DEM Derived Products

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DEM Derived Products

• RNR/GEOG 420-520
• Spring, 2000

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DEMs and Derived Products

• Derived Products are Nearly Always Raster Based
• Based on Principles of Map Algebra and
  Cartographic Modeling
• Can be Divided into Point and Area Operations

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Point Functions

• Computes an output grid in which the output value
  at each location is a function of the input value
  at each location
• Accept grids, numbers and scalars as input
• Called Local Functions in GRID

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Point Operations in GRID (contd.)

• Trigonometric
• Logarithmic
• Reclassification
• Selection
• Logical
• Statistical

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Spatial Operations

• Interpolation
• Spatial Filtering
• First and Higher Order Derivatives
• Surface Topology Drainage Networks and
  Catchment Delineation
• Contiguity Assessment Clumping
• Cost Surface Calculation Non-Linear Dilation
• Visibility Analysis, Shaded Relief, Irradiance

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Spatial Filters
Total
Range
Diversity
Mean
Min
Max
Mode
Std. Dev.
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Spatial Filters in ARC/INFO

• Focal Functions
• compute an output grid based on input cell(s) in
  a specified neighborhood of the location
• Focalflow, majority, max, mean, median, min,
  range, std, sum, and variety

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Spatial Filters in ARC/INFO

• Zonal Functions
• These functions compute statistics or identify
  features on the values from the value grid that
  intersect the zones defined by a zone grid
• Zonal Functions same as Focal Functions

Zone GRID
Value GRID
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Low Pass Filter

• Low Pass Filter smoothes the entire DEM and
  reduces the significance of anomalous cells
• Mean filter for continuous surfaces
• Mode filter for nominal data
• FILTER ltin_latticegt ltout_latticegt LOW HIGH
  FILE iterations filter_file

Mean
Mode
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High Pass Filter (Edge Filter)

• Original Surface minus Low Pass Surface equals
  High Pass surface
• Has the effect of enhancing areas of rapid change
  or complexity -- Edges
• FILTER ltin_latticegt ltout_latticegt LOW HIGH
  FILE

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Measures of Relief

• Relief calculated with combinations of focalmin,
  focalmax, and focal range
• DOCELL
• temp focalmin (dem, circle, 5, data)
• relief dem - temp
• end

Relief Above max Target 950 810
140 Relief Below Target Min 810 780
30 Range max min 950 780 170
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Measure of Roughness

• Terrain roughness can be seen as a factor of
  elevation change, and measured as standard
  deviation within a neighborhood
• roughness focalstd (DEM, circle, 5, data)

Std. Dev.
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Using Volume to Gauge Exposure

• Measured by passing an imaginary cylinder at a
  set height over each cell
• Cylinders with high volume are exposed, cylinders
  with low volume are sheltered

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Making a Shelter Grid
/making a grid with all cells equal to 1killme
(dem 0) 1/ making the shelter
gridDOCELL temp1 focalsum (killme,
circle, radius, data) temp2 (dem 20)
temp1 temp3 focalsum (dem, circle,
radius, data) sheltergrid temp2 -
temp3end
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Slope and Aspect in GRID

• GRID fits a plane to the local neighborhood
• The angle and direction of the plane supply slope
  and aspect measurements

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A Problem With Aspect

• Aspect is measured in Degrees
• 1 degree and 359 degrees are essentially the same
  aspect
• The value 359 is much greater than the value 1

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North South Aspect Grid

• DOCELLif (aspect gt 180) aspectNS - aspect
  360 else aspectNS aspect360END

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Slope and Aspect for TINs

• TINs must be converted to obtain slope and aspect
  values
• Conversion to a lattice allows GRID commands for
  slope and aspect
• TINARC with poly option converts to polygons that
  carry slope and aspect values

• PAT for polygon created with TINARCAREA
  6811.91209PERIMETER 553.27638TINPOLY
  5TINPOLY-ID 4PERCENT_SLOPE
  14.822ASPECT 260.762SAREA
  6886.33275

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Hydrologic Modeling

• DEMs allows automated modeling of hydrology
• Hydrologic modeling is a process that begins with
  a DEM
• Types of models flow direction, flow
  accumulation, watershed delineation, and flow
  length

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Flow Direction

• Calculates flow direction for every cell in the
  GRID
• Based on direction of steepest slope in local
  neighborhood
• Several rules address potential problems
• FLOWDIRECTION(ltsurface_gridgt, o_drop_grid,
• NORMAL FORCE)

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SINK

• Sinks are bad!
• GRIDs SINK command creates a grid identifying
  all areas of internal drainage, i.e. sinks
• SINK(ltdir_gridgt)
• GRIDs FILL command fixes sinks

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Flow Accumulation

• Based on flow direction grid
• Creates a grid of accumulated flow to each cell
• Can be used to create a stream network
• FLOWACCUMULATION (ltdir_gridgt, weight_grid)

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Flow Accumulation -gt Streams

• Use a conditional statement to create a stream
  network from a flow accumulation grid
• streams con (flow gt 100, 1)

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Delineating Watersheds

• Determines the contributing area above a set of
  cells in a grid
• Needs a flow direction grid and sources
• WATERSHED (ltdir_gridgt, ltsource_gridgt)
• BASIN
• SNAPPOUR

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Flowlength

• Calculates distance along a flow path for each
  cell
• Goes either upstream (to the nearest ridge) or
  downstream (to the nearest sink or outlet)
• Primary use is to determine the length of the
  longest flow path in a basin
• FLOWLENGTH (ltdir_gridgt, weight_grid,
  DOWNSTREAM UPSTREAM)

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Visibility Analysis

• Requires
• DEM
• Viewpoint/s
• What it tells you
• Line of sight
• Viewshed
• Combined Viewshed
• Cumulative Viewshed

Tall al-Umayri