Reflectance Map/Hillshade

The reflectance map computes the illumination from the sun (or a synthetic illumination direction), but does not consider shading by other topography in the line of sight toward the sun. The program first computes the normal vector to the earth's surface, which will be normal to the slope/aspect direction vector, and then computes the angle between this vector and the scene illumination.  For small angles, there will be a large reflection, and a light color will be applied.  Large angles imply little reflection, and a dark color.

The reflectance map is also called a hillshade.

2D representation of the relationships among topography and illumination.  In actual practice the vectors are all 3D. The slope/aspect vector will be tangent to the ground surface and point downhill at all locations, and will be perpendicular to the normal to the topographic surface. Because of the actual distance to the sun or scene illumination (essentially infinitely far away), the illumination vector will have a  constant orientation throughout the scene. The angle between the illumination direction and the surface normal will determine the shading used in the hillshade/reflectance map.  When the angle is small, as at location B, a large amount of light will reflect back and the point will have a light color.  Where the angle is large, as at location A, little light will reflect back and the point will have a dark color.

Geologists like reflectance maps with various illumination angles to emphasize lineaments or faults, and many people like them as a base map upon which to display other information while showing the relationship of the second layer to the topography.

The map of Mt. St. Helens below shows the effect of changing the sun position for the reflectance map.  Because of the artistic convention of top-left lighting, the best results will usually have to the sun to the NW and shadows going to the SE, even though this is never physically possible in the United States and most of the northern hemisphere.  Shadows to the right as a cartographic convention dates back to the 15th century.

Sun Azimuth 335, Elevation 45 Program default	Sun Azimuth 335, Elevation 45	Lower sun elevation increases shadows	Sun Azimuth 135, Elevation 45 For most people this leads to terrain inversion, ridges and valleys reversed.

Multi direction hillshades

Reflectance map directions in MICRODEM

Last revision 11/26/2017