发表机构
University of Glasgow（格拉斯哥大学）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究审计卫星基础模型AlphaEarth对全球城市嵌入的表示，发现其支持跨区域比较但压缩城市内部变化，且年度差异未验证用于时间比较。
AI 中文摘要
城市在建成形态、土地覆盖和发展历史上存在差异，这使得跨地点和跨时间的比较变得复杂。卫星基础模型将地球表面映射为共同的数值表示。然而，用于塑造这些模型的任务和目标通常不聚焦于城市：全球一致的城市功能标签并不存在，许多数据集（尤其是土地覆盖和土地利用分类）将建成环境压缩为少数几个类别。在此，我们审计了这种表示，重点关注AlphaEarth，但其结论对其他地球嵌入也具有更广泛的适用性，通过探测162个国家中1000个城市区域的嵌入的几何和地理分布。我们发现，城市占据超球面上一个偏移但重叠的区域，距全局平均方向62.7°，且大陆和气候可预测排除国家中城市中心平均方向之间24.3%的变化。在城市内部，城市化程度承载了8.9%的变化，而它们所遗漏的部分具有共享方向，其局部方向各异，而非单一的城市化普遍轴。保留的变化本身是不平等的：城市中心内的离散度每增加一个标准差的国家发展水平就增加14.1%，即使在调整人口、土地面积和大陆之后也是如此。进一步的控制表明，发展中国家的城市在植被和纹理方面对比度较低，而离散度随这种对比度变化：完全调整后，梯度最多剩余6.4%。每年，一个城市表示的变化幅度几乎是重新绘制其像素所解释的近八倍，并在2022年Sentinel-1B丢失导致一个通过方向收缩的地方发生收缩。因此，AlphaEarth的表示支持跨区域的比较，而其年度层之间的差异尚未被验证可用于跨时间的比较。
英文摘要
Cities differ in built form, land cover and development history, complicating comparison across places and time. Satellite foundation models map Earth's surface onto common numerical representations. Yet the tasks and targets used to shape them typically do not focus on cities: globally consistent labels for urban function do not exist, and many datasets - especially land cover and land use classifications - collapse the built environment into few classes. Here we audit the representation, focusing on AlphaEarth but with broader applicability to other Earth embeddings, by probing the geometry and geography of embeddings for 1,000 urban areas in 162 countries. We find that cities occupy a shifted but overlapping region on the hypersphere, 62.7° from the global mean direction, and continent and climate predict 24.3% of variation among the mean directions of urban centres in excluded countries. Inside cities, degrees of urbanisation carry 8.9% of the variation, and what they leave holds shared directions whose local orientation varies, not one universal axis of urbanisation. Retained variation is itself unequal: dispersion within urban centres is 14.1% greater per standard deviation of national development, even after adjusting for population, land area and continent. Further controls suggest cities in developing countries present less contrast in vegetation and texture, and dispersion follows that contrast: full adjustment for it leaves at most 6.4% of the gradient. Annually, a city's representation moves nearly eight times more than redrawing its own pixels explains, and contracts where the 2022 loss of Sentinel-1B removed a pass direction. AlphaEarth's representations therefore support comparison across regions, while the differences between its annual layers are not yet validated for comparison over time.
