Module 7 - Special Topics in GIS - TIN's and DEM's

Symbology in a TIN elevation image of a landscape, with 5x exaggeration of relief.

This week in Special Topics, we began a 3-module unit on surfaces. In the first lab, we had an introduction to DEM's or Digital Elevation Models, and TIN's or Triangulated Irregular Network Surfaces.

DEM's are raster-based, and made of regular grids of pixels, each of which holds a value.

TIN's however, are based on vector data.  Data points values can be turned into a TIN by means of triangulation: the points are connected to their neighbors by edges, and the edges form triangles, which make up facets in the surface in question.  Vector lines and polygon boundary lines can be incorporated into a TIN, to show the positions of roads, rivers, lake shores, and other significant features that may affect the shape of a landscape.  These linear features are included in the network of triangles, with series of small triangle edges defining the linear features.

In an elevation TIN, each triangle or facet has a uniform slope and aspect, while elevation changes across the triangle. In the figure above, different degrees of slope are shown by different tones of gray.  The surface can also have contours overlaid, as well as the original mass points (red).  Aspect (azimuthal direction) can also be symbolized.

The figure above shows a close-up example of a TIN.  Many features of its can be shown in the symbology, including the nodes (points), edges of the triangles, slope, aspect, and surface.

ArcGIS uses Delaunay triangulation.  This process avoids long skinny triangles by requiring that a circle that includes the three vertices of any triangle does not enclose vertices of  another triangle.