I have been doing NDVI (Landsat 4-5) analysis for my research area for some time now. I recently decided to try out EVI (MODIS). But the EVI values I get are a bit different from NDVI. For example maximum NDVI values reach upto 0.9 but maximum EVI values only reach 0.6. I read that EVI does not get saturated in dense biomass regions like NDVI does so maybe that is a reason for lower maximum EVI values? In any case, is there some conversion factor or some other way to compare the two values for the same region? For example if I say NDVI in a park changed from 0.3 to 0.6 in 5 years, the changes in EVI for the same park ofcourse wont be within the same range. This is a great confusion for the people who await my results as they only understand NDVI values and would think NDVI of 0.6 is equal to EVI of 0.6.
Well this is not exactly a GIS (definitely not arcgis) question, but remote sensing is somewhat related. Anyway, EVI (enhanced vegetation index) is similar to NDVI and to SAVI (Soil-adjusted vegetation index). To wit it corrects for atmospheric and soil distortions. As stated in the NASA website "While the EVI is calculated similarly to NDVI, it corrects for some distortions in the reflected light caused by the particles in the air as well as the ground cover below the vegetation".
Conversion is not as straight forwards as can seen by the formula of EVI provided by Wikipedia. I do not recommend conversion or comparison at all, since EVI and NDVI are two different indices. It would be more appropriate to use EVI, in particular in areas rich in Biomass, or in areas with scarce vegetation where soil might influence your results. Additionally, cross satellite comparisons would be wrong due to issues of spatial resolution (250 m in MODIS and 60 m in LANDSAT 4-5) and might be due to other wavelength issues of specific sensors.
If you want to compare NDVI and EVI anyway, I would at least recommend to calculate EVI and NDVI (with atmospheric correction) using data from only one satellite.