I have created a NDVI image from Landsat 7 data using ERDAS (veg_NDVI.gmd). This model was already in ERDAS 9.3. After creation the final output visually highlighting vegetative areas but the value showing 0-255. But theoretically it is known that the NDVI value ranges from (-1) to (+1). Please let me be clear about this confusion and suggest the right way.
Erdas provides an option to stretch the NDVI values from the typical floating point NDVI range (-1 to 1) to 8-bit unsigned integer (0 - 255). This can be handy to reduce file size from floating point to 8-bit unsigned integer (0 - 255) and stretch the values to the full pixel depth of the image. However, it also reduces the sensitivity of the index. You can create NDVI doing the following:
Interpreter > Spectral Enhancement > Indices. In this dialogue box, make sure to not select "Stretch to Unsigned 8-bit". There is also a you tube video that can walk you through the process.