Using GIS to Determine Horizon Belts[2]

Create the ridge viewpoints file
The ridgeline data was used to indicate what portions of the road viewshed
corresponded to ridges. A ridge viewpoint file was created that corresponded to the back
edge of the viewshed traversing a ridge (Figure 1). The ridgeline was used only as a
horizontal guide for the location of the ridge. The back edge of the viewshed, and the
corresponding “ridge” viewpoints could have extended beyond or fell short of the actual
ridgeline. At least three ridge viewpoints were selected, two from near the ends of the
ridge and one from a highpoint along the ridge. Additional viewpoints were added for
divides, as indicated by ridgelines. They were also added at viewshed junctions where
the back edge of the viewshed changed direction sharply. These viewshed junctions
represent locations viewed by more than one road viewpoint. The ridge viewpoints were
converted to a 3D file, with elevations derived from the TIN model. Both target offsets
and observer offsets (OffsetA) of 35 feet were assigned as attributes to all viewpoints.
Create an analysis mask
Analysis masks were used to constrain the viewshed analysis from the ridge
viewpoints to a limited area (Figure 2). This served two practical purposes. Areas
“behind” the ridge viewpoints were not of concern and eliminating them from
consideration speeds up processing time. Analysis masks also were used to segment
concave ridges and create separate horizon belts for each ridge segment.