Hillshading

Stacy Hoppen

Hillshading is variation in tone from light to dark to delineate the form of the land. The shading should be used to suggest the surface material, such as snow or vegetation.

The three types of hillshading:

  • slope shading: Tonal strengths are proportional to the angle of slope (the steeper the darker).

  • oblique light shading: The pattern of light and dark on relief surface is produced by an oblique light source.

  • combined shading: A combination of slope and oblique shading.

    Contour lines and hillshading are often used together because they complement eachother. Hillshading gives an overall impression of the terrain but without mathematical accuracy while contours give heights mathematically, but have information gaps. Yoeli demonstrates the skill of visualizing terrain from contours by comparison to the difficulty of imagining statues or sculpture described only by contour lines rather than photographs.

    The level of detail used to produce a hillshaded image should be more detailed than the level of detail needed to produce contour lines because shading should reveal the terrain between contour lines.

    Hand hillshading methods

    There is general agreement among cartographers that the skill, imagination, and judgement required to hillshade a map cannot be completely replaced by a computer...yet. But those who are working toward an algorithmic solution would like to see the end of the many "unscientific" and arbitrary decisions made by a cartographer.

    The Swiss relief method, characterized by clear, simple forms, is considered the highest form of hand hillshading:

  • A dominant direction for the light source is chosen, but the cartographer also makes small local adjustments to the direction of light in order to best reveal the terrain.

  • Atmospheric perspective is produced

    Photographing an illuminated 3D model of terrain, as done by Ernst Hoffman at Hammond, and others, can produce dramatic images, but has several disadvantages according to Yoeli (1965):

  • The model is built from contour maps, which already have generalization and error built in.

  • The photograph of the model is a central projection which is later superimposed upon an orthogonal projection of the contours.

  • The light source is fixed so that features parallel to the light rays may disappear.

  • Relief features throw unwanted shadows and reflected light upon eachother.

    Analytical approach to hillshading

  • Lehman (1799) developed the theory of hachures in an attempt to express the slopes of relief by differences in light intensities. Because printing technology of the time did not allow shading, he developed hachures, many small lines for shading. The thickness of and distance between lines are varied so that variations in light and dark so as express variations in the slope of the terrain. These strictly defined rules were difficult to render and reproduce.

  • Wiechel (1878) suggested basing the method of oblique shading on a mathematical foundation. He developed an equation for the light intensity at every point that uses the cosine of the angle between the light incidence and a normal to the surface. This method wasn't feasible until computers could handle the bulky calculations.

  • Yoeli, using Wiechel's idea, introduced the first algorithmic means of hill shading (analytical hill shading)

    - The dot size is proportional to elevation.

    - The reflectance is proportional to the cosine of the angle of incidence with line normal to surface.

  • Brassel expanded on Yoeli's work by incorporating computer capability to make small adjustments to shading (slight variation in the direction of the light source) to best reveal the landscape. This incorporates "judgment" into Yoelli's method.

    - Illumination is adjusted to local relief.

    - Atmospheric perspective is simulated: diminished contrast far away (valleys) and enhanced contrast in upper areas (ridges).

  • Hugli did work on the general problem of depicting the shading produced by parallel rays on any mathematically defined surface. (no publications in English)

    Cartographic hillshading vs. computer graphics shading

    Both cartographic and computer graphics produce analytically-based shading, the application of uniform rules to depict light and shadow. While some of the approaches to shading are similar, graphics shading is more concerned with the overall effect the shading produces and evaluates the result by how well it simulates reality. Cartographic hillshading always maintains a location attached to each elevation, permitting further surface analysis such as slope and aspect, and is most concerned with revealing the most possible information about the terrain.

    Algorithmic concerns that cartography and computer graphics have in common are: creating a diffused light source to soften shadows, atmospheric perspective, shading of convex and concave surfaces, and optimally revealing a surface shadowing by local light adjustments. Areas applicable mainly to computer graphics are depicting shadows, transparency, multiple fixed light sources, surface textures, and surface smoothing. Although shadows are taboo in cartographic hillshading, shadow algorithms are similar to visibility algorithms used to calculate viewsheds.

    References

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    URL http://everest.hunter.cuny.edu/terrain/hillshadtxt.html
    Last Update: January 2, 1996