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Can't reproduce NDVI values

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/202558349-Menu-View-Index-Calculator-Sidebar-1-Reflectance-Map

https://support.pix4d.com/hc/en-us/articles/115002441843-Inputs-and-outputs (output folder specified here)