GEOL 404504 Advanced GIS

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GEOL 404/504 Advanced GIS

• Sudhanshu Sekhar Panda, PhD
• Lecturer/GeoSTAC Coordinator
• GIS Training and Research Center
• Idaho State University
• Spring 2006

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Lecture 7Digital Terrain Modeling
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What is it?

• Digital use of computers and digital (digitized)
  data
• Terrain Topography, relief, hypsography
• Modeling Addressing a complex problem using
  only 1 or 2 variables known or believed to
  influence the problem

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Applications

• Slope
• Aspect
• Inflection
• Viewshed
• Analytical Hill shading
• Water flow
• Watershed
• Data visualization

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Slope
Rise / Run Slope
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Slope Map Calculation Algorithm
?e
?y
?x
?e sqrt ((?x)2 (?y)2)
(S is the size of the pixel)
Percent of slope (?e/2s) x 100 (when ?elt2s)
Percent of slope 100 - ((1 (?e/2s) x
100) (when ?egt2s)
Degree of slope tan -1 (?e/2s) x 180) / ?
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Aspect Map Calculation Algorithm
? tan -1 (?x / ?y)
Where ? slope in degrees ?x length wise
average elevation difference ?y width wise
average elevation difference
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Assumptions

• The mean slope will be used.
• The maximum slope will be used.
• The minimum slope will be used.
• How many pixels are involved in the calculation?
• How is RUN calculated?

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Aspect
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Inflection

• Second derivative
• Slope rate of change

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Viewshed Analysis
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Analytical Hill Shading
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Flow models
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Watershed Models

• Quantitatively determine the extent and
  boundaries of watersheds.
• Requires
• DEM
• sinkriver or stream

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Data Visualization
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Understanding the Roving Window Process

• Row-wise OR
• Column-wise

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A Scenario Instance

• Locate all areas with elevation above 1675 meters
• Why?
• Lodgepole Pine forest types
• Why?
• Species selectivity
• Why?
• Security cover

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The Cartographic Model
Z gt 1675
DEM
DEMIDRIS
IMAGE CALCULATOR
LPP Elev Criteria
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Using Image Calculator

• Similar rules to SQL (Query by attribute).
• Symbols of inclusion are important.
• Mathematical OR logical expressions.

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A Simple Expression
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Step-by-Step

• Here is what the computer is doing
• 1. Open the Idrisi raster DEM
• 2. Create a new blank output raster layer
• 3. Apply roving window to evaluate each pixel
• 4. Write the result of each evaluation to the new
  output raster layer

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Applying the roving window
0
0
1600
1610
1625
1640
1620
1610
1630
1680
1640
1630
1620
1685
1675
1660
1635
1625
System Rule 1 Begin with the pixel in the upper
left corner.
System Rule 2 Advance the roving window in a
row-wise fashion.
Continue this process until all cells have been
evaluated!
Does the value of this pixel satisfy the
criteria? (DEM gt 1675).
No. The value for this pixel in the output layer
will be 0 (false).
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Another Scenario

• Locate all areas with an elevation above 1675
  meters and a northerly aspect.
• Why?
• Precipitation regime better favors LPP
• Problem
• What does northerly mean?

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Defining Northerly
360 or 0
90
270
180
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Cartographic Model

• The more complex the analysis, the more you need
  to use a cartographic model.
• Make it a habit to develop cartographic models!

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Source Data
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Three Simple Expressions

• The first (elevation criteria)

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Three Simple Expressions (contd)

• The second (northerly aspect 1 of 2)

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Three Simple Expressions (contd)

• The third (northerly aspect 2 of 2)

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A Compound Expression
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Which one is Correct?

• Version two or three?
• Are they the same?
• Lets check.
• GIS Analysis---Database Query---AREA

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How do we know?

• Combine results of simple expressions
• How?

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Overwhelmed?Confused?Lost?

• The solution is obvious now
• The process was painful, confusing, and
  tedious.

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The Key to this Solution

• Every expression must be bound by parenthesis
• Just like an SQL statement, you must watch your
  symbols of inclusion closely!

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Questions?
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Key Concepts

• This topic provides you with knowledge in
  GIScience.
• You will learn/experience new techniques, tools,
  and applications.
• You will experience new ways to think critically
  about GIS data.