Introduction
You are already familiar with the points, lines and polygons of vector GIS datasets. Today we'll explore a different type of GIS dataset: raster data. In the raster data model, the earth is first divided up into a regular series of grid cells rather like a sheet of graph paper. Each cell is then assigned a value based on the attributes of the earth at that grid cell. If you were mapping vegetation, then each grid cell would contain a unique vegetation code corresponding to the type of vegetation found at that cell's location on the earth. If you were mapping elevation, then each cell would contain a value equal to the earth's elevation at the center of the cell. Some data, such as point features, line features, and uniform polygons can usually be mapped more accurately using a vector approach. However, continuously varying surfaces are usually mapped more accurately using the raster approach.
In this exercise you will work with a grid dataset derived from a digital elevation model (DEM). In a DEM, the earth's elevation is measured at precise points along linear transects. With the dataset you'll be using, the transects were 30 meters apart and a sample elevation along each transect was taken every 30 meters. The result was a collection of elevation values that could be converted into a raster data structure where the cell sizes were 30 meters by 30 meters.
In this brief exercise, you will learn how to:
Setup
Follow the usual setup procedures:
You should see a network of major roads and freeways for north
Adding a Grid or Raster Layer
In order to fully utilize a grid or raster layer in your project, you must first add the Spatial Analyst extension. This is a two step process. First, activate the Spatial Analyst extension. Second, open the Spatial Analyst toolbar.
Using Colors to Visualize Elevation
The default legend for the DEM grid is a solid color gradient where darker colors indicate lower elevations and lighter colors indicate higher elevations. You can change the colors just as you would any other layer:
Notice how the areas of higher and lower elevation can now be distinguished more clearly.
How does this method compare to the previous one?
Close the Layer Properties dialog.
Using a Hillshade Method to Visualize Elevation
Hillslope shading is a standard technique designed to make a flat surface appear to look "raised" by applying differential illumination based on the slope angle and slope aspect. By default, an imaginary source of light is applied from the northwest direction. Slopes facing the northwest are brighter, slopes facing the southwest and northeast have medium illumination, and slopes facing the southeast along with any slopes that are in the shadow of other slopes are darkest. To apply hillshading to your DEM grid:
Note how the valleys cutting down into the hills in the upper right of the map are accentuated by their dark shadows compared to the brighter slopes above them. Also note how the northwest facing slopes are brighter than the southeast facing slopes.
Using Contours to Visualize Elevation
Another common technique for visualizing elevation on a flat map is to represent a surface by lines of equal elevation (contour lines). To convert the elevation grid to contour lines:
Zoom in on the hilly areas to see the results of the contour operation
Experiment with these different ways to visualize elevation.
Can you see advantages and disadvantages associated with each method? Write a brief (about 1 typewritten page) paper comparing these three methods for visualizing elevation.
If you have finished your 1 page paper, try this.
Last modified 11/16/2021