Lab 11: TINs and 3-D Viewing

The advantage of a Triangulated Irregular Network is that it can represent a surface with varying levels of resolution, placing many points where more detail is needed and few points where it is not. However, it takes more computation to work with a TIN than a raster. In ArcGIS, TINs require the 3D Analyst extension.

1. Getting to know ArcScene

a. Defining height of a layer

While ArcMap produces only 2D maps, ArcScene presents a 3D display. First, the height of the terrain must be defined, starting with the hypsography layer, which has contour lines from the USGS topographic maps. (Caution: layers provided with this lab are in the NAD1983 datum; layers from Lab 9 or before were in the NAD1927 datum.) The horizontal coordinates are in feet, but the SPOT field is in meters (see Lab 10). However, according to the Attribute Coding Standards for Digital Line Graphs, the MINOR2 field contains elevation in feet, so there is no need to recalculate it. Instead of using a Z Unit Conversion as in the lab instructions, I'll just define the height of the layer using MINOR2 directly.

These are just lines, not a surface, so lighting and hillshading don't apply, but already they have a convincing 3D appearance to the eye. Inevitably though, the display is usually slower to pan and zoom than the 2D display in ArcMap, because of the computation required (especially when TINs are displayed).

b. Creating a TIN

A TIN can be created from the contours: the vertices of the contour lines will be the nodes of the TIN. Again, MINOR2 is the height source.

The TIN is a large and complex data structure:; its folder takes about 2.4 MB of disk space, compared to 0.4 MB for the contour shapefile. Below, a small area of nodes and edges is shown. The "hard edges" (blue) are the original contour lines.

TINs often contain flat triangles, where there is a sharp bend in a contour line so that several vertices at the same elevation are each other's closest neighbors. This is a potential problem for hydrologic analysis, and there are algorithms for eliminating the flat triangles. Some flat triangles in the TIN are shown in white below.

The faces can be colored various ways, for example with a single color:

Light and shadows are shown, with a default position of the sun. I chose a light color so there would be contrast between lighted and shaded areas.

A color ramp can also be used. Note that since this TIN does not have an attribute table, only height-related properties can be symbolized: elevation, slope, aspect.

Strangely, it seems to display elevation upside down by default, with the highest values listed first. In the scene below, I reversed the order so that lower heights are listed first, and applied the color ramp named "Surface", which has the conventional so-called "hypsometric tints" running from green at the lowest elevations through yellow to red, then white at the highest. Edges are not shown because there are so many of them that they obscure the faces. 

c. Controlling light and shadow

The position of the light source can be controlled, but I thought that the default altitude of 30 degrees was generally the best.

Altitude = 15 degrees. Low altitudes make most of the scene too dim.

Altitude = 50 degrees. High altitudes make it too bright and wash out details.

d. Draping vector layers on a surface, and converting to 3D

It's also possible to write a new shapefile where the points have 3 coordinates, not 2, using Convert > Features to 3D on the 3D Analyst menu. Here the 3rd dimension is taken from the TIN again.

The new feature layer has the same attribute table as the original; the only difference is that the objects in the shapefile are 3-dimensional points.

2. Suitability Analysis

All the same surface operations can be done with a TIN as with a raster: slope, aspect, hillshade, viewshed, contours, as well as others that we haven't looked at such as cut-and-fill volume, drainage basins, etc. I'll use the Spatial Analyst and 3D Analyst toolbars to do a suitability study for My New House Site in Claremont Canyon.

3. Other Applications of 3D Analysis

The 3D Analyst extension has other fun tools not used in this lab, such as

• finding steepest path
• creating contours from surface
• Profile graph: height of a line feature vs. distance. For example, here's the elevation of the main segment of Claremont Creek as it flows from the top of the canyon to the bottom.