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I'm using GrassGis 7.0.0 for Landsat imagery. In the description of i.Landsat.toar under dos3 and dos4 is:

TAUz = exp[-t/sin(e)]

What is "t"? I cannot find any description about what it is. Can anyone help?

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The GRASS GIS Landsat.toar module relies heavily on the article by Chavez from 1996 and you ought to read that for a full understanding on what happens.

The implementation in GRASS GIS doesn't directly match the one seen in the paper, but I expect that this is a result of an aversion to the use of the trig-function 'sec' in GRASS GIS.

'TAUz' is the atmospheric transmittance along the path from the sun to the ground surface.

't' is referred to as 'del' in the original paper - where it is described as 'the optical thickness values measured in-situ at the given wavelength'

I would expect that GRASS GIS takes look-up-table values for this, based on standard atmospheres, selected based on location and time of year, but I will have to look into this to be certain.

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