Viewshed analysis determines the visible area or viewshed from one or more viewpoints using a digital elevation model (DEM) or triangulated irregular network (TIN). It requires a point layer containing viewpoints and a DEM/TIN representing the land surface. The line-of-sight operation connects each viewpoint to targets on the landscape, determining if any land blocks the view. Parameters like viewpoint location, viewing angle and radius influence the results. Viewshed analysis helps locate facilities like towers to maximize coverage while minimizing overlap and can inform land development and path planning.

1. Viewshed Analysis
SUBMITTED BY
DHANYA SENTHIL

2. Definition
• View shed is a portion of land
surface that is visible from one
or more view points.
• View shed analysis can be based
on a digital elevation model or a
triangulated irregular network.

3. Viewshed Analysis
• A viewshed refers to the portion of the land surface that is visible from
one or more view points.
• The process of deriving viewshed is called viewshed or visibility
analysis.
• A viewshed analysis requires two input data sets.The first is usually a
point layer that contains one or more view points.
• If a line layer is used the view points are points that make up the linear
features
• The second input is a DEM or a TIN , which represents the land
surface.

4. • The basis for viewshed analysis is the line of sight operation.
• The line of sight , also called sight line , connects the viewpoint and the
target.
• If any land , or any object on the land , rises above the line , then the
target is invisible to the view point.
• If no land or object blocks the view , then the target is visible to view
point.

5. • Parameters of Viewshed Analysis
• A number of parameters can influence the result of a viewshed analysis.
• The first parameter is the viewpoint.
• A viewpoint located along a ridge line would have a wider view point
located in a narrow valley.
• There are at least two scenarios in dealing with the viewpoint in GIS.
• The first scenario assumes that the location of the point is fixed.
• If the elevation at the point is known , it can be entered directly in a
field.
• If not it can be estimated from an elevation raster or a TIN.

6. • The second scenario assumes that the viewpoint is to be selected.
• The second parameter is the viewing azimuth, which sets horizontal
angle limits to the view.
• Viewing radius is the third parameter , which sets the search distance for
deriving visible areas.
• Other parameters include vertical viewing angle limits , the Earth’s
curvature, and tree height.
• Viewing viewing angles can range from 900 above the horizontal plane
to 900 below. The Earth’s curvature can be either ignored or corrected in
deriving a viewshed.

7. • Applications of Viewshed Analysis
• Viewshed analysis is useful for the site selection of facilities such as forest
look out stations, wireless telephone base stations, and microwave towers for
radio and television.
• The location of these facilities is chosen to maximize the viewable areas
without having too much overlap.
• Viewshed analysis can help locate these facilities , especially at the
preliminary stage.
• Viewshed analysis can also be used for housing and resort area developments
, although the objective of the analysis can differ between the developer and
current residents.
• Viewshed analysis can also be integrated with least – cost path calculations to
provide scenic paths for hikers and others.
• Finally , viewshed analysis can be a tool for preparing 3- D visualization.