topic Re: Raster calculator in ArcGIS Spatial Analyst Questions

Raster calculator

FelixSchwarz — Mon, 29 Jan 2018 14:22:05 GMT

Hello,

is it possible to combine 2 rasters with the raster calculator defining the value of the rasters? It should look more or less like this but at the moment i'm getting an error for the expression.

Thanks for answers

Re: Raster calculator

XanderBakker — Mon, 29 Jan 2018 14:26:31 GMT

Can you try a Con statement like thois:

Con("AlnusSpp" + "BetulaSpp" > 20, 1, 0)

Re: Raster calculator

FelixSchwarz — Mon, 29 Jan 2018 16:05:26 GMT

Thanks for the quick answer. This one works but I try to retain the exact values from the data and not the define them

as 1 or 0, because the exact values are important for further procedures.

Re: Raster calculator

DanPatterson_Retired — Mon, 29 Jan 2018 16:39:35 GMT

Once you have your indicator array (the 1's) you use that array/raster to extract all the locations where a 1 is found.

In essence 1 indicates the locations where the sum of the two rasters is > 20. Use the 1's to extract the values. from the 2 input rasters.

Re: Raster calculator

XanderBakker — Mon, 29 Jan 2018 17:56:03 GMT

So what Dan is explaining, you could use:

Con("AlnusSpp" + "BetulaSpp" > 20, "AlnusSpp")‍

... in case you want to create a new raster with the original values of "AlnusSpp" where the sum of "AlnusSpp" and "BetulaSpp" is higher than 20 (rest of the pixels in this case will be assigned NoData)

And you can use:

Con("AlnusSpp" + "BetulaSpp" > 20,"BetulaSpp")‍

... to create the same type of raster but now with the original values of "BetulaSpp" when the sum of the two raster is higher than 20 (rest of cells will be NoData)

Or you can use something like this:

Con("AlnusSpp" + "BetulaSpp" > 20,"AlnusSpp" + "BetulaSpp")‍

... when you want to create a raster with the sum of the values of the two raster when the sum is higher than 20 (rest of cells will be NoData)

Re: Raster calculator

FelixSchwarz — Sat, 11 Dec 2021 16:10:42 GMT

The last one is exactly the one I needed, thank you very much!

One last question: Overall I got 9 species and want to create a raster where at least 2 or more of the species with >20 are displayed, is there maybe one expression for it?

Or is it possible to combine expressions maybe like this:

Con("AlnusSpp" + "BetulaSpp" > 20,"AlnusSpp" + "BetulaSpp") OR 
Con("AlnusSpp" + "CarpinusSpp" > 20,"AlnusSpp" + "CarpinusSpp")

Re: Raster calculator

DanPatterson_Retired — Tue, 30 Jan 2018 10:41:20 GMT

No. You would have script a solution or try modelbuilder (which has its own issues)

Re: Raster calculator

XanderBakker — Sat, 11 Dec 2021 16:10:44 GMT

As Dan mentioned, you will need a script to accomplish this. Find below an example of what you could use:

# load modules
import arcpy
import os
from arcpy.sa import *

# settings
ws_species = r'path to your folder of fgdb workspace'
species = ["BetulaSpp", "CarpinusSpp", "other species"]
specie_compare = "AlnusSpp"
output_postfix = 'Cmp'

# check out the SA extension
arcpy.CheckOutExtension("Spatial")

# create raster object from comparison raster AlnusSpp
compare_ras = arcpy.Raster(os.path.join(ws_species, specie_compare))

# loop through species
for specie in ws_species:
    specie_ras = arcpy.Raster(os.path.join(ws_species, specie))
    out_path = os.path.join(ws_species, specie + output_postfix)
    # calculate raster and store result
    result = Con(compare_ras + specie_ras > 20, compare_ras + specie_ras)
    result.save(out_path)
‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍‍

This script will loop through species based on a list defined on line 8 and create the output raster for each comparison. Output name will be specie name with "Cmp" at the end (see output_postfix on line 10). It assumes that all rasters are stored in the same workspace (folder or fgdb) as defined on line 7.

Re: Raster calculator

JayantaPoddar — Tue, 30 Jan 2018 15:41:16 GMT

You could try

SetNull(("AlnusSpp" + "BetulaSpp" )<= 20, ("AlnusSpp" + "BetulaSpp"))

Re: Raster calculator

FelixSchwarz — Thu, 01 Feb 2018 10:55:09 GMT

Thanks for the script, I will try it.

In general I was trying to create a map that shows a 'Mixed Forest' containing the particular species. So there have to be at least 2 species ( or more - with >20) in one grid. Isn't there another solution or tool for this one?

Re: Raster calculator

DanPatterson_Retired — Thu, 01 Feb 2018 11:31:02 GMT

There are multiple conditions Felix, to handle them all it requires you break things down into steps. separating mixed forest is one, With one species, that can be handled by a single Con statement. It is the multiple conditions that necessitate the many steps.

Hence Xander's last lines in his script looping through the species and doing the Con

Re: Raster calculator

XanderBakker — Thu, 01 Feb 2018 12:05:13 GMT

In addition to what Dan Patterson mentioned, in case a "Mixed Forest" requires the existence of more than 1 species and a certain value (>20, is this % coverage?) a situation o 0% of one species and 30% of another will yield a value > 20, but does not mean that there are actually two or more species.

Apart from validating "AlnusSpp" + "BetulaSpp" and "AlnusSpp" + "CarpinusSpp", the script will not validate "BetulaSpp" + "CarpinusSpp" nor "AlnusSpp" + "BetulaSpp" + "CarpinusSpp", when this may also yield a value > 20 and could represent mixed forest.

It could be better to create for instance a binary raster per species that represents its existence and sum those rasters. Those pixels that have a value of 2 or higher will have more than 1 species. Use a sum of all original rasters to validate the condition > 20.