Atmospheric correction
Before you can compare a Wyvern scene with another date, another sensor, or a library spectrum, the atmosphere has to be taken out of it. This page explains the difference between L1B top-of-atmosphere radiance and L2A bottom-of-atmosphere reflectance, which one you're likely to have, and what to do if you're starting from L1B.
L1B top-of-atmosphere (TOA) radiance is the light signal the sensor measures, including the effects of the atmosphere such as scattering and absorption by air, aerosols, and cloud. L2A bottom-of-atmosphere (BOA) reflectance is a corrected product that estimates how much light the surface itself reflected, as though the atmosphere were not there. L2A is what you want for studying vegetation, water, and soil, because it's comparable between scenes, dates, and sensors.
Which product you get
Wyvern delivers both L1B and L2A. The Open Data Program publishes L2A surface reflectance unless a scene is explicitly noted otherwise, and also carries a small L1B collection for anyone who wants to work from radiance. Check the processing:level property on the STAC item, or look for the _l1b or _l2a suffix on the item ID.
The two products differ in more than the atmosphere. See processing levels for the full comparison of data type, scale factor, NoData value, and coordinate reference system.
Use L2A for absorption-feature work such as spectral matching, continuum removal, and narrowband indices. In L1B radiance, the shape of the solar spectrum and atmospheric features, including the oxygen absorption near 760 nm, are larger than the surface features you are trying to measure, so those features are not recoverable without correcting the scene first.
Correcting L1B yourself
If you're working from L1B, convert radiance to surface reflectance before any quantitative analysis. The first step is converting at-sensor radiance to TOA reflectance, which accounts for the solar spectrum and the sun angle. Wyvern publishes code for this in top-of-atmosphere-processing. Getting from there to surface reflectance means modelling what the atmosphere did to the light, which is what the tools below do.
These tools read and correct Wyvern data directly:
| Name | Type | Details |
|---|---|---|
| ATCOR | Paid | Established atmospheric correction tool. |
| ENVI QUAC | Paid | Built into ENVI. A scene-based approach that needs no atmospheric input parameters. |
| ACOLITE | Free | Open source, developed for coastal and inland water applications. |
Other tools work with Wyvern data but need manual setup to get the inputs and outputs right. These include Py6S, SICOR, SIAC, SL2P-CCRS, and libRadtran.
More resources
NASA's Earthdata has a clear reference on data processing levels.