With the arrival of large galaxy surveys such as KiDS, DES or HSC and their ability to observe millions of galaxies, statistical errors have shrunk, making cosmology a precision science. Now, systematic effects are becoming the main source of uncertainty, so dealing with them will be one of the main challenges that next-generation surveys, such as Euclid, DESI or LSST. In this presentation, we will focus on two sources of systematic uncertainty: observational systematics and atmospheric conditions. For the former, we will explore the methods and lessons learnt from the DES-Y3 galaxy clustering analysis, the ongoing work for Y6 and the prospects for LSST-DESC. For the latter, we will introduce LSST’s Auxiliary Telescope (AuxTel), whose purpose is to measure atmospheric transparency and to derive color corrections based on spectroscopic observations.