Download - DTM DEM Generation
DTM/DEM Generation
SUG541
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
Introduction → DEM & ortho-imagery
DEM provides height, referenced to position
Ortho-image provides metrically correct map for feature extraction
Ortho-image draped over a DTM
Modern photogrammetric process for geospatial information collection
DTM Generation: Contents
• Terminology• DTM Acquisition• DTM Editing• DTM Representation• Accuracy Issue• From DSM to DTM/DEM• DTM Manipulation
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)
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
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
DSM to DTM
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
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
Triangulated Irregular Network (TIN)
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
Raster vs. TINs
Shaded DSM
Source image (1 of stereo pair) Shaded DSM
Accuracy of Photogrammetric DSM from IKONOS
DSM to DTMMorphological operations
Morphological operations: removal of above-ground features
DSM
DTM
DSM to DTMMultiple height bin method for object detection & removal
Multiple height in method for object detection
image DSM Detected above-ground objects
DTM Manipulation
• Nearest neighbour assignment• Linear interpolation• Bilinear interpolation• Cubic convolution
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
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
Bilinear Interpolation
Cubic Convolution
• Determine the Z value of a point using 16 neighbouring grid points
• Convolve with a smooth curve function