How to remove abnormalities from the Green vegetation fraction

Question

Study area was clipped by using the shape file from Band 3 and Band 4. First, Band 3 and Band 4 were converted to at-satellite brightness temperature by using the built-in function in semi-automated tool in QGIS (semi-automatic classification plug-in). NDVI was calculated by b4-b3/b4+b3 in landsat 5 images, the range of NDVI was 0.0968299 - 0.433846. Green Vegetation fraction were also needed to examine the role of vegetation in mix land cover that was derived from the equation:

GVF = NDVI - NDVImin / NDVImax - NDVImin

but the problem here was as you would see the range of the fraction is -0.631684 - -0.0011024) that is due to some abnormalities in the data that don't know how to remove it. Is there something wrong in "NDVI" values as there is no data values (no data values = -3.4020 + 38) by seeing the properties of image in QGIS. I have taken the NDVImin and NDVImax as 0.0968299 and 0.433846 respectively but literature has shown that the values 0.05 and 0.70 as NDVI threshold values that were derived by global Advanced Very High Resolution Radiometer (AVHRR) 10-day composite data (Zeng et al. 2000).

Answer 1

Given the forumla:

GVF = (x - min) / (max - min)

(Notice the brackets.) It is impossible for the values to be outside the range 0 - 1 for all values of x between min and max, which by definition it is. In fact, it should occupy the full range from 0 - 1. So we can conclude that either your min and max values aren't representative of the actual range of values in the image (e.g. if you used the display min and max values used to scale the palette rather than the actual data min and max) or else that there was a mistake in the form of the equation when you entered it. Solving for x, we get:

x = GVF * (max - min) + min

Substituting in your minimum and maximum output values (-0.631684 and -0.0011024) we get a range of input values from -0.116057778 to 0.096458373. You'll notice that that upper range value is actually within three significant digits of your stated minimum value. (Coincidence? Possibly.) However, a range of NDVI values in an image from -0.116057778 to 0.096458373 does seem unlikely and you can confirm if this is the case by performing a histogram analysis on the image. It is much more probable to me that you simply had mixed up the equation slightly. Was it in the Raster Calculator that you performed this analysis? Given you dropped out the brackets in the equation in your question, I explored various combinations of missing brackets and wasn't able to come up with a form that would replicate your output range. I did find that the equation:

OUT = (x - max) / (max + min)

for your input min value (0.0968299) resulted in the output value -0.635069541, which is very similar to your stated output minimum value. Unfortunately it won't give you your output max value, so that wasn't it...but it's likely some similarly mis-typed equation resulting from substituting + for - or leaving out a bracket here or there.

Note that the error is unlikely to be the result of a NoData value, as has been suggested, since 1) NoData values are generally very large negative values that would result in an output far outside the small negative values found in your output range, and 2) NoData values should really be ignored by any GIS that utilizes the construct.