[talk-au] Coastlines from admin boundaries

James Livingston doctau at mac.com
Tue Jul 14 11:27:07 BST 2009

On 14/07/2009, at 7:55 PM, Matt White wrote:
> occurs to me that the ABS data that was imported is probably
> substantially better quality than we will ever get either with a gps  
> or
> tracing off yahoo (some areas excepted).

I'd been noticing the same thing, in many places the ABS boundaries  
are fairly close to the PGS and a lot more detailed. Whether it's more  
accurate I don't know, but definitely more detailed. Also, if you're  
going to mark something else (e.g. a park) where the coastline is the  
boundary, it'd be nice to have one to use rather than two ways in  
slightly different locations.

> But I wasn't sure if I should re-align the PGS way with the ABS admin
> boundary way, or nuke the PGS way and tag the existing ABS boundary  
> with
> the coastline tags, or just dance a jig. I know admin boundaries can
> change, but (global warming aside) the odds are pretty good those
> boundaries set against coastlines probably aren't likely to change.

I'd actually started re-doing the coastline on some bits of Tasmania,  
and I have seen instances of other people doing so on the Queensland  
coast. I would imagine there are other places too, but I haven't  
looked. For example left of the Brid river is changed, right isn't at http://osm.org/go/uJHmdiSm-

What I've done (and also seen elsewhere) is simply deleting the PGS  
way and tag the ABS boundary with natural=coastline. if you re-align  
the old coastline ways, you wouldn't be using the old nodes and  
source=PGS would be wrong, so why not just tag the ABS ways?

You'd obviously want to check that the PGS and ABS ways aren't  
significantly different, and if so find out why. For example, in  
Bridport the ABS suburb boundary went out around the pier where some  
commercial fishing boats dock, and the coastline obviously doesn't.

If people don't think that's a good idea I can revert my changes, but  
I think it makes sense.

Also if you are going to do this, remember to ensure the ways are  
correctly oriented with natural=coast - Land on the Left, and  
natural=coastline must go across river mouths to form closed shapes.

-- James

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