[GIS] Raster Calculator making incorrect calculations in ArcGIS Desktop

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I am trying to use raster calculator (ArcMap 10.2.2.3552) to apply a poisson generalized linear model to estimate species abundances based on elevation. The outputs don't make any sense to me, and I'm hoping someone can clarify why.

The best fit model (generated elsewhere) is:

Exp( 3.394 + ( 4.717 * "elev" ) + ( -2.602 * ("elev" ^ 2) ) )

Input raster below:

Raster Calc inputs

The output, however, doesn't reflect the math I input, and I am wondering if I have screwed up the syntax, or am missing something basic about raster calculator (which works fine for other models I have used on this dataset). The "elevmllw" raster ranges from -84 to +5 in values (meters) and the output ranges from 0 (which is correct) to 3.49972e+011 (I'm not sure where that number comes from, solving my model equation for that solution gives an imaginary number).

Output:

Output Raster (note high/low values)

When trying to figure out what was going on, I poked around noticed all the values made no sense. Using identify, it's clear that the calculated values for given cells (from "poisson") do not align with what I am trying to calculate for the elevation value from that cell, so it seems like I must be missing something about how to specify this equation.

enter image description here

The calculation runs without any error messages and the environments are shown below (sorry I'm not sure which are most helpful in troubleshooting, so I showed them all) enter image description here

Best Answer

Exp( 3.394 + ( 4.717 * "elev" ) + ( -2.602 * ("elev" ^ 2) ) )

In Raster Calculator syntax, the '^' operator is for 'Boolean XOr', not 'raise to power of' (see Raster Calculator operators here). Instead, you could use:

Exp( 3.394 + ( 4.717 * "elev" ) + ( -2.602 * ("elev" ** 2) ) )

or

Exp( 3.394 + ( 4.717 * "elev" ) + ( -2.602 * ("elev" * "elev") ) )

or

Exp( 3.394 + ( 4.717 * "elev" ) + ( -2.602 * (Power("elev",2)) ) )

Related Solutions

[GIS] reclass, raster calculator

If you provide more detail on how you reclassified and the specific raster calculation used to get the empty data raster (usually indicated by -3e20 to +3e20) that will be helpful in troubleshooting the problem.

Assuming ArcGIS with Spatial Analyst, if reclassify and raster calculator are giving you empty output then try the following two tools to achieve the result you need:

Set Null can be used to set all other values not equal to the one you're interested in as NoData - a much more straightforward method than reclass to change values to NoData.

Extract by Mask can then be used to isolate cells within the other raster wherever there are values in the set null output. And then you can use this masked raster when calculating the corresponding equation (or more efficiently use the SetNull output as a mask in the environments and calculate on the original land raster).

Perhaps a more simplified approach will be to use the Con tool to apply your separate equations inside conditional statements for different land types. Basically, they may look something like this (substitute with your file names):

Con(LandRas == 1, (type equation 1 here), 0)
Con(LandRas == 2, (type equation 2 here), 0)
Con(LandRas == 3, (type equation 3 here), 0)

[GIS] ArcGIS Raster Calculator output raster has one band but cannot build attribute table

Here's something I have done many times in the past. You can use the Raster Calculator and Convert your values to Integer. However, before doing so, I strongly recommend looking a bit deeper into your raster values.

For example, if you know how many significant digits you want your data in, then you can just multiply the current rasters by a factor then convert that value to Integer.

To illustrate, say I a raster type float, and I determine that the data values are only necessary up to 4 decimal points, I can use the Raster Calculator to do:

Int(raster * 10000)

This will multiply all of my raster values by 10,000 (moving the decimal 4 places to the right) then convert the number to an integer.

After you create your raster attribute table, add a field to it, and you can divide the raster value by 10,000 if you need a field to show a more accurate value for each raster pixel.

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