I'm new to QGIS and I would like to convert SPOT(2-5) images from DN to TOA. How do I go about doing this? I've an idea of how to do it for Landsat images however I'm not sure about SPOT.

  • 1
    do you want to transform it to TOA radiance or TOA reflectance?? – Pau Aug 18 '14 at 12:57
  • TOA reflectance – Barry Aug 18 '14 at 13:13

There is two ways you can do this:

  1. Inside the toolbox:

    • Grass: i.atcorr: performs atmospheric correction (i don´t know if it works with spot)
    • Orfeo Toolbox: Optical calibration (works with spot 5)
  2. Inside toolbox --> band math

    • you can read the metadata, look for the coeficients for the following equations ( for example: <SUN_ELEVATION>, <PHYSICAL_BIAS>, <PHYSICAL_GAIN>)
    • you can find d in chander et al. 2009
    • you can find ESUN in SPOT ESUN
    • transform DN to TOA radiance (L=DN/Ak; A= bandgain, B=bandbias)
    • transform TOA radiance to TOA reflectance (p=[PI*d^2*L]/[ESUN*cos(theta)]; PI=3,14..., d= distance to solar to earth, L = TOA radiance, ESUN = Mean exoatmospheric solar irradiance, theta = Solar zenith angle (90-SUN_ELEVATION))

    • you must do this for every band

| improve this answer | |
  • The equation works however for Spot it should be (p=[PI*L]/[ESUNd^2*cos(theta)] – Barry Aug 30 '14 at 23:41

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