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I am trying to make a soil erosion risk model using raster overlay in ArcGIS Pro.

I have calculated soil erosion risk in raster calculator by combining the following risk factor rasters:

  1. slope
  2. soiltype
  3. rainfall

The risk from these factors is calculated based on the following table:

enter image description here

From this I derived 5 risk rasters which have been reclassified as follows:

slight-1

lower-2

moderate-3

high-4

very high-5

I have then added these into a new raster to make a risk model (values ranging 1-5):

enter image description here

The next step requires this model to be moderated based on a landcover raster(with risk categories ranging from 0 to 4)

The relationship of the risk values in these two rasters is determined by the following table:

enter image description here

What operation is required in raster calculator to achieve this result?

Following the last answer here is what I have after multiplying the original risk layer by 10 and then creating a new raster adding (+) the multiplied risk layer to the land category layer:

enter image description here

I then reclassify as per the screenshots below:

enter image description here

enter image description here

This however seems to not change the risk categories compared to the original risk layer, as the count and location of pixels is identical between the 2 layers, so there has been no moderation of a class 5 to class 3 based on the land category layer if this makes sense, I must be doing something wrong here, could you point me in the right direction please?

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  • Just use Combine tool, it designed for this task.
    – FelixIP
    Commented Jun 7, 2023 at 3:47
  • You are almost there you are running the tool correctly, just not apply the correct values. So looking at your Table 4, a very high erosion (5) moderated by a land cover risk of (2) should calculate out as a pixel value of "52", that recodes to the matrix value of "Moderate" which if as I have assumed is following the erosion class is 3. Now look what you have type into the reclassify tool for "52", you just typed in 5 for the new value, so you are not moderating the new value. A matrix value for example of "21" should now be 1, but you have entered 2.. and so on.
    – Hornbydd
    Commented Jun 7, 2023 at 15:11
  • Right! cause the new classes prior to reclassifying are all the possible combinations between the 2 layers.. it does make sense now I'll try reclassifying again, thanks a lot!
    – Bill
    Commented Jun 7, 2023 at 15:28

1 Answer 1

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A trick I have used in the past to answer this sort of question is to multiply your erosion risk layer by 10 so you have values 10,20,30,40 & 50. You then add your land cover raster to this raster. So a value of 32 for example is a moderate erosion/LC risk 2 combination. Then run this raster through a reclassify tool and for example you would convert a 14 value back to 3 (moderate).

Having seen @FelixIP comment the Combine tool does the first two steps I suggested in one step which is undoubtedly much faster! You would still need to reclassify the output if you want the same range of values as in your table.

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  • Hi! thanks for the answer.. I have tried this but after reclassifying I am getting the same pixel values as in the original risk layer.. am I reclassifying incorrectly? I converted everything in the 10 range to 1, 20s to 2, 30s to 3 etc
    – Bill
    Commented Jun 7, 2023 at 14:20
  • Would that not be what you want? The Table 4 matrix value happens to be the same names as the erosion risk categories. So very high erosion which is value 5 is converted to a 3 if the land cover risk is 1 or 2?
    – Hornbydd
    Commented Jun 7, 2023 at 14:47
  • Hi again,.. yes, converting that 5 to a 3 would be what I want but I don't seem to have done that, as what was 5 in the original layer is still 5 in the final reclassified one. so I must be reclassifying it wrong I guess
    – Bill
    Commented Jun 7, 2023 at 14:51
  • Sounds like it :) Typically you would update your question with an update section to expand on the issue, in this case how exactly are you running the reclassify tool? A screen shot would help.
    – Hornbydd
    Commented Jun 7, 2023 at 15:01
  • yep just added the screenshot let me know if you need more!
    – Bill
    Commented Jun 7, 2023 at 15:09

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