How Reliable Are Geospatial Foundation Model Embeddings for Cross-City Urban Mapping?

Abstract

Frozen geospatial foundation model (GeoFM) embeddings are increasingly used as general-purpose features, yet their reliability under cross-city transfer remains unclear. We evaluate AlphaEarth and TESSERA for building-function and local-climate-zone mapping across 31 cities using leave-one-city-out (LOCO) transfer, comparing them with handcrafted Harmonized Landsat–Sentinel-2 (HLS) and Sentinel-1 features and annual band medians. When all representations are summarized over the same 90-m spatial support, both GeoFMs are competitive with the handcrafted baseline, although their average gains are modest: across both tasks and all three classifiers, the mean city-paired differences in macro-F1 are +0.011 for AlphaEarth and +0.012 for TESSERA, with both 95% confidence intervals spanning zero. Relative GeoFM performance also varies with label availability and spatial-support setting. Despite these small average differences, performance varies substantially from city to city. For AlphaEarth, this variation shows a clear contrast between Global North and Global South cities across both tasks; in building-function mapping, the association persists after adjustment for measured differences in urban form and reference-data characteristics, whereas the corresponding pattern is weaker for TESSERA. These findings show that competitive average accuracy does not necessarily imply reliable transfer across cities and highlight the importance of evaluating GeoFM embeddings across diverse deployment geographies.