Ground Vegetation Index

[Cristina Isabel Seabra [Ambiente]]

Hello GRASS users!

I've been trying to classify a LANDSAT TM image in terms of land use. 
I think it would be interesting to have a Ground Vegetation Index map and combine it with
2 other bands. I've used r.mapcalc to define the GVI. What I did was:
GVI = (band4 - band3) / (band4 + band3) but the result I get is a map with only 2
categories (-1 and 1). Shouldn't this GVI layer created by r.mapcalc have more
information or am I doing something wrong when I write the expression in r.mapcalc?
Is there someone that can help me in this matter?

Thanks,

Cristina Seabra
New University of Lisbon
Lisbon, Portugal


cis at fct.unl.pt