[Talk-ca] Missing islands and coastline

Bégin, Daniel Daniel.Begin at RNCan-NRCan.gc.ca
Thu Sep 23 18:52:43 BST 2010


Hi,

>From my understanding, natural=coastline is just a way to map large water areas.  

- The advantage of using this tag is that it can be composed by multiple ways without the need of a relation type=multipolygon or doesn't need that all ways be joined togeter to get a single closed way.
- The problem using this tag is that processing is a "complicated matter" and  "It has not been updated since several months" (!)

http://wiki.openstreetmap.org/wiki/Coastline
http://wiki.openstreetmap.org/wiki/Coastline_error_checker

So, you can delete a coastline without problems as long the way(s) you are deleting create a closed feature (like small island or lake).

If you delete a larger coastline (to replace it by a Canvec natural=water polygon for example) you must make sure to join both undeleted coastline extremities to remove all gaps.

Actually, for large water area, I find it easier to replace segments of the original coastline with corresponding geometry of the Canvec water feature...

- Cut both coastline/Canvec water feature overlapped segments;
- Delete the overlapped coastline segment;
- Change the natural=water tag of the Canvec segment for natural=coastline;
- Join both Canvec segment extremities to the original coastline;
- Check for the direction of the coastline. The rule is "water on the right".

Once every overlapped Coastline/Canvec water feature are replaced, remove unused Canvec segments.  Use the Validator (including upload check option) to find problems in the coastline.

Hope it helps

Daniel 

-----Original Message-----
From: talk-ca-bounces at openstreetmap.org [mailto:talk-ca-bounces at openstreetmap.org] On Behalf Of G. Michael Carter
Sent: September 23, 2010 11:56
To: Nakor
Cc: talk-us at openstreetmap.org; talk-ca at openstreetmap.org
Subject: Re: [Talk-ca] Missing islands and coastline


  The mix of natual=water and natural=coastline is because their dual 
objects.   The natural=coastline is needed as the great lakes (as far as 
I know) is connected to the ocean.  So deleting the coastline would delete portions of the Atlantic Ocean.

What I'm doing is enclosing the Canadian side of the great lakes, (object http://www.openstreetmap.org/browse/relation/1120169  (which needs to be loaded in sections in JOSM)

My reasons:
1.  Coastline's need to be complete to render properly.  So if your loading Toronto island (Lake Ontario) into a system, you have to pull half the worlds oceans to get it to render properly... as with incomplete data a rendering engine can't till which side contains the 
water.   Having a enclosed relation allows you to pull just that area.
2.  It's currently impossible to tell if a coastline object is fully enclosed inside JOSM editing.  But if you have a enclosed relation object (with type natural=water) where just one side is the coastline.  
You can easily tell by downloading the relation (aka 1120169)
3.  Naming.   Can't name a sting of coastline as easy as a single relation.

As for coastlines inland (like Lake Simcoe) make absolutely no sense to me as it's not a coastline.  So my thought, if you have a natural=water object that more accurately represents the body of water... use it to replace the interior coastline.

But that's just my take...


On 22/09/10 12:46 PM, Nakor wrote:
>     Michael,
>
> The relation in question is
> http://www.openstreetmap.org/browse/relation/1124369 but hit a wall 
> here. I cannot modify it (both Potlatch and JOSM time out). Isle 
> Royale (which was my initial concern) is still missing on a couple 
> zoom levels.
>
> Before I continue trying to fix this it seems there are a mix of 
> natural=water and natural=coastline for the Great Lakes. I'd like to 
> have this consistent over the Great Lakes but am not sure which one to 
> use. Please comment which one would be better/worse and why?
>
>   Thanks,
>
> N.
>
>
>
> On 9/20/2010 10:20 AM, G. Michael Carter wrote:
>>  It was brought to my attention there was some problems in Lake 
>> Superior area, but the problems seem to be all over the great
>> lakes.   There's a user, who's name I don't have handy, creating 
>> massive relationship objects of the great lakes.  I think this might 
>> be sinking a lot of the islands.   The island objects were last 
>> modified by this user in the cases I checked.
>>
>> However, if you refresh the mapnik (aka /dirty) the tiles everything 
>> seems to be refreshing ok.   Just wanted to let people know.    If 
>> you find some area underwater refresh the tiles, before investigating.
>>
>> Michael
>>
>>
>>
>> _______________________________________________
>> Talk-ca mailing list
>> Talk-ca at openstreetmap.org
>> http://lists.openstreetmap.org/listinfo/talk-ca
>
>
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