Hallo,
I have stitched my Sequoia data with Pix4Dmapper. I am using the values in the generated Red and NIR orthomosaics tiff files to calculate NDVI values. However, when I compared the NDVI values which I get with that of the pixel in the same position of the exported NDVI geoTiff I do not get the same values.
For example
NIR RED Calculated_NDVI Exported_NDVI
39528 11872 0.538054475 0.8469724
12475 9586 0.130955079 0.61869067
31652 30197 0.023525037 0.4404439
Can you please advise?
Marike
Hi Marike,
The orthomosaic maps do not have radiometric correction (they are just irradiance and not corrected for the sunlight, scattering etc) The reflectance maps are generated for NDVI or any other index calculation. If you use the reflectance maps of NIR and red for creating the NDVI, you will get the same results as Pix4D.
Thanks Momtanu
If the reflectance maps have radiometric correction, why would I see “ghost” images of my GCP checked boards?
The reflectance maps are radiometrically corrected. The reflectance maps are generated for each band for a multispectral camera.
Some useful links: https://support.pix4d.com/hc/en-us/articles/202739409-Reflectance-map-vs-orthomosaic
https://support.pix4d.com/hc/en-us/articles/115002441843-Inputs-and-outputs (output folder specified here)
I have the same issue.
I exported each band separately from step 3, so I am not using the reflectance band but the index GREEN NIR RED RED-EDGE and also NDVI. When I compare the results they do not match. Why is it so?
I need to calculate many more index in my excel. Thanks in advance.
@jzanelli which camera are you using? Are you calculating the NDVI from the index maps of each band in excel? To confirm, the values in excel do not match the NDVI map generated by Pix4D.
I am using a SEQUOIA. I am exporting each band + NDVI index. Then in excel, I am checking if calculations with the bands match NDVI of pix4d, and they do not.
Hi @jzanelli, Can you please upload the quality report, the index map of each band and also the NDVI map for me? It would be good to have a look, you can upload them in google drive and share the link with me. If it is confidential, you can write us a personal support request, with the link to the files.
Ok, I will do that. I uploaded in the folder 84546 under NDVI calculation error title. Thanks
Hi @jzanelli, I could not reproduce your issue. I checked in Matlab, QGIS and excel, however, the NDVI values from Pix4D matched with the NDVI I generated with NIR and red tif files. Could you let me know how you are converting the tif to csv? If the conversion is not correct, it will mess up the values of the pixels