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Correct computation of NDVI


Does the correct computation of NDVI index require absolute reflectance or total amount of Irradiance (for which the sun-shine sensor is used )?

The document from Pix4D Camera Requirements For Precision Agriculture mentions that Total Irradiance is important for absolute reflectance and not very important for NDVI calculation.

The question here is can we compare two NDVI indices computed after radiometric calibration without using the Irradiance information?. Does the above statement form Pix4D document essentially mean that Irradiance is not required for NDVI?



There are several ways to calculate the absolute reflectance. Each way has a certain type of correction applied to the images: In Pix4D desktop there are three types of corrections: 

  1. Camera only: you would need either a radiometric calibration target or the sunshine sensor (for both the albedo values should be known). 
  2. Camera and Sun Irradiance: you would need the sunshine sensor and the measure of Irradiance
  3. Camera, Sun Irradiance and Sun Angle: sunshine sensor with Irradiance and Sun Angle. 

Hence, the irradiance is not necessary to compute NDVI maps with absolute reflectance, but it can give additional accuracy. 



Why would we need sunshine sensor for Camera Only  calibration? I thought we only require it when we want to add the effect of sun irradiance. 

@Muzq: You are right, the sunshine sensor is not needed for Camera Only , this was a mistake from our side when posting.  

We would like to add some more information on NDVI maps: 
If an NDVI map is created for a one time analysis of a field. The reflectance maps can be on a relative scale to create an NDVI map, due to the division in the Index (NIR-RED)/(NIR+RED), i.e. no need to have absolute reflectance in that case.

However, to compare several NDVI maps of the same area, there is the need for either a radiometric calibration target or a sunshine sensor that measures Irradiance values. The need comes from the fact that the Irradiance profile can change for each band depending on the day, this can be captured by the radiometric calibration target or the sunshine sensor. 

We recommend to use a radiometric calibration target for all projects. This enables you to compare several NDVI maps from the same camera, and compare NDVI maps taken with different cameras as well, because the radiometric calibration target allows one to obtain absolute reflectance values.


How would the absolute reflectance values change if a reflectance panel image from a different day was used instead of the option of no calibration at all?

Hey Richard,

It is not ideal to use a reflectance panel image from a different day however if the weather conditions were similar, it may still work. 

@Richard, Selim

The calibration target has to be from the same day of the flight, moreover before or after the flight otherwise the results are biased. Please note that in the target are stored the date of the flight and the hour which we do use for the sun angle corrections based on the sun position on that specific day at that particular hour.



For example, say my agricultural field has multiple flights to complete (area >10 ha). For each flight, I will take capture an image of the reflectance panel before the UAV starts its mission. What is the best option for choosing which panel images for calibration if my field takes 3 flights to complete?

Hi Richard,

Depending on the weather conditions, If the flights are taken at a considerable amount of time apart then, to ensure the best quality of the results I would recommend processing them individually each with each own target.

Otherwise, if the flight condition did not change from one flight to the other (sky coverage, illumination, near sunset time…) then you could use on a representative flight for all of them.

However processing them individually would lower the amount of time for processing as well as data manipulation.