What software I can use to plot the spectral signature of a Landsat 8 or Sentinel 2 pixel to compare it with the spectral signature of a field spectrometer (HR2000+)?
How can I do this?
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If you have access to ENVI, which has been a widely used software for remote sensing, before the days of Google Earth Engine, here's a link to a set of instructions to view the same.
In ArcMap, I recommend using Zonal Statistics as Table tool to obtain the spectral values for all bands.
First thing is that you probably want to convert the top-of-atmosphere L1C product to bottom-of-the-atmosphere to remove atmospheric annoyances. You can do this with sen2cor, or you may just want to use the combined 30m Landsat+S2 product from NASA.
Then you need to extract whatever pixel you want from the data (GDAL, QGIS and so on will all do that and are free software), which gives you the surface directional reflectance over the S2/L8 bands, which you can compare with your ground spectra, possibly after you have applied passband functions to the data (so you go from very fine spectral sampling to the broad-ish bands in the satellite sensors).