Image Difference Thresholding - NV5 Geospatial

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Last Post 10 Jun 2010 02:22 AM by anon

Image Difference Thresholding

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anon

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10 Jun 2010 02:22 AM
Dear Friends, I have NDVI image of two dates (e.g. 1992 and 2008) and am looking for changes through image differencing and PCA. I have used band math to create the diffrence image (b1-b2) + 100 to find the pixel based diffrence and to convert all negative values to positive. I have created the statistics and got the mean and SD (standard deviation) values along with min and max values. I want to create change threshold values based on 0.5, 1, 1.5, and 2 SD above and below mean. Could you please help me how to do this in Envi 4.5 Is it possible to see the range (max and min) after each SD value (e.g. SD =1) and also finding the changes like High Loss, moderate loss, No change, moderate gain and High Gain in NDVI values. Thanks, PK

MariM

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10 Jun 2010 07:08 AM
If you know what threshold values you want to use, then one way to apply them visually is to use a Density Slice under the display menu under Tools ->Color Mapping.

Deleted User

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21 Jun 2010 09:10 PM
I dont know how to determine the threshold value (range) for changes. I have seen in literature that researchers use +- 1, 2 or 3 SD values from the mean as a thresholding measures. I dont know how to do it in ENVI 4.5. Please help me out Thanks

Deleted User

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14 Jan 2011 07:07 AM
This can be solved in arcgis by classify in image properties/symbol menu, but i wonder to know how did you determine the appropriate threshold for the changed area, i could not find the best way to determine the threshold ,especially for large area,because i did not have ground truth data to assess the accuracy, please tell me your way,thank you very much.

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