OSM TAGS Surface
Can be seen in more detail here: http://wiki.openstreetmap.org/wiki/Key:surface
Possible values: (all in lower case)
- asphalt
- concrete
- concrete
- concrete
- paved
- cement
- compacted
- fine_gravel
- paving_stones
- metal
- bricks
- grass
- wood
- sett
- grass_paver
- gravel
- unpaved
- ground
- dirt
- pebblestone
- tartan
- cobblestone
- clay
- earth
- stone
- rocky
- sand
- mud
