A shaded relief image provides an illustration of variations in elevation. Based on a user-specified position of the sun, areas that would be in sunlight are highlighted and areas that would be in shadow are shaded. Shaded relief images are generated from an elevation surface, alone or in combination with an image file draped over the terrain.
It is important to note that the relief program identifies shadowed areas—that is, those that are not in direct sun. It does not calculate the shadow that is cast by topographic features onto the surrounding surface.
For example, a high mountain having sunlight coming from the northwest would be symbolized as follows in shaded relief. Only the portions of the mountain that would be in shadow from a northwest light would be shaded. The software would not simulate a shadow that the mountain would cast on the southeast side.
Shaded relief images are an effective graphic tool. They can also be used in analysis, for example, snow melt over an area spanned by an elevation surface. A series of relief images can be generated to simulate the movement of the sun over the landscape. Snow melt rates can then be estimated for each pixel based on the amount of time it spends in sun or shadow. Shaded relief images can also be used to enhance subtle detail in gray scale images such as aeromagnetic, radar, gravity maps, and so forth.
Use Shaded Relief function to generate a relief image.
In calculating relief, the software compares the user-specified sun position and angle with the angle each pixel faces. Each pixel is assigned a value between -1 and +1 to indicate the amount of light reflectance at that pixel.
- Negative numbers and zero values represent shadowed areas.
- Positive numbers represent sunny areas, where +1 is assigned to the areas of highest reflectance.
The reflectance values are then applied to the original pixel values to get the final result. All negative values are set to 0 or to the minimum light level specified by you. These indicate shadowed areas. Light reflectance in sunny areas falls within a range of values depending on whether the pixel is directly facing the sun or not. (In the example above, pixels facing northwest would be the brightest. Pixels facing north-northwest and west-northwest would not be quite as bright.)
In a relief file, which is a DEM that shows surface relief, the surface reflectance values are multiplied by the color lookup values for the image file.