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?


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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