Author Topic: Reflectance calibration and orthomosaic export with digital numbers  (Read 68 times)


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I saw two posts that touched on this subject, but I did not see a resolution.
When processing Micasense RedEdge (3 or Dual) 16-bit images in Agisoft Pro, the Calibrate Reflectance tool is implemented and the program successfully finds the images of the calibration panel and the corresponding calibration values are displayed for each band. I check both use reflectance panels and use sun sensor options.
The rest of the workflow is normal, with DTM, DSM and orthomosaic being produced.
When the orthomosaic is exported as a geoTIF and imported into QGIS, the histogram produced the pixel data is clearly still in digital numbers (DN), ranging from 0 to 65,535.
I have seen at least two posts where it is suggested that the Raster Calculator in Agisoft be used to divide each band by 32768. For instance,
"Metashape Professional is performing the reflectance calibration operation according to MicaSense recommendations. So the values in the output bands would still be 16 bit integer values like the input values, but 100% reflectance for each band would correspond to the middle of the available range, i.e. to 32768 value. In case it is necessary to export the reflectance normalized to 0 - 1 range, then it is required to create Output bands in the Raster Calculator dialog and for each one of them input the formulas that divides the source value by the normalization factor: B1/32768; B2/32768; B3/32768; B4/32768; B5/32768"
Since in this Micasense post on processing raw images ( the raw pixel values (and the black level values) are supposed to by normalized (divided by 65536 for 16-bit images) prior to calculating adjusted reflectance, using the radiometric calibration coefficients, the gain and exposure time, and the vignetting model.
If the output from Agisoft is still in DN, it is not evident that the radiometric calibration has been fully carried out as proposed by Micasense.
Also, the exposure time and gain (or ISO speed) shown in the image information are different from the values in the metadata extracted from the images using exiftool.
Can you please provide some clarity on this?