I want to calculate NDVI for 50 Landsat 7-TM images . I have done a stack layer for each band of 50 images using envi software and then calculate NDVI for the stack of two bands demanded in NDVI equation. Is this correct? Or is there an other method?
You need NIR and Red band for the calculation of NDVI. These bands corresponds to the 3rd and 4th bands of Landsat 7 Images. You can use Spectral Indices tool (Spectral > Vegetation > NDVI) for the calculation. But you need IDL for batch processing. If you want to do it as a batch process and don't know IDL you can use another software like QGIS (open source) or ArcGIS. Another option is downloading Landsat NDVI Images using USGS ESPA Services.