Openness Maps

Upward openness maps show ridge crests in white, while downward openness maps show valleys in white.  They provide dramatic depictions of terrain.

Reflectance	Upward (Positive) openness	Downward (Negative) openness

Choice of upward (positive) and downward (negative) openness.  For each select a region size in meters, because openness is defined as a maximum or minimum angle over a specified region.  The region size should be at least as large as the ridge/valley spacing in the region.  The Graph parameter by Region size  option can help you pick the value by seeing the inflection points.

Openness can be computed from, and set on the Analysis tab of the options form,:

• Points on the group
• Points on an antenna a fixed distance above the ground
• At a constant flying height

Openness calculates an angle in each of 8 radials in the principal compass directions. The diagrams to the left shows two of the angles computed for upward openness.  The peak in the top diagram will have a high value for openness, while the valley in the bottom diagram will have a small value. The Value L shows the distance used for the computation; for the peak, visibility extends all the way out to L, while for for valley, the steep walls block visibility well before the L distance. If you want, you can only use some of the 8 radial directions.  This is set on the Analysis tab of the options form.  The final value is the average of the values in each of the directions used.

Openness maps correlate strongly with maximum and minimum curvature.

Short distance openness maps can be very effective with urban lidar DSM.

References for openness maps

Last revision 11/26/2017