What a map vertex costs
ยท 7 min read
Here is a building in the centre of Warsaw the way our previous offline map format stored it, :
Grid step 73 cm. Corners move by up to 44 cm, 28 cm on average.
The dashed line is the outline as it was mapped in OpenStreetMap. Every vertex of a GLMap tile sits on a grid laid over the tile, and the tiles that serve street-level zooms cover about 3 km of Warsaw each. With 12 bits per axis the grid step there is 73 cm, and the orange lines show how far each corner moved to reach it. At zoom 13 nobody sees that. At zoom 17, where these buildings fill the screen, the corners visibly go astray. the step is 18 cm.
gom5, our new map format, moved to 14 bits. Those two extra bits per axis cost bytes in the file, bytes in the download and time in decoding, and we did not see all of it at once.