I have downloaded MODIS NDVI images of 12 months. I would like to create just 1 image with the mean values. I tried using IDRISI's 'NDVI compositing utility', but I get an error. Is there a way to do this in QGIS?
You can use the GRASS r.series command in QGIS to accomplish this. Make sure to select the "average" statistic.
r.series - Makes each output cell value a function of the values assigned to the corresponding cells in the input raster map layers.