2. Introduction → Map/GIS Layers
Spatial information layers require a
base comprising a DTM/DEM &
imagery
The ortho-image layer: a critical
element, which is draped over a
DTM
3. Introduction → DEM & ortho-imagery
DEM provides height, referenced
to position
Ortho-image provides metrically
correct map for feature extraction
7. 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)
8. 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
9. 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
11. 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
12. 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
Delaunay
Triangulation
15. 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 & TINs
TINS:
• Each vertex must have the following information
– Height
– Connectivity information
– Surface normal
• The triangle legs can be forced to coincide with the break lines
24. 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
25. 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
