Understanding the shape of a surface
One good way to get a general understanding of the shape of a surface is to view it in 3D. You can zoom in and out and rotate the surface to see it from different angles. When you need to show a surface on a printed map, it can be useful to create contour lines to represent the surface. In the following illustrations, the same area is displayed using a 3D perspective hillshaded view of terrain (left) and a planimetric contour line representation (right).
ArcGIS 3D Analyst provides tools that let you create individual contours or contours for a whole surface. An experienced map reader can tell that the surface is steeper where the lines are close together and identify ridgelines and streams from the shape of the contours. Contour lines can give you a feel for the shape of a surface, but they are not very useful as input for analysis.
ArcGIS 3D Analyst also has tools that allow more quantitative analysis of the shape of a surface. Slope is often used in analyses to find areas with low slopes for construction or areas with high slopes, which may be prone to erosion or landslides. Aspect is often used to determine how much sun a slope will receive—for instance, to model how vegetation will grow or snow will melt or how much solar heat a building will receive.
Slope and aspect in rasters and TINs
The following illustrates slope and aspect. In the picture on the left, darker shades of red indicate steeper slopes. In the picture on the right, west-facing slopes are dark blue and southeast-facing slopes are yellow.
Rasters and TINs model a surface's slope and aspect in different ways. In a raster, slope and aspect are calculated for each cell by fitting a plane to the z-values of each cell and its eight surrounding neighbors. The slope or the aspect of the plane becomes the slope or aspect value of the cell in a new raster. In a triangulated irregular network (TIN), each triangle face defines a plane with a slope and an aspect. These values are quickly calculated, as needed, when you query or render the faces.
Hillshades are the patterns of light and dark that a surface would show when illuminated from a particular angle. Hillshades are useful for increasing the perception of depth in a 3D surface and for analysis of the amount of solar radiation available at a location.
There are two ways to derive a hillshade of a surface. The first technique, which is most useful for adding depth for 3D visualization, is to turn on shading for the layer in ArcScene using the Rendering tab on the Layer Properties dialog box. This shades the layer (any surface or aerial 3D feature layer) on the fly using the scene's illumination settings. Hillshading is on by default for TINs. You must turn it on for rasters, because not all rasters are surfaces.
The following illustrates a 3D perspective of terrain without hillshading (left) and with hillshading (right).
The second technique, which is useful as input for analyses and for enhancing depth in 2D surfaces in ArcMap, is to use the Hillshade Surface Analysis tool on the 3D Analyst toolbar. This creates a new hillshade raster that you can make partially transparent and display in 2D (or 3D) with an elevation layer. The following illustrates (from left to right) a 2D elevation raster, a transparent hillshade raster, and a shaded relief map.
Deriving contour lines from rasters and TINs
Contours are lines that connect all contiguous locations with the same height (or other) value in the input grid or TIN. There are two ways to create contours with 3D Analyst:
- By clicking the surface with the Contour tool . This creates a single contour line, which exists as a 3D graphic in a scene or map.
- By using the Contour Surface Analysis command. This creates a series of contours with a given contour interval for the whole surface. These contours are saved as a feature class with a height attribute.
When you create contours from a grid, the contouring function interpolates lines between the cell centers. The lines seldom pass through the cell centers and do not follow the cell boundaries. In contrast, when you create contours from a TIN, the function interpolates straight lines across each triangle that spans the contour value using linear interpolation.
The following illustration on the left shows contours created from a raster surface. The illustration on the right shows contours created from a TIN surface.