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I have a NDVI that I created from a Landsat 5 image. Is there a way to extract all the pixels above a certain threshold and save it as a shapefile? I am trying to create a vegetation mask of sorts to filter out areas in a city that have vegetation, leaving only the structures.

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The simplest option would be the use of Raster Calculator with a conditional statement. Your statement may look something like the following with 'threshold' replaced with whatever value you like. The resulting raster will give you pixels with value=1 where values are greater than threshold, and NoData where values are less than threshold.

Con("NDVI_img" >= threshold, 1, "")

You could also do something like the following, which will result in all values greater than threshold retaining original values, where cells less than threshold have NoData.

Con("NDVI_img" >= threshold, "NDVI_img", "")
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    then you use "feature to raster" and you have your shapefile – radouxju Jan 2 '14 at 19:37
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By adding more conditions in the syntax of conditional statement as @Barbarossa nicely explained the answer of what your requirement is, a more evaluation threshold can be added by adding a multiple conditions in this statement. I was to assign the some constant values like 0.99 where NDVI < 0.20, 0.97 where NDVI > 0.5 and between this threshold i had to give another derivative formula in it and I had accomplished it with multiple conditional statement conditional evaluation with Con in ArcGIS raster calculator in ArcGIS raster calculator, a very nice tool.

Con(NDVI < 0.20, 0.97, Con( (NDVI >= 0.2) & (NDVI <= 0.50), 0.004*((NDVI - 0.20)/(0.50-0.20)) ^ 2 + 0.986, 0.99 ) )

If anyone knows about the free tool like in QGIS, or GRASS using map calculator tool. share it how to do it.

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