On 13/02/2020 00:15, Martin Koppenhoefer wrote:
Il giorno 13 feb 2020, alle ore 00:05, Colin Smale <colin.sm...@xs4all.nl> ha scritto:Locations are stored in OSM as pairs of {lat,lon} and I assume these are both 64-bit floats in the database.AFAIK they are stored as integers (shifting the decimals)
They are stored with 7 places of decimals (i.e. multiple by 1e7). https://wiki.openstreetmap.org/wiki/Node
Do not use IEEE 32-bit floating point data type since it is limited to about 5 decimal places for the highest longitude.
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A 32-bit method used by the Rails port is to use an integer (by multiplying each coordinate in degrees by 1E7 and rounding it: this allows to cover all absolute signed coordinates in ±214.7483647 degrees, or a maximum difference of 429.4967295 degrees, a bit more than what is needed). For computing projections, IEEE 64 bit floating points are needed for intermediate results.
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The 7 rounded decimal places for coordinates in degrees define the worst error of longitude to a maximum of ±5.56595 millimeters on the Earth equator, i.e. it allows building maps with centimetric precision. With only 5 decimal places, the precision of map data would be only metric, causing severe changes of shapes for important objects like buildings, or many zigzags or angular artefacts on roads.
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