topic Re: Raster Calculator in ArcGIS Spatial Analyst Questions

Raster Calculator

WersusBarckowich — Tue, 24 Jul 2012 06:27:51 GMT

Hello!
I try to calculate NDVI using Raster Calculator:

But result has only three values (and three colors): -1, 0 and +1.
What am I doing wrong?
Thanks!

Re: Raster Calculator

SheaMeyer — Tue, 24 Jul 2012 14:42:24 GMT

You must first convert your .TIF bands from an integer to a float.

The integer using an 8 bit pixel type for DN values from 0 - 255 in whole numbers.

The float (32 bit) allows for decimals.

The map algebra is Float ("%file.tif%")

Re: Raster Calculator

WersusBarckowich — Tue, 24 Jul 2012 15:45:33 GMT

Thanks!
But how can I do this?

Re: Raster Calculator

SheaMeyer — Tue, 24 Jul 2012 16:07:01 GMT

Use the same exact NDVI equation in raster calculator as you have now, but instead of the band 3 and band 4 .tif you are currently using from landsat, replace them with the output after converting them to a float.

You can convert them in raster calculator with the expression i.e. Float ("%Band 3.tif%") and repeat for band 4.
Or you can use the Float tool in the toolbox Spatial Analyst -> Math.
I recommend converting them to GRID files as well when converting to the float. Simply enough, just don't add the .tif extension when you name the output raster.

Good luck.

Re: Raster Calculator

WersusBarckowich — Tue, 24 Jul 2012 16:27:48 GMT

I try to add "%L5015041_04120090613_B30.TIF%" in raster calc, but I have error:

Executing: RasterCalculator "%L5015041_04120090613_B30.TIF%" C:\Users\Admin\Documents\ArcGIS\Default.gdb\rastercalc2
Start Time: Tue Jul 24 21:27:02 2012
"%L5015041_04120090613_B30.TIF%"
ERROR 000539: Error running expression: rcexec() <type 'exceptions.AttributeError'>: 'str' object has no attribute 'save'
Failed to execute (RasterCalculator).
Failed at Tue Jul 24 21:27:02 2012 (Elapsed Time: 0,00 seconds)

Re: Raster Calculator

SheaMeyer — Tue, 24 Jul 2012 16:51:09 GMT

Try to calculate statistics for the .tif file. Might be why it can't find a "table."

Re: Raster Calculator

SheaMeyer — Tue, 24 Jul 2012 16:52:19 GMT

Try to calculate statistics for the .tif file. Might be why it can't find a "table."

*Attribute I mean not table

Re: Raster Calculator

AnnetteWielemaker — Thu, 26 Jul 2012 10:01:07 GMT

Have you tried:
Float(Float("L5015041_04120090613_B40.TIF" - "L5015041_04120090613_B30.TIF") / Float("L5015041_04120090613_B40.TIF" + "L5015041_04120090613_B30.TIF"))
??

Re: Raster Calculator for NDVI

PeterHarrel — Wed, 10 Oct 2012 15:29:27 GMT

We do this all the time in our GIS class. If you input image bands are integer, ArcGIS will default to an output of Integer. Since NDVI has a range of -1 to +1, you end up with only -1,0, +1. To fix this, simply multiply one of the layers in the equation by 1.0, with the decimal. ArcGIS then defualts to float for the output.

Re: Raster Calculator

JeffreySwain — Thu, 11 Oct 2012 12:23:01 GMT

If you are using version 10 or higher, you can also create the NDVI output with the mosaic dataset as well.

Re: Raster Calculator for NDVI

curtvprice — Sat, 11 Dec 2021 06:18:36 GMT

This is a feature of Python 2.x. When you divide two integers, you will will get integer output:

>>> 1 / 2
0
>>> 1 / 2.0
0.5
>>> float(1) / 2
0.5

(post #1000!)