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In ArcGIS for Desktop I have to Derive and symbolise areas considered susceptible to avalanche risk based on the following simple rules:

  1. areas with a slope between 20 and 45 degrees,
  2. facing towards the prevailing wind (west in this case)
  3. not in forested areas.

By far I manage to display the Slopes and Aspects using tools from Spatial Analyst.

But how I can display the slopes in 3 different categories where they all facing west?

Photo attached shows what I manage to come up with

enter image description here

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    Use raster calculator to exclude non-western slopes, something like Con(aspect=1, slope). What if there is no snow at all or it is very thin layer? – FelixIP Dec 1 '14 at 1:18
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    Why only the western slopes? surely down is down regardless of which way it's facing... FelixIP has it right, use Raster Calculator with a Con but it might be easier to use Extract by Attributes Value = 1 to create your mask, followed by Extract By Mask, build up slowly, observing and validating each step before moving on. What format is your forestry data in? Do the cells from slope and aspect match (I would assume yes if you made them from the same DEM). – Michael Stimson Dec 1 '14 at 1:32
  • Thx for answering, This is the part of my assignment which involves defining Avalanche Risk Areas. The teacher give us some dummy files to work with. So I have the Slopes and Aspects on a separate files but there is no table of attributes added to it. I've only the values produced by default in "Symbology". Slopes has round numbers but in Aspect it displays as "Northwest (292.5-337.5)" or"West (247.5-292.5)" for example, is there any chance I can change it somehow so the calculator will recognize it? thx – user3277558 Dec 1 '14 at 2:51
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    It sounds like attributes have been built for the raster (the data stores a number but shows text), you will need to find the value that matches 'west'. The slope should be a floating point regardless of what the symbology says, unless it too is classified, you can find this in the data type in the Raster Properties. It is absolutely vital you determine if the cells align and are the same size, if not you will need to resample the slopes to match the aspect (not the other way around as it's classified data). Pls go to Catalog, right click and select properties on rasters and include image. – Michael Stimson Dec 1 '14 at 4:20
  • What format (image/vector) are your forests in? Usually when one is processing raster all the inputs are converted to raster, conversely, if the problem is vector then all inputs are converted to vector... depending on what format the forests are in you might want to extract rasters, convert to vector and do vector overlays. – Michael Stimson Dec 1 '14 at 4:22
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Here is how I would set this up using raster classification and raster calculator:

1: Reclassify each raster dataset to the following:

Slope between 20-40 degrees = 1 , all else 0

Rasters facing West (what ever your degree range is) = 10, all else 0

Cells in an un forested area = 100, all else 0

2: Raster calculator and add all 3 rasters together. The resulting cells with the value of 111 are your target area

3: Reclassify again so:

Cells valued 111 = 1, all else 0

The result is a raster dataset with 1's (good) and 0's (bad). Symbolize as needed.

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