Index Values, what do they mean?

PIX4Dmapper PIX4Dmapper Questions/Troubleshooting
1

I have generated orthomosaics of multispectral and rgb images and have generated reflectance and index mosaics for the two bands I would like to use (NIR and Blue). The index values for each pixel in the NIR index orthomosaic range from 0-1 (seemingly normalized). The index values for each pixel in the Blue index orthomosaic range from 14-255 (seemingly not normalized). 

What is happening behind the scenes that causes this difference? How can I correct one of the bands in order to have the pixel values represent the same thing? 

Can I use the method described here https://eclass.teicrete.gr/modules/document/file.php/TP283/Lab/03.%20Lab/lesson3Notes.pdf to change the pixel values for one of the two indices? 

Any additional information you can provide will be helpful. 

Thank you!

2

Hi Christa,

For he orthomosaic generation, the colour blending is applied while for the reflectance map not. More than that if you apply radiometric corrections to your dataset the orthomosaic will be generated from the images that you used as input whereas the reflectance map from the radiometrically corrected version of the images. That is why the reflectance map has values between 0-1. 

That being said to have the true value of the pixel you will have to look at the reflectance map and not the orthomosaic. 

You can also find the following document interesting

https://support.pix4d.com/hc/en-us/articles/204894705-Camera-Requirements-For-Precision-Agriculture

https://support.pix4d.com/hc/en-us/articles/202739409-Difference-between-the-Reflectance-Map-and-the-Orthomosaic#gsc.tab=0  

Cheers,

Ina

3

Hi Ina, 

Thanks for the input, but I’m still a little confused. 

If I multiply the multispectral photo pixel values by 255, will I obtain values equivalent to the pixel values from the RGB images? That is, equivalent in what each pixel represents, not equivalent numbers. 

 

4

Hi Christa,

 

The Reflectance Maps contain the reflectance values of each pixel and not the color of it. The reflectance and the coloring of the orthomosaic are different in algorithms and not related. If you want an RGB representation of your multispectral images you will have to generate the reflectance map individually for each band and then composite them using a third party software as ArcGIS or QGIS.

You might find the following video useful. Keep in mind that is a third party software, so we have not tested it yet:
https://www.youtube.com/watch?v=YZulyAk6yRU

Cheers,

Ina