[Tagging] generalized survey and consequences

[SomeoneElse]

Martin Koppenhoefer wrote:
>
>
> Actually the "source:maxspeed" tag was discussed years ago (2009 IIRR) 
> on this list and by the time approved. The idea to use maxspeed:type 
> instead is very new compared to this, and there wasn't any actual 
> proposal to see whether this was backed by the community, rather then 
> what appears to be a national initiative of some folk in the british 
> comunity in order to make up something different to how it is done 
> elsewhere:
> http://taginfo.openstreetmap.org/keys/maxspeed%3Atype#map
> http://taginfo.openstreetmap.org/keys/source:maxspeed#map

... another interpretation as to what actually happened is that 
"source:blah" was in widespread use since the beginning of time for "the 
source of the blah key" (survey, local_knowledge, whatever). Then at 
some point people decided to use "source:maxspeed" to refer to the zone 
(e.g. if a road's in an urban area, and urban areas in your country have 
a 30 km/h speed limit regardless of signage, then it's OK to say that 
the "source of the maxspeed" is something like "DE:zone:30" or similar).

In the UK "maxspeed:type" is usually used with "national" speed limits, 
not zone-based ones.  The source of the maxspeed is therefore "survey", 
"local_knowledge" or similar, and "maxspeed:type" is used to indicate 
that it is a "national" maximum speed limit, not a numeric one.  It's 
important to record what the sign actually says, not just the number 
that that sign happens to currently represent - historically the 
national speed limit for different classes of road for cars has changed, 
and it may again in the future.

There was (a couple of years ago) a significant usage of 
"maxspeed=national" or similar in the UK.  this caused a certain amount 
of "toys being thrown out of prams" from people (mostly from countries 
where the posted limit is always numeric) who expected the "maxspeed" 
value to always be a number.  The compromise was to store store the 
actual posted sign value in maxspeed:type, and what number that happens 
to currently correspond to in "maxspeed" (and the actual source, if it 
needs to be stored, in maxspeed:source for compatibility with other 
source keys).

Cheers,

Andy