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I'm really happy to find this website. It is very helpful for the learners of Arcgis. Now I wanna ask a question. When i open NDVI image in ENVI, its value is still within the range (-1 to 1). But when I open this NDVI image in ERDAS and Arcgis, its value is 0-255. What does it mean? Is the NDVI image some problem?

best regards

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Do the negative NDVI values in ENVI correspond to ERDAS/ArcGIS values in the range 128..255? –  whuber Jun 17 '11 at 14:13
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3 Answers

What method did you use to generate this NDVI? It may be that ArcGIS and ERDAS make a normalization of their data by comparing the range of -1 to 1 for 0 to 255 so that its value to become integer and not floating. But you must know how NDVI was generated.

a hug.

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NDVI ranges from (theoretically) -1.0 to +1.0. If you got data ranging from 0..255 then you have data to 8 bit integer which need to be correctly rescaled to -1.0..+1.0 based on the related metadata.

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The VI Calculator by default applies a biophysical mask to values that are unreasonable. It is likely that the atmospherically corrected data has some values that are outside a reasonable range (because some pixels are not modelled well) and thus return unreasonable NDVI values. These should be masked.

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Are you suggesting that some values in the range [-1,1] are invalid for NDVI? –  whuber Jun 17 '11 at 14:13
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