I've used a Landsat 8 image for vegetation analysis in an urban area. I've got a large maximum number of EVI value as shown in this image:
Is this normal in calculating EVI and NDVI? Because I know that value range in NDVI and EVI are between -1 and 1, although only 1 DN has max value.
And 2nd question is, what are the best vegetation indices for analyzing in urban areas with Landsat 8 images? I use ENVI apps to calculate band math.
Best Answer
You are right: the range of the NDVI is limited to values between -1 and 1 due to its' normalization properties. The negative limit of -1 will be reached if you encounter maximal reflectance (1) in the red wavelength region and zero reflectance in the NIR. The positive limit will be reached by maximal reflectance in the NIR region and zero reflectance in the read wavelength region.
The EVI:
uses coefficients to correct for canopy background (
) and aerosol influences (
, 
) (see e.g. Huete et al. (2002) for details.
Especially for artificial surfaces you can encounter values out of the range between -1 and 1. For example, blue rooftops, which show high reflectance (e.g. above 0.6) in the blue wavelength region will generate EVI values outside the "typical" range. This is caused by the use of the blue band to correct for aerosol influences in the red band.
Consequently, you can encounter values greater 1 or below -1 for the EVI but for the NDVI the values will always range between -1 and 1. Regarding your second question: you should try different vegetation indices and analysis techniques according to your problem at hand. If you want to estimate vegetation abundance in urban areas you could for example use spectral mixture analysis.