Using GIS to Determine Horizon Belts[2]

The main tasks of the methodology are listed and annotated as follows:
Create a triangulated irregular network (TIN) model and viewshed ridgeline data
DEMs are a type of raster data representing terrain as a series of discrete steps, 10
meters squared. TIN models can be derived from DEMs, and are a more realistic
representation of terrain with a surface of continuous elevation change. The ArcView 3D
Analyst Extension was used to generate a TIN for the town of Kent. Lakes are features
of constant elevation and were used as breaklines for refining the TIN model, assigning
those areas of the model a constant elevation. Viewshed ridgelines were derived from the
boundaries delineated in the subbasins data, after eliminating subbasin boundaries that do
not correspond with topographically distinct ridges. Some topographically distinct ridges
not corresponding to a subbasin boundary were manually added.
Create the road viewpoints file
The road viewpoints were obtained from public roads in the town of Kent.
Viewpoints were chosen that corresponded to intersections of these road segments with
other roads, the town boundary, subbasin boundaries (an indicator of high points along
the road), or known vistas (Figure 1). The viewpoints were converted to a 3D file, with
the elevation derived from the TIN model, and a target offset (OffsetB) value of 35 feet
was assigned as an attribute to each viewpoint.