# SPOT 2-5 convert DN to TOA using QGIS

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.

• 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 donĀ“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

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