I'm using MOD13Q1 product of MODIS for estimation soil moisture. As I know, this product is made up of the maximum value of NDVI of each pixel over 16-day calculated NDVI values. The problem is that my soil moisture in situ measurements don't correspond to the NDVI values. For example, where NDVI is 0.75, the calculated soil moisture is 0.2; in another case, where NDVI is 0.4, the soil moisture is 0.5, which is the highest value of soil moisture calculated. Of course, this problem doesn't occur for all the pixels.
For working with MODIS product, I use ENVI software. I first use MODIS Conversion Toolkit(MCTK) to transform its sinusoidal coordinate system into Geographic Lat/Long system. Then, I examine the NDVI values based on the location of my soil moisture ground measurements.
I know that my ground measurements are point-wise and the NDVI product covers around 250m * 250m on the ground, but soil water content over this area doesn't differ significantly. What's the problem in this case? where I'm making a mistake?