Knowledge of the ground state of lanthanides (Ln) in Ln-based materials is fundamental for understanding their properties, particularly those related to magnetism. In crystalline materials, the magnitude and orientation of 4f magnetic moments are often strongly influenced by the crystal electric field (CEF), making them challenging to predict ab initio. This complexity is intensified in low-dimensional systems and nanostructures, where the CEF may vary significantly near surfaces and at interfaces. The related variations of the 4f ground state should be probed by methods with adjustable surface sensitivity. Here, we apply x-ray absorption spectroscopy (XAS) at the Ln 4d edge, along with modeling of XAS spectra, to probe the 4f ground state of Ln atoms both at the surface and in the bulk of LnRh2Si2 layered crystals. Our results explicitly reveal significant changes of the 4f ground state, which may lead to the reorientation of 4f moments at the surface compared to the bulk, driven by alterations of the CEF. We also provide a comprehensive database of calculated Ln 4d XAS spectra in numeric format for all lanthanides. Our findings will facilitate studies of Ln-based materials and molecular complexes by enabling comprehensive analysis of XAS data, including those obtained through magnetic linear and circular dichroism techniques, as well as related spectromicroscopy studies.