I have a 6 channel proprietary camera and I need to create a NDVI map. I created a map from each band, so now I have 6 files each one on a different band width.
How can I proceed now using QGIS?
To create a NDVI image you need to use the channels for red and near infrared (NIR) reflectance. The red channel is usually in a wavelength range between 0.6-0.7µm and the near infrared channel in a range between 0.7-1.10µm. So if one of your 6 files is in one of these widths you can use it for the calculation.
In QGIS there are two ways to calculate the NDVI:
--> 1. For the raster calculator way you need to know the NDVI equation: NDVI=(NIR-RED)/(NIR+RED). If you loaded your red and your NIR band file into QGIS they will appear in the 'raster band' content table in the raster calculator. When you chose one band file it will appear in the 'raster calculation expression' field. Use the 'operators' to express the equation. But be careful with the brackets! Following define the output file and go!
--> 2. Within the radiometric indices you don't need to write any equation but this tool can only work with layer stacked files. So you need to create a layer stack that contains all of your 6 bands first. You can do this with the 'merge' tool (raster-->miscellaneus-->merge). Take care that you enable 'layer stack'. The radiometric indices tool is part of the Orfeo Toolbox (OTB) that is part of the Processing framework. Hence OTB have to be installed and the Processing framework have to be configured properly (see --> http://docs.qgis.org/2.6/en/docs/user_manual/processing/3rdParty.html). When you open the Processing Toolbox go to OTB-->Feature Extraction-->Radiometric Indices. The input image is your layer stacked image. Now you have to define the red and the NIR band regarding to your layer stack. Then just choose NDVI at 'available radiometric indices' and go! You also have the opportunity to calculate other vegetation indices - just have a look at the 'help' tab or at http://www.orfeo-toolbox.org/SoftwareGuide/SoftwareGuidech12.html#x38-2060011.