Introduction

The hillshade process in ArcView can cause an inverse perception of depth if the azimuth, color ramp, and brightness theme are not chosen correclty. In ArcView 3.2, the default hillshade assigns a color ramp that makes it appear as if the sun is on the opposite side of what you specified as the azimuth. In this example we have applied the default hillshade to a cone (Figure 1) and to a Digital Elevation Model (DEM) of the Sierra Vista Ranger District (SVRD). The examples displayed below provide a good reference for those who are unclear about what azimuth, color ramp, or brightness theme to use.

Fig. 1. A 3D view of the cone

Application of Hillshade to a Cone

In ArcView 3.2, the default hillshade assigns a sun azimuth of 90; however, the default color ramp gives user the perception that the sun is at an azimuth of 270 (Figure 2). If you flip the color ramp, then the hillshade provides an image with the light coming from the specified azimuth (Figure 3). In figures 2 and 3, the elevation at the center of the cone is higher than at the the perimeters.

Fig. 2. The default hillshade of a cone (Azimuth = 90)

Fig. 3. The hillshade of a cone with a flipped color ramp (Azimuth = 90)

If you use the advanced function in the ArcView Legend Editor to specify the brightness theme, you can make the hillshade surface smoother (Figures 4 - 7). If you use this option, the perception of the source of light is consistent with the specified azimuth. However, it becomes impossible to change the appearance of the hillshade by flipping the color ramp.

Application of Hillshade to an Actual DEM

Figures 8 and 9 show the hillshade of an actual DEM with the default color ramp (Fig. 8) and the flipped color ramp (Fig. 9). The advanced function in ArcView's legend editor was not used to specify the brightness theme.

Fig. 8. Hillshade of an actual DEM with default color ramp (Azimuth = 270)

Fig. 9. Hillshade of an actual DEM with flipped color ramp (Azimuth = 270)

Figure 10 shows the same hillshade with the brightness theme specified. Figure 11 shows the 3D view of the SVRD area.

Fig. 10. Hillshade of an actual DEM with the brightness theme specified (Azimuth = 270)

Fig. 11. 3D view of the SVRD area

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