Lab 7 - TINs and DEMs

The objective of this lab was to learn the difference between TIN and DEM elevation models, as well as examine some of their properties. The first part of the lab was to drape an image over a terrain surface. I was provided a TIN elevation model and a satellite radar image showing the land surface roughness. Using ArcScene, I added both layers and saw that since the image has a base elevation of 0, it is hidden by the terrain surface. Using the base heights tab, I chose to float the image over the terrain surface, and I was now able to see the whole scene. To get a better idea of the elevation changes, I used vertical exaggeration.

In the second part, I used a DEM to develop a ski resort suitability model. I had 3 categories to reclassify in terms of which got the most snow: elevation, slope, and aspect. After reclassifying the categories as described in the lab, I used the Weighted Overlay tool to create a suitability raster with the following weights: 25% aspect, 40% elevation, and 35% slope. I added an exaggeration factor to show the elevation changes a little better and used lighting effects.

The next portion of the lab allowed us to explore TINs in some more detail. The first thing we did was look at different visualization options, including elevation, slope, aspect, and nodes options. Next we created and analyzed TINs, comparing them to DEMs. The nice thing about TINs is that to see contours, one only needs to modify the symbology, whereas with the DEM one needs to use the Contour tool. Basically, a DEM is a better option with continuous grid spacing, where a TIN is good with data that has higher and lower density grid points. When investigating the elevation data, both models seem to do a good job, with the DEM doing a little better with the smaller details toward the edges of the map where there are fewer grid points. Below are two screenshots of the DEM contours and the TIN contours:

TIN contours:

DEM contours:

TINs derived from elevation points or contours don't necessarily display sharp boundaries such as streams or lakes well. So we learned to modify TINs to better define those sharper boundaries. I loaded the TIN and the lake shapefile, changing the offset of the lake layer so that it would just show up on top of the TIN. Using the edit TIN tool, the lake boundaries were changed to hard breaklines so that the TIN was forced to use the exact boundaries and elevation of the lake polygon.
Overall, I found this to be an excellent lab in learning not only what a TIN is, but also when to use a TIN vs. a DEM.