Using GIS to Determine Horizon Belts[2]

idge viewpoints, and would have a backdrop of only land from the perspective of the
road viewpoints. Thus, the purpose of the ridge viewshed analysis is to subtract the areas
from the road viewshed analysis that would have a backdrop of land rather than sky. The
output of the ridge viewshed analysis was reclassified to indicate whether a cell was
visible to any (value = 1) or none (value = 0) of the ridge viewpoints.
Create horizon belts
The horizon belt corresponds to the intersection of the road viewshed and ridge
viewshed analyses. This was obtained by multiplying the two grids. Any area common
to both grids receives a calculated value of one, while an area just common to one grid
receives a calculated value of zero. These belts delineate the area where at least some
portion of the backdrop to a 35 foot structure would be open sky, as viewed from the
public roads.
Implement horizon belt constraints
Horizon belts are a subset of a viewshed, the area within a viewshed where
structures have a backdrop of sky rather than land. Further constraints were added to this
subset for the town of Kent. A 1000’ buffer from the viewshed ridgelines was created
and given a grid value of 1. A slope gradient map was generated from the DEMs and
reclassified to indicate slope gradients >= 15% (grid value = 1), or slope gradients < 15%
(grid value = 0). A dataset of steep slopes OR ridgeline buffer was combined. A subset
of all horizon belts was derived from an intersection with this grid of constraints.