Red Band Problem


#1

Hello, My name is Emre from Turkey. I use Sequoia multispectral camera. I calculate reflectance value by this means reflectance= radiance(dn)/irradiance(dn). For irradiance dn, I take a photo of spectralon reflectance panel using sequoia. Then I use envi to averaged of reflectance panel values in photo. This average value is irradiance for me. But problem is occured in this step. Because in Red band, reflectance values of the some area is higher than 1.0. As a consequence of this NDVI value is negative in this area. Because red value is higher than Nir value. İt is not possible to red value is higher than nir in soil area. I take a photo from s1000+ about 50.0 m above ground level and cloudless condition and when sun angle is 45 oC.
Also, I try pix4d to convert raw image to reflectance image. Same problem was occured.
How can I solve this problem.


#2

Following ! I have several doubt about datas acquired by sequoia. I have datasets of satellite at the same day and same hour of day, I will compare them but at at this time I have a lot of difference between 2 datasets.

Did you apply vignetting correction ?


#3

No I didn’t, I only try to pixel value to irradiance, but result is same. In soil area, red values higher than nir value. How can I calibrate it ?


#4

Here you have : Vignetting Correction Sample Code

Python script proposed by Sean based on GDAL and parrot vignetting correction.


#5

Hi @tuncaemre ,

Please note that you cannot simply take the pixel value as radiance.
Sequoia only produces raw data accompanied by the required metadata to perform the needed corrections.
It never performs any corrections by itself.

Therefore, as @kikislater has pointed out, corrections for things like vignetting need to be accounted for.

Please refer to this thread and ask there if you still have questions regarding estimation of reflectance using Sequoia (for any of its monochrome bands):

Best regards,