Quantization of lon/lat coordinates

[gmaclennan]

We should quantize the coordinates to a fixed precision to avoid database bloat and potential floating point rounding errors at the spatial index. Decimal degrees precision:

decimal	degree	@ 0° Lat	@ 30° Lat	@ 60° Lat
2	0.01°	~ 1 km	~ 0.9 km	~ 0.5 km
3	0.001°	~ 100 m	~ 90 m	~ 50 m
4	0.0001°	~ 10 m	~ 9 m	~ 5 m
5	0.00001°	~ 1 m	~ 0.9 m	~ 0.5 m
6	0.000001°	~ 0.1 m	~ 0.09 m	~ 0.05 m
7	0.0000001°	~ 0.01 m	~ 0.009 m	~ 0.005 m
8	0.00000001°	~ 0.001 m	~ 0.0009 m	~ 0.0005 m

8 decimal places would get us 1mm precision, I think that's more than enough. The GeoJSON Spec recommends 6 decimal places for <10cm precision. There may be use-cases for greater precision, e.g. drawing precise building footprints. I think 8 decimal places (1mm precision) would be more than enough. Perhaps configurable at a DB level? Would it need to be kept consistent across the whole db?

[hackergrrl]

I think as long as all levels of quantization fit into the chosen datatype in kdb-tree-store it will be fine.