digital image processingerdas-imaginemodisremote sensing
I'm using MOD09GQ product of MODIS to calculate vegetation indices in Erdas 2014, and I was able to extract NDVI using Erdas Model Builder.
What is the formula I should use in model builder for calculating Normalized Difference Water Index (NDWI) in Erdas? Do i need to download another products of MODIS for computing NDWI?
Your problem is likely due to how your model handles bit-depth. EVI, like NDVI, ranges from -1 to 1 and has a practical range of 0 - 1. Therefore, if you try and perform the EVI (or NDVI) calculation and save it as an integer type raster (e.g. signed 8 bit), the output will appear to be black, or all one value. Rather, you need to save the output as a float type, which allows decimals. Alternatively, you can stretch the data to the full range of, for example, an 8-bit unsigned raster (0 - 256).
Rescaling an NDVI or EVI from -1 to 1, to 0 to 1, uses the Rescale function (under Raster, Radiometric).
Clipping the top and bottom 0.5% is a percentage linear contrast stretch. To do this in ERDAS 2013, click on Panchromatic, then General Contrast. This brings up the Contrast Adjust window (seen below), and you can choose a variety of different methods of contrast adjustment.
You want to use Percentage LUT and set the % accordingly.
(I'm not sure of the specific steps in other versions unfortunately.)
Best Answer
the original (GAO 1996) NDWI is based reflectance for the wavelength of 0.86 (NIR) and 1.24 µm (SWIR). This corresponds to bands 2 and 5 if you use MODIS. The equation for MODIS is thus (b2-b5)/(b2+b5).
There are derived equation with other SWIR bands (e.g. for Landsat OLI the SWIR is around 1.6 µm), as well as NDWI2 (Mc Feeters 1996) which uses green (band 4 of MODIS) and NIR if you don't have SWIR bands.
MOD09GQ only provides bands for the red and NIR (the one you need for NDVI), but not the SWIR that is necessary for NDWI. Therefore you should use another product that includes band 5, such as MOD09GA. Note that the resolution of band 5 is 500 m, and so is MOD09GA.