DEM provides height, referenced to position
Ortho-image provides metrically correct map for feature extraction
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Ortho-image draped over a DTM
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Modern photogrammetric process for geospatial information collection
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DTM Generation: Contents
• Terminology• DTM Acquisition• DTM Editing• DTM Representation• Accuracy Issue• From DSM to DTM/DEM• DTM Manipulation
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Terminology• DEM (Digital Elevation Model)
– Refers to regular array of elevations• DTM (Digital Terrain Model)
– More complex concept involving elevations and other GIS features– (e.g., rivers, ridges, break lines, etc.);– Encompasses terrain relief, planimetric, and derived data (slope,
aspect, visibility, etc.)• DHM (Digital Height Model)
– Similar as DEM, but less commonly used terminology• DGM (Digital Ground Model)• More emphasis on digital models of the solid/continuous surface
of the earth (used in the UK)
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Terminology
• DTED (Digital Terrain Elevation Data)– Term used by the US Mapping Community; comes
from military specs.– Usually refers to gridded/regular arrays– DTEDs come in different ‘levels’
• DSM (Digital Surface Model)– Refers to digital model including features above
surface of the earth (e.g., trees, buildings)– Very important for orthophoto generation
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DTM Acquisition
• Photogrammetric data capture (passive sensor)• Aerial photography• Digital satellite imagery
– Image matching used to automatically extract a dense– point cloud of 3D surface points from stereo image pairs and
potentially multi-image coverage; DTM derived via ‘regularization’ of the point cloud.
• RADAR: RAdio Detection And Ranging (active sensor)• LIDAR: LIght Detection And Ranging (active sensor)• Digitized contour maps• Ground surveying
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DSM to DTM
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DTM Editing
Modification and refinement of DTMs, and derivation of intermediate models.
• Editing: correcting errors and updating DTMs• Filtering: smoothing, enhancing, compression
and resampling• Merging and joining DTMs: – combining DTMs from several sources (possibly from
different dates)– Converting DTMs from one data structure to another
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DEM Representation• Raster DEM– Elevations are available at equally spaced grid points
• TIN (Triangulated Irregular Network)– Elevation data at irregular points that are formed into triangles– The TIN is generated in such a way that the summation of the
lengths of the triangle legs is a minimum
DelaunayTriangulation
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Triangulated Irregular Network (TIN)
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Raster DEM & TINs
RASTER: Effect of Grid Size on Surface Representation• Sampling interval will affect
– Amount of detail captured (accuracy)– Amount of storage (redundancy, efficiency)
• Optimum sampling interval depends on the nature of the terrain• Raster DEM & TINsTINS:• Each vertex must have the following information
– Height– Connectivity information– Surface normal
• The triangle legs can be forced to coincide with the break lines
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Raster vs. TINs
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Shaded DSM
Source image (1 of stereo pair) Shaded DSM
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Accuracy of Photogrammetric DSM from IKONOS
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DSM to DTMMorphological operations
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Morphological operations: removal of above-ground features
DSM
DTM
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DSM to DTMMultiple height bin method for object detection & removal
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Multiple height in method for object detection
image DSM Detected above-ground objects
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DTM Manipulation
• Nearest neighbour assignment• Linear interpolation• Bilinear interpolation• Cubic convolution
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Nearest Neighbour Assignment
• Assigns the values of the nearest grid point to the output grid cell
• No actual interpolation is performed based on values of neighbouring points
• Does not create a continuous surface
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Bilinear Interpolation• Determines the height of a point based on a
weighted average of the 4 grid points• Generated surface is continuous, but not smooth
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Bilinear Interpolation
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Cubic Convolution
• Determine the Z value of a point using 16 neighbouring grid points
• Convolve with a smooth curve function