Viewshed and Watershed

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Viewshed and Watershed

• Exercise 7 Background and Process

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Overview

• Viewshed how does it work?
• Watershed how does it work???
• Exercise 7

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Viewshed

• Exercise 7 has two parts
• locate microwave towers using Viewshed
• determine size and characteristics of watersheds
  above certain bridges
• Viewshed finds all the cells which can be seen
  from a target cell called a viewpoint, by
  establishing lines of sight to all cells in the
  image along radial lines from viewpoint cells.

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In Concept, nice. But

• Viewshed (and Watershed) use a DEM (
• We know there is error with DEMs
• So you have to be careful
• You can use Monte Carol Techniques to estimate
  the probability that a cell can be seen
• But lets look at DEM error first

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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 of elevation into
  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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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
  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!
• Target 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 gtmin tan acannot be seen

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(A sees B) (B sees A)?
Where would you rather be in a gun fight?
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To Use Viewshed (Ex7)

• 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
• Make the Surface Image
• we also have to add structural height to the
  elevation!
• Trees, buildings, shrubs, etc

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To Use Viewshed

• Viewer Height
• Tower size e.g. 100 feet
• Person e.g. 5 feet
• Be careful this should be our value units!
• Search Distance
• Range of antennae
• Same units as distance(reference) units
• 5 miles needs to be converted to 26,400 feet

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Process
INITIAL
Viewshed 10k 5
DEM
VIEWSHED
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UPDATE DIALOG
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DEM
VIEWSHED COVERS DEM
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Watershed

• Idea in watershed is to find all the cells which
  drain into a selected drain
• It first finds the cells which drain into the
  drain
• These are cells which have an aspect facing the
  drain (A cell to the W of the drain must have
  an aspect of E to qualify
• But it is NOT that simple!

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Not So Simple

• 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 cell which
  drain, eventually, into the drain!
• For example

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Overview-Viewshed

• Setting-up images
• Structural Elevation
• Viewpoints
• Running Viewshed
• Analysis
• Collection Editor

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Viewshed

• Objectives
• Find and suggest 1 acceptable location where we
  can locate the tower
• Provide support to demonstrate that it cannot be
  seen from the beach
• What parcels?

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Viewshed

• Set-up
• Landuse, Elevation MSL and Lake to get
  effective/structural elevation
• How?
• Also Update beach with its structural height
• The Viewpoint Images
• Initial and Update
• For tower1, tower2 and the beach

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Structural Elevation
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Viewshed

• Running Viewshed
• Boolean overlays
• Collection Editor to query
• Confirm
• Extract

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Overview-Watershed

• Setting-up images
• Viewpoint Image
• Running Watershed
• Analysis
• Extract