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I'm calculating Simple Ratio (SR) from NDVI using Raster calculator in Arcgis. Here is expression I wrote in Raster calculator.


But when I evaluate this expression, the following error always appear on the screen.

GRID ERROR-Unable to open the input grid: 1+FLOATC:\document\... 
Error in executing grid expression.

What is wrong with this expression?

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are you giving spaces between all operators? ( 1 + float[NDVI_apr.tif] ) / ( 1 - float[NDVI_apr.tif] ) – A.R Dec 2 '12 at 9:57
Yes, I did. But result is the same, showing me the above error. The NDVI_apr.tif image is raster image. I want to ask 2 things. 1. Can the raster image be used directly in raster calculator in Arcgis. I read it can, but I'm not sure. Or Is it necessary to convert it into grid. 2. The NDVI image is in D disk on my computer (not in GIS working directory). Is it ok? – user3063 Dec 2 '12 at 11:58
Doesn't Float need a capital F in Raster Calculator as per example here? – PolyGeo Dec 3 '12 at 1:56
I'm not sure how you can calculate the SR from an NDVI. Isn't the simple ratio nIR/red? – Aaron Dec 4 '12 at 4:35

write your expression using raster calculator buttons . don't write / + - * using pc or laptop keyboard.

your expression must be like this :

sr =(1 + Float("NDVI_apr.tif")) / (1 - Float("NDVI_apr.tif"))

this expression work properly in ArcGIS 10

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Float appears to need a capital F in Raster Calculator as per example here which is probably why @wetland Answer has expression working.

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