On Tue, Jul 09, 2019 at 02:44:08PM +0200, Martin Trautmann wrote:
On 19-07-04 20:14, Greg Troxel wrote:
osmand live provides files that contain differences from published
map files. So with a fairly recent map (and it seems to not have to
be the most recent one), and the delta updates, you have up to date
data.
If this mechanism would be that easy, why would it be so time
consuming to create new map updates for every month?
Likely because the base monthly maps are not generated from the delta
difference files, but by reprocessing the entire OSM dataset anew each
time (the planet osm datafile, when uncompressed, is about 1 terabyte).
And, anticipating a "why" question, one would process the data this way
to prevent a common problem when handling long chains of delta data.
If any piece of delta data in the middle of the chain is missed
everything after the missed data that depends from the missed data
turns into garbage.
So one would regenerate anew for some cycle (in OsmAnd's case, that is
monthly) then build differencing files for a month. So any errors in
the delta chain propagate for no more than one month before they get
cleaned up (by the "start anew" montly cycle processing).
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