Exercise 4: elevation

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Exercise 4 elevation
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Overview, Ex4 and PP

• Basics of DEMs
• Quite a bit of playing with palettes
• A bit of arithmetic
• Making an IDIRISI Profile
• Calculate Slope and Aspect
• RECLASS function
• Modeling

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BASICS

• A DEM has values in each cell that are distance
  units (ft,m, furlongs. ) above some arbitrary
  zero plane
• Usually height above MSL
• Most operations on DEMs do not care much about
  the height-above-what.
• BUT, some do! (Booby Trap)

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Slope and Aspect

• Slope in or degrees
• Aspect azimuth in which slope faces
• Azimuth Degrees from North, 0 to 360 degrees,
  0 and 360 are the same
• A BOOBY TRAP HERE IS THAT YOU PRODUCE SLOPE IN
  DEGREES RATHER THAN - MAX VALUE SHOULD BE LIKE
  31.

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Compass Rose
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Aspect
An EAST facing slope
N
A WEST facing slope
W
E
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Calculating slope aspect
Launching SURFACE opens this dialog
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And you get this
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Now that is straight forward

• Only one booby trap what is it?
• Now you have images of raw slope and aspect but
  they are not very useful
• Why not?
• So we fix that by RECLASSifying the images.

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RECLASS

• RECLASS allows you to change the values of cells
  regardless of where in the raster they are
  located.
• This is done by specifying a new value and a
  range of values to be changed to that new value
• The RECLASS dialog has three fields

New value
Start value
To just less than
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Reclass into slope classes

• Unless you are doing something where individual
  slope values are needed you will usually use
  slope classes.
• Arent that accurate anyway
• Here are the classes
• Slope class 1 0 to JLT 5
• Slope class 2 5 to JLT 10
• .
• Slope class 5 25 to JLT 1000

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How is that done?

• Click on the RECLASS icon

• Enter Values

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What does a .rcl file look like?
IDRISI EDITOR
OPEN
Or you could do the same thing by?
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Quant
Reclassed (to integer) Qual
My 5 class Qual Palette And edited legend values ?
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Reclassing Aspect

• Aspect is a little more complex but the process
  is the same
• Reclass into point of the compass
• Edit legend
• How is the Reclass done?...

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Range for East
The range of values forEast is From 67.5 To
Just Less Than 122.5
0
22.5
45
67.5
90
122.5
North is more complicated.
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Range for North
North has to be donetwice! 0 to JLT
22.5 AND 337.5 to JLT 361
0
22.5
337.5
45
315
67.5
90
122.5
Why JLT 361?? WHY NOT 360???
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RECLLASS Aspect
SlopeClasses.rcl
RECLASSing raw ASPECT into points of the compass
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Change Legend!
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Summary so far

• Elevation is a basic layer of most applications
  of raster GIS
• Elevation images are called DEMs
• Slope and Aspect are derived from DEMs
• Slope and Aspect are always more generalized than
  the original elevation layer the kernel bit
• Edges of an image never have correct slope or
  Aspect values
• Both slope and aspect are usually classified
• The units of a classified image are Classes

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Lets do this in the modeler

• To use the Modeler we have to launch it

• Once loaded you build the model, testing as you
  go
• Then you save it

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Launch the Modeler
Delete
Run
Insert map
Display
Insert Module
Insert link
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Insert DEM
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Insert Surface Module
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Set Surface Parameters
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Link input and module
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Name the output
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Run
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Do the whole at once
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AND
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Legend?

• Cant update the legend automatically
• BUT I can open the metadata while in the modeler
  and do it manually!
• Advantage you have a record of what you
  did!!!!!
• Disadvantage might take a little longer unless
  you make a mistake somewhere in the manual
  processing and have to reconstruct what you did!

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Summary

• DEM are basic to most analyses
• Slope and Aspect are calculated from DEMs
• Resolution is 3x that of DEM and edges are
  screwed up
• Slope and Aspect are usually classified
• Life can be better if you use the modeler