Grid Algebra

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Grid Algebra

• FE 423 - Lecture 3a

2
From Last Week

• Use a sun-angle calculator from the web to
  identify the sun angle for the beginning and end
  of this class period. Make a hillshaded image of
  the UW area for these times.
• Using a DEM of steep topography, map the areas of
  steep and convergent topography.

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Practice Problem 1 Identify the slope of each
square
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Practice Problem 2 Identify the aspect of each
square
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Practice Problem 3 Estimate slope for each
cell
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Practice Problem 4 Estimate aspect for each
cell
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Practice Problem 5 Shade convergent
topography
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Practice Problem 6 Shade convergent topography
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SCHEDULE
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OUTLINE

• Map Query
• Reclassify
• Map Calculator
• Handling Results
• Reading/Revising Equation

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MAP QUERY

• grid vs. number (dem gt 500)
• grid vs. grid (snow gt melt)
• grid vs. equation ((snow-melt) gt 10)

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RECLASSIFY

• Beyond just yes/no
• allows low/medium/high

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MAP CALCULATOR

• Put in any equation
• SWEz1d1d2Ez d3Ez2

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OPERATORS

• ! anotherGrid Grid
• anotherGrid Grid
• anotherGrid Grid
• anotherGrid Grid
• anotherGrid Grid
• - anotherGrid Grid
• / anotherGrid Grid
• lt anotherGrid Grid
• ltlt anotherGrid Grid
• lt anotherGrid Grid
• ltgt anotherGrid Grid
• anotherGrid Grid
• gt anotherGrid Grid
• gt anotherGrid Grid
• gtgt anotherGrid Grid
• And anotherGrid Grid
• Grid
• Negate Grid
• Not Grid

Any unmelted snow? (snow gt potential
melt) Total runoff? (rain
melt) Volumetric soil creep rate? (soil creep
rate soil depth)
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Numbers-to-Grids .AsGrid

• Numbers must be converted to grids
• SWEz1d1.AsGrid d2.AsGridEz d3.AsGridEz2

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PARSING

• Our notation is not specific
• SWEz1d1d2Ez d3Ez2
• Precedence is what we want
• SWEz1d1(d2Ez) (d3(Ez2))
• Left-to-right is what ArcView does
• SWEz1(((d1d2)Ez) d3)Ez2

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HANDLING RESULTS

• Naming Outputs
• Finding disc location
• Other useful stuff

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READING/REVISING EQUATION

• What equation did I use?
• What happens if I change it?

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Reading from ArcDoc

• Spatial modeling
• Cell-based Modeling with GRID
• Functions locals, focals, and zonals
• From ArcView Help
• Grid (Class)