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I have water supply and water demand rasters.They are both of the same cell size and spatial reference.

I need the ratio of water use to water demand.

Then I want to classify this output in the following way for each raster cell:

If water demand value is greater than water supply value by 40% I assign it a value of 1

If water demand value is 20 - 40% greater than water supply value, I assign it to class of 2

If water demand less than or equal to 20% greater than water supply, I assign it to 3.

2

The first step is to calculate the water usage percent. I'm assuming your units are the same (gallons, kilolitres, cups... whatever, so long as they're the same). Divide your supply by demand (100 kl supply and 25 kl demand = 25%, 1kl supply and 4kl demand = 400%) in Raster Calculator (you must have spatial analyst license)

Supply / Demand

If you're doing this in catalog you might need to fully qualify your rasters:

Raster("c:\full\path\to\Supply.tif") / Raster("c:\full\path\to\Demand.img")

Next Calculate your statistics or the next tool will refuse to work.

To reclassify your raster into the three ranges (as a one-off) use the Reclassify tool and enter the bands manually; if you're likely to be doing this again with the same parameters consider creating a remap table and using Reclassify by Table (instructions on the table are there).

  • Hi Michael, please could you simplify your answer, I am new to arcGIS. So when I divide the 2 layers, where does the percent come into play? The two rasters are in the same unit. – veesilouette Dec 2 '14 at 0:22
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    To calculate percentage (as a decimal) you divide one by the other. As in the example 100 kl supply and 25 kl demand = 25% which is a value in the percent raster of 0.25, mathematically this is the same. If you want to get percent (0.25 to 25) then multiply by 100 i.e. (Supply / Demand) * 100 in raster calculator. – Michael Stimson Dec 2 '14 at 0:41
  • Ohh, thanks a lot. So to summarize your answer into steps: 1. use the formula (supply/demand)* 100 to get the results in percentage, 2. Calculate statistics of output raster 3. reclassify output raster. I don't understand the arcGIS desktop help, I never do. Kindly explain the reclass by table part. Thank you so much – veesilouette Dec 2 '14 at 0:59
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    Reclassify by table is a means of performing a reclassify with a set value, the table gives the output class and range to cover then the tool reads the table and applies the classes. It works the same way as Reclassify except that the remap values are read from the table instead of being entered manually. – Michael Stimson Dec 2 '14 at 1:46
  • The correct division is demand/supply which has given me the right ratio values. Thank you. – veesilouette Dec 2 '14 at 16:49

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