Viewshed

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Viewshed

• Exercise 7 Background and Process

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Overview

• Viewshed how does it work?
• Viewshed finds all the cells which can be seen
  from a target cell called a viewpoint, by
  establishing lines of sight from the viewpoint to
  every other cell in the image and then
  determining if the cells can be seen that is,
  no obstructions
• Introduction to Ex7

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Overview
Viewshed is the same concept as Watershed

• Viewshed how does it work?
• Viewshed finds all the cells which can be seen
  from a target cell called a viewpoint, by
  establishing lines of sight from the viewpoint to
  every other cell in the image and then
  determining if the cells can be seen that is,
  no obstructions
• Introduction to Ex7

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Viewpoint
Viewshed
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What is viewshed used for?

• Any line-of-site phenomena
• Telephone microwave communication between
  microwave towers
• Cell phone communication with the nearest Cell
  Antenna
• Visual observation like for fire towers
• More advanced raster systems (ArcGIS) can also
  model the refraction of radio waves over
  obstacles

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Like
Cell Tower
Terrain
No signal
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Simple Concept, But

• Viewshed uses a DEM
• We know there is error in DEMs
• So you have to be careful
• You can use Monte Carlo techniques to estimate
  the probability that a cell can be seen
• More on this later

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VIEWSHED Defined

• Finds all pixels that are in view of a viewpoint
• A viewpoint is any number of pixels that in an
  image that is otherwise 0 (zero)
• If you have more than one viewpoint what is the
  resultant 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

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

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Nits and Grits of Viewshed

• Not as straight forward at it seems!
• Pays no attention to size of cells!
• Viewpoint cell tops are transparent to viewshed
• So shape of terrain is not really represented!

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Nits and Grits of Viewshed

• Calculates the Tan of angle a for each cell on
  each radial line from the view point
• As it goes out along the radial line it saves the
  minimum tan a
• Any pixel with a tan a min tan acannot be seen

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Nits and Grits of Viewshed

• Tan opposite/adjacent
• Min Tan a is red
• tan a of cell E4 is seen.

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(A sees B) (B sees A)?
Where would you rather be in a gun fight?
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Using Viewshed (step 1)

• Make Viewpoint Images
• 1) use INITIAL to make an image of all zeros for
  each viewpoint needed
• 2) use UPDATE to change the location of the
  viewpoint to a 1
• 3) You will do this several times because we want
  to do one viewpoint at a time

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Using Viewshed (Step 2)

• Make a Surface Image over which radio waves or
  view propagates
• Light and microwaves (cell signals) do not
  propagate very well through buildings or
  vegitation
• So we have to add a Structural Height to the
  terrain (DEM) to produce a Strucural Elevation
• This is done by adding the landuses average
  structural height to the DEM

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Setting Viewshed parameters (3)

• Viewer Height
• Tower height above structural elevation e.g. 100
  feet
• Person e.g. 5 feet average
• Be careful, this must be in value units!
• Search Distance
• Range of antennae or eyeball (Horizon is about 11
  miles in flat terrain or water)
• Same units as distance(reference) units
• If value units of DEM are feet you cant use
  miles!

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Process
INITIAL (0)
Viewshed 10k 5
DEM
VIEWSHED
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(No Transcript)
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DEM
VIEWSHED COVERS DEM
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UPDATE?

• Changes the values in any rectangular area in an
  image

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Booby Trap!!!!

• REMEMBER THAT COLUMN AND ROW NUMBERS START WITH
  ZERO (0)!!!

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DEM Error!

• Many DEMs come from USGS quad sheets with contour
  line errors of or - 20 feet!
• Depending on how measured the elevation may be of
  the ground surface or it may be the elevation of
  vegetation on the surface
• There is a high probability that the error in
  elevation will be large where the slope is steep.

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Back to Monte-Carlo

• Determine probability-of-being-seen (Felleman)
• introduce known random error into the DEM
• create 100 random surfaces with mean 0 and SD of
  that of the estimated DEM error
• Add these to the DEM ? 100 perturbed DEMs
• Run VIEWSHED 100x, then tally how many times the
  known target cell is in the viewshed
• Result is estimated probability that the cell can
  be seen.
• Results around 50 will have to be investigated
  on the ground.

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Liu Method

• Didnt like the method above because the
  resulting DEMs didnt have reasonable spatial
  autocorrelations for elevation
• so, made the random layers so that they had 2 to
  3 times the estimated SD of the surface
• add those to DEM as before and smooth it with a
  low pass, average filter
• Calculate SD subtracting the original DEM from
  the smoothed DEMs
• Calculate spatial autocorrelation ( went up to
  like .96)

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Thank Goodness

• You wont worry about that but you to be should
  be aware of the pitfalls of using viewshed.
• You should always ground truth before making big
  decisions

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Collection Editor

• You can find the site(s) that work using
  techniques you have already learned . But there
  is an easier way.
• The collection Editor to the rescue!
• It allows you to see the values in multiple
  images when you use the query tool

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Collection tool

• First set up a collection
• Click on the tool ?
• This opens the collection tool

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Collection 2

• Add layers you want in the collection to the
  window on the right

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Collection 3

• When you have added the layers you want to be in
  the collection
• SAVE it!

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Collection 4

• This is what that does to the pick list (like in
  Display)
• The images are copied to the Class collection

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Collection 5

• To use the collection files you must DISPLAY one
  of the images FROM THE CLASS COLLECTION!
• When the image displays click on the Feature
  Properties tool

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And you get this
When you click on the query tool
In this case I had the Struct_elev image
displayed and I can see the values for the
viewshed and the landuse for that cell