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I have an NDVI raster that I would like to reclassify into some arbitrary bins i.e. all values between 0 - 0.2, 0.2 - 0.4 etc. for spatial analysis. Could anyone advise how to:

1) reclassify a raster in QGIS?

2) then calculate spatial statistics e.g. in the context of NDVI i would like to be able to calculate ground cover by "live" vegetation and then ground cover relating to the "bins" described above and produce some maps and histograms?

  • Why do you want to bin the data? The analysis goal you later define would better be performed on continuous data. You may want to recenter the data so its minimum value is 0. The term "spatial statistics" is a bit vague. Please state some specific goals and what you have already tried. We are a help forum and not here to define your analysis for you! Also, please search the forum. There are plenty of posts regarding reclassifying raster data in QGIS. – Jeffrey Evans Mar 2 '16 at 15:23
  • Thanks Jeffrey. Apologies - my intention was not to get you to define my analysis for you so perhaps i should attempt to explain what i am trying to do again. I have an NDVI (raster) derived from airborne multispectral imagery over a city which itself is useful for empirical observations but i'd like to take it one stage further and get some basic quantitative data from it in terms of vegetation cover (based on an arbitrary threshold) and then vegetation cover at various other thresholds broadly relating to "stressed"; "moderately healthy" and "healthy' which would be the new data classes. – Rob Lednor Mar 3 '16 at 0:33
  • I have tried some of the reclassification methods in the posts but i think i'm running into issues because NDVI is decimal data rather than integers... the solution is beyond my capability at the moment hence seeking guidance! – Rob Lednor Mar 3 '16 at 0:37
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I achieved this by: 1) Reclassifying in SAGA (via QGIS Processing Toolbox)using "Reclassify Grid Values" 2) Using the classification report function in the Semi-Automatic Classification Plugin to report area coverage by classification.

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If you are working with decimals try the GRASS tool from the processing toolbox called 'r.recode'. I did it in this case study on the QGIS website.

The 'r.recode' tool will require you to create a txt file outlining the range of old values and their new value. Its a pretty simple file in the format of "range start:range end:new value", for example the values you mentioned above could be written like below.

0.00:0.20:1
0.21:0.40:2

You could then generate zonal stats from you reclassified raster, or convert it to a polygon for further work.

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