Module 2.1: Surfaces - TINs and DEMs

The purpose of this week's lab was to create 3D visualizations of elevations models, create and modify TINs and compare TIN and DEM elevation  models. Digital Elevation Models (DEMs) are raster based models with information stored as a grid array with topography in equally spaced intervals. Triangular Irregular Network (TIN) models are vector based with elevation points (vertices) as a triangulated surface of overlapping triangles. TINs  also include information about altitude, slope and aspect that can be used to extract and analyze study areas. Which model is most useful in GIS analyses depends on the purpose of the analysis.

I explored various TINs and DEMs in this lab but the exercise that demonstrates the differences and similarities between the two models is when I compared the contour lines between a TIN and a DEM created from elevation points.

To create the TIN, I used the Create TIN tool with the elevation points as the input using the mass points type and the study area as a second input using a soft clip type. I changed the symbology of the resulting TIN to show only contours at 100 meter intervals. The TIN is below:




To create the DEM, I used the Spline tool with the elevation points as the input with a tension spline type. To create the contours of the DEM, I used the Contours tool with 100 meter intervals. The DEM is below:



To compare the contours of the TIN and DEM, I overlaid the two sets of contours in a local scene with the TIN as the elevation source. This is below, using the same symbology as above:

The key differences between the two sets of contours is that the contours from the TIN are sharper and more jagged than the contours from the DEM, which appear smoother. In addition, there are contour lines in the DEM that do not exist in the TIN. The greatest differences between the two sets of contours are in areas of high elevation. The smallest differences between the two sets of contours are in areas of lower elevation. The differences between the two sets of data can be explained by the distribution of points. The areas where the contour lines agree are areas where the points are densest, whereas the areas where the contour lines have less agreement – and also areas where contour lines are missing on the TIN – are where the points are sparsest. It would appear that the more accurate set of contour lines belong to the DEM, as these capture more of the subtle differences in the terrain.


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