I’ve used custom indexes in Pix4D Fields and it works in the orthomosaic, for example when using EVI and TCARI/OSAVI index the image looks the same, the trouble comes with the histogram, according to the values practically are all 0 to 0 and I don’t know why. The formulas that I researched for are:
EVI:2.5*((nir - red )/(nir + 6* red - 7.5* blue + 1))
TCARI/OSAVI:(3* (rededge- red )- 0.2* (rededge - green )* (rededge / red ))/(1+0.16)* ((nir - red )/ (nir + red+ 0.16))
According to the index calculator, there are no syntax mistakes, so I want to know the reason of the histogram values.
Another curious fact is that the bands used with EVI and TCARI/OSAVI or other custom index is GREY.
I´ll appreciate your comments.
Can you send us some screenshots of the maps in Pix4DFields and some more information regarding which camera you are using?
Could you also give a screenshot for “Another curious fact is that the bands used with EVI and TCARI/OSAVI or other custom index is GREY.” so that we understand better?
I use the camera from a DJI P4M also linked with an RTK base station.
I just read that Pix4D now has support for this type of camera in order to develop radiometric corrections.
So it’s curious to me the histogram values practically going 0 to 0 and the section that claims the band used in the generated index is “grey”. This is something I need to worry about?
Hi, For EVI, SAVI, OSAVI, the reflectance values of each band must range within 0 to 1 and that is possible only when you use radiometric targets. If you are not using radiometric targets, I would recommend calculating only the indices that do not have this criteria.
Sample documentation found while browsing: https://www.microimages.com/documentation/TechGuides/81VegIndices.pdf
I did not know this thing about the radiometric target, that’s sad because DJI doesn’t offer a reflectance target only the corrections with the sunshine sensor of the P4M.
Being said this, I would like to read any suggestions of yourself about some reliable index I can generate or apply using the P4M camera, obvious this includes the index database by default in Pix4D fields, but, I normally have been used this collection of indexes (image) in the past working over maize, medicago sativa, sorghum, and pecan trees.
What do you suggest?
I appreciate your time and attention Momtanu, thanks a lot.
Greetings from Mexico
Hi Davila, Greetings from San Francisco
You can use CCCI and NDRE for sure. I do not think you can use anything that has the constant value (L, for example 0.16, 2.5 etc). Some clients use other targets (for example, if you have the micasense targets) with DJI p4m to get absolute reflectance values and they range from 0-1.