Subject
- Railroads17
- Utilities and Communication17
- Altitudes16
- Cities and towns16
- Roads16
- Biology and Ecology15
- States14
- Air quality—Standards13
- Community development13
- Rivers13
- Hydrography11
- Oceans11
- Planning and Cadastral11
- Counties10
- Geomagnetism10
- Mines and mineral resources10
- Provinces10
- Employment9
- Historical geography9
- Income9