[Tagging] Handle with care

Greg Troxel gdt at ir.bbn.com
Fri Sep 11 18:02:35 UTC 2015


Colin Smale <colin.smale at xs4all.nl> writes:

> How does the chap with the GPS on his smartphone know that the old
> coordinate of 50.000 is "more correct" than his own measurement? 

This is generally a hard question, but if you're using a phone, and the
value you see is within 20m (or maybe 10m) of what's in the db, then you
shouldn't change it unless you've thought about why your new value is
better.  And arguably data with higher-than-normal accuracy deserves a
note.

Around me, town boundaries are defined by the actual positions of
granite markers.  Massaschusetts publishes a dataset with coordinates
determined from a surveying long ago, reckoned forward to a modern
coordinate system.  Those coordinates are in OSM.  I've visited a number
of these, and of course my handheld GPS receiver gets different numbers.
But they are all close, and I have no reason to believe my measurements
are better.  In fact, I think my measurements are worse.  So I'm
certainly not changing the values in the database.

So really, people just have to understand the expected errors of their
measurements.
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