ACOLITE forum

Hello Quinten and to all members of this forum,

What can one look for to know if ACOLITE has properly applied atmospheric correction on a specific scene?

Just asking since I believe it would be helpful to know some tips regarding the said matter for someone that isn’t well informed with the concept of atmospheric correction such as myself.

Thanks in advance!

Vince

Hi Vince

My apologies for the very late reply! Usually we evaluate the reflectance after atmospheric correction through matchup analysis with in situ measurements. When you do not have in situ measurements, you'll have to judge the shape and magnitude of the reflectance spectrum in another way. This can be done either through modelling (e.g. Hydrolight) or just by expert judgement, and familiarity with water spectra, in particular for your region of interest.

One giveaway that something went wrong in the atmospheric correction is the retrieval of negative reflectances. These are masked by default by most processors. Another good place to look for cues is the NIR and SWIR end of the spectrum, that is if the sensor you are using has SWIR. Due to the high absorption by water the reflectance should be zero (SWIR) or very well behaved (NIR, see e.g. Ruddick et al. 2006). If you see a strong signal in the SWIR this is usually caused by clouds, glint, or adjacency effects.

The ACOLITE L2R files can be opened in SNAP to explore the spectrum for each pixel. You'll see rhot_* and rhos_* data, which represent the top-of-atmosphere and surface level reflectances.

I hope this helps!

Quinten