[OSM-dev] Source tag on each node

[Alan Mintz]

At 2010-12-19 02:21, Wyo wrote:
>Alan Mintz wrote:
>>At 2010-12-19 00:59, Wyo wrote:
>>>I've just realised that rather many nodes have their own source tag, even
>>>if they are just part of a way with identical tag. So I guess a rather
>>>large part of the data consists of these source tags. While this isn't a
>>>problem for files (thanks to compression) it enlarges the database quite 
>>>a bit.
>>>
>>>Are these tags really necessary or could they be remove as long as the way
>>>has a source tag? Could anybody provide an estimate how much space
>>>reduction could be gained? Could anybody create a report on how many nodes
>>>have identical tags as their way?
>>
>>Do you have an example?
>See http://www.openstreetmap.org/edit?lat=47.17473&lon=4.71559&zoom=17 
>where the nodes of the canal way have identical values.

Oddly, only some of them. I agree that these seem duplicative (and I don't 
see the need for such a long value, where cadastre_2009 would convey the 
same info).


>>I tag intersections of ways as I survey them with source=*, source_ref=*
>>among other things, like signalling, turn restrictions, etc. Sometimes, the
>>source value is the same as that of the way, but that is only coincidental
>>- it's not a duplication.
>Sorry I don't understand that. Since nodes obviously inherit (by 
>understanding) anything from their ways, the values either must be 
>different to show different info or they don't carry any info.

I don't agree that they necessarily inherit from their ways. A node along a 
street is not named, not functionally classified (i.e. highway=primary), etc.

The situation I was referring to was this:

I tag a street: { source="survey;image;usgs_imagery" + 
source_ref="AM909_DSCN1234" + name="Foo Street" } after tracing it from 
satellite imagery, then surveying it and taking a pic of the street sign 
(id'd by the source_ref) somewhere along the way.

Somewhere along this same street may be a node tagged { 
highway="traffic_signals" + source="survey;image;yahoo_imagery" + 
source_ref="AM909_DSCN5678" } indicating that I surveyed it, took a pic, 
and positioned the intersection according to (different) satellite imagery.

While the two may share the survey;image sources, that is only 
coincidental, not duplicative. There may be other nodes along the way that 
I have not surveyed or taken pics of, which do not have tags.

--
Alan Mintz <Alan_Mintz+OSM at Earthlink.net>