Thematic Map of Photogrammetric compiled DEM available
Thematic Map of 5M contours available (Bye product of the photogrammetric compiled DEM).
DEM (Digital Elevation Model) data describes the surface of the earth through a regular rectangular grid which can be visualised as a wire grid framework draped over the surface of the earth. Each grid intersection is defined by two components, i.e. a coordinate reference grid location(y, x) and associated height (H). The coordinate and associated height components are referenced to separate earth reference systems, i.e. a two dimensional (y, x) and one dimension (H) reference system. Collectively, the reference system can be described as a Compound Coordinate Reference System (CCRS) [ISO 2002]. The y and x coordinates are referenced to the Datum's and Coordinate Systems while the height component is reference to mean sea level.
DEM data is typically sourced, by the NGI, from a stereo pair of photogrammetric images or from contour data sets. The stereo pair of images is of higher accuracy and the preferred source. The NGI has full National DEM coverage sourced from hybrid methods, i.e. from photogrammetric or vectorised contour datasets. The data is supplied in either ASCII text or ERSI grid formats at 25m x 25m spacing or multiples thereof.
DEM data can be used for, creating visualisation models and hill shading maps, hydrology analysis, view shed analysis, 3D path profiling, line of sight analysis and for orthorectifying imagery, etc. The primary use, at the NGI, is for orthorectifying aerial photography and hill shading.
|Figure I – 3D Visualisation model
||Figure 2 – 3D Wireframe|