How to reduce number of raster bands of a RGB picture?

Question

For my Master Thesis in Ecology I want to calculate an NDVI Index. The tricky part is, that I don't have sattelite pictures with bands and so on, but RGB pictures. The pictures where taken with a normal digital camera and the NIR pictures were taken by using a filter to block the light from 680 nm onwards. The information of the red channel were obtained by extracting this channel out of a normal RGB picture, taken with the same digital camera as the NIR pictures. I stored this extracted channel as a TIF-file.

Now I loaded these two pictures in QGIS, render type "platted". Starting the raster calculator per picture 3 bands appear.

How can I reduce these raster channels in raster calculator to only one raster channel per picture?

Could I combine the information of the 3 bands to one somehow?

Could a simple addition of all three bands per picture help?

Answer 1

The Green-Red Vegetation Index is a useful one for ecological purposes.

From the following publication:

Motohka, T., Nasahara, K. N., Oguma, H., & Tsuchida, S. (2010). Applicability of green-red vegetation index for remote sensing of vegetation phenology. Remote Sensing, 2(10), 2369-2387.

We chose GRVI because the response of GRVI to various ground covers may be simple to interpret. In terms of the balance between green reflectance (wavelengths of around 500–570 nm) and red reflectance (wavelengths of around 620–700 nm), we can find three groups of spectral reflectance patterns for major components of ground cover (Figure 1):

• Green vegetation (conifers, deciduous trees, and grass): ρgreen is higher than ρred.
• Soils (brown sand, silt, and dry clay): ρgreen is lower than ρred.
• Water/snow: ρgreen and ρred are mostly the same.

Vegetation has positive values, soil has negative values and water/snow has near zero.

Answer 2

If I get you right you want to merge - stack your bands related to each picture. You can use Raster -> Miscellaneous -> Merge menu for this. You need to check the layer stack in the opening window. For this steps and NDVI calculation I've used this tutorial: http://arset.gsfc.nasa.gov/sites/default/files/land/webinars/Advanced_NDVI/ARSET_Advanced_NDVI_Week2_Exercise_Final.pdf which will give you an idea of all the process with Landsat Images. You can apply the same for your picture bands while calculating NDVI.