Viewshed and Watershed

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Viewshed and Watershed

• Exercise 7 Background and Process

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Viewshed

• Viewshed is all cells that can be seen from a
  view point or collection of view points
• By human or animal eyeballs
• By Cell tower radiation
• By microwave radiation
• Use INITIAL and UPDATE to create the target images

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Watershed

• Watershed is all the cells above a target or
  drain cell that drain water into that cell 2
  methods
• Automatic for entire image by cells
• User-defined from seeds in an image that is 0
  elsewhere
• Use INITIAL and UPDATE to create the seed images

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VIEWSHED Defined

• Finds all pixels that are in view of a viewpoint
• A viewpoint is any number of pixels that are in
  an image that is otherwise 0 (zero)
• If you have more than one viewpoint what is the
  resulting viewshed?
• All cells viewable from all viewpoints?
• All cells viewable from any viewpoint?
• Thats why you should be careful of more than one
  target cell -- like roads

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Viewshed parameters

• A DEM (modified for barriers)
• A View Source image the viewpoint(s)
• A Viewer height value the height of the viewers
  eyeball (5 feet?) or the height of an antenna
  above the surface over which the viewshed is
  being run
• A Search distance cant see forever and cell
  phone microwaves only go about 10 miles or so

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Viewshed What is the surface?

• In our case we are looking at the propagation of
  microwaves over the vegetated surface of Nifkin
• This surface is the DEM plus the average height
  of the vegetation in the area
• Height of vegetation can be based on
• Landuse
• Satellite imagery
• Orthophotos
• The image of vegetation height is called the
  Structural Height

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Viewshed

• From book
• Area in view
• Area NOT in view
• (this done at higher resolution than indicated
  here!)

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(A sees B) (B sees A)?
Where would you rather be in a gun fight?
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Viewer Height

• Tower height e.g. 100 feet
• The viewer height parameter for towers is its
  height ABOVE the vegetation.
• If forest has a height of 40 and the tower is
  100 tall then for viewshed the viewer height is
  60 above the structural surface

Earths surface
40 forest
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The Dialog

• Structural Surface
• Viewpoint
• Point
• Line
• Area
• Output
• Viewer height
• Distance
• Boolean? Proportional?

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Basic Process
INITIAL
UPDATE R12C28
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ViewShed
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Boolean vs. Proportional
Boolean Any pixel seen by any viewpoint is 1,
rest are 0
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Water
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Watershed

• Idea in watershed is to
• find all the cells in a DEM which drain into a
  selected drain
• OR
• Find all the watersheds in the DEM image
• The additional functions are
• Flow calculates flow direction FROM a pixel
• Runoff calculates the accumulation of flow

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Basins or ArcHydro

• Basins is a Hydrologic modeling system based on
  raster images
• Pit fill
• Watersheds
• Flow direction (Flow in IDRISI)
• Flow accumulation (Runoff in IDRISI)
• Rain drop tracing (Not in IDRISI)
• So these are pretty basic tools for the
  hydrologist and pollution modeler

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Watershed Automatic

• Does a pit fill automatically
• Finds all the watersheds in a DEM image that are
  enclosed by the image
• That is, their outlets are in the image
• You have to specify the minimum size of
  watersheds to be identified
• Each watershed is given a unique serial

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Automatic
Watersheds of minimum size 50 pixels
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Seedy (drain point)
100 pixel sheds
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Flow
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Runoff (flow accumulation)
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Runoff streams?
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How does watershed work?

• First find the cells which drain into the
  startingdrain
• Next find the cells which drain into the previous
  set of cells which drained into the drain
• And so on until there are no more cells that
  drain, eventually, into the drain!
• For example

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Problems with watershed

• Sometimes you get only ½ of the watershed
• If that happens pick two cells for drains OR pick
  the stream if one available
• The old way to get watersheds was to draw them on
  a Topo map and digitize the vector outlines
  based on height of land
• Now watershed delineation on a DEM is more common
• Despite the fact that both Topo maps and DEMs
  have unknown accuracy!

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Summary

• Both Viewshed and Watershed functions are in
  common useage
• Best done in a raster system
• Both are based on the accuracy of a DEM and that
  is a problem
• LIDAR is probably the best but costs lots of
• In Exercise 7 you get to use both!