Good morning everyone
We have recently purchased a NGB converted camera (NIR, G and B channels) with the goal of observing different vegetation indices (DVI, NDVI, SAVI, etc) from UAV-captured images. We were hoping some of you may have some tips on calibrating the camera settings to optimise these images (NDVI in particular).
We have found that the NDVI (and DVI, as expected) images vary immensely in different light conditions and for different camera settings. This is far from the ideal normalised index, where the range of values should be environment independent. To put it simply, based on the literature we would expect the NDVI values to fall between 0.2 and 0.8 for vegetation, where the range represents stressed to healthy vegetation. However we are finding that the range of our values vary with different light conditions. While we could obviously manipulate a single image (e.g. it's colormap) to display a nice looking NDVI plot, the day-to-day variance does not allow for comparisons over time.
Below are two NDVI images of the same scene, one day apart with the same colormap, which illustrates the paragraph above.
We currently use a blue card, in the same light conditions as the image, for the white-balance, a shutter speed of 1/1000 and an ISO of 400.
We have analysed the individual NGB channels and noticed a few surprising results. While the NIR and G channels show expected values, the blue channel is producing quite different results to that in a RGB camera. The vegetation is showing up particularly bright in the NGB blue channel, opposed to the respective RGB channel. This would explain why there are problems with the NDVI values as we are using the blue channel to calculate the index. Perhaps calibrating the white balance differently would overcome this variance?