[OSM-dev] Coastline shapefiles

David Groom reviews at pacific-rim.net
Mon Jan 30 15:28:37 GMT 2012

----- Original Message ----- 
From: "Paul Norman" <penorman at mac.com>
To: <dev at openstreetmap.org>
Sent: Monday, January 30, 2012 5:09 AM
Subject: [OSM-dev] Coastline shapefiles

>I have been experimenting with generating the coastline shapefiles locally
> using http://svn.openstreetmap.org/applications/utils/coastcheck/ and 
> ended
> up with a few questions
> 1. Is this the same code that is currently used to generate the 
> processed_p
> files?
> 2. It took my server about one hour from the start of extracting the
> coastline data to creating the shapefiles. My understanding was that this
> process took about a day. I'm using a different route to extract coastline
> data than osm2coast, could this account for the difference?
> 3. If I were to run the coastline generation daily and upload the files
> somewhere, could someone then host a slippymap showing coastline errors?

Firstly , is a slippy map showing coastline errors really necessary?  If you 
could upload the error point shapefile somewhere daily I'm sure you would 
find these errors were corrected within a few hours.  Michal Migurski has 
been generating coastline error files every 3 - 4 weeks 
http://metro.teczno.com/#coastline, and recently I've been finding that by 
the time I try and correct the errors someone has got there first.

Secondly , if my memory serves me correctly, the original coastline error 
map showed both points where there was a disconnect in the coastline, and 
also places where there were geometry issues (mainly the small artefacts 
left over form the PGS import).  These errors were I believe displayed on 
the slippy map by way of  transparent tiles, and obviously there was time 
taken to generate these tiles, and then bandwidth / space requirements 
involved in deploying them.

Recently ( last 3 - 4 months)  the number of disconnects has been reduced to 
a low number, and it is now possible to deploy a slippy map much more 
simply, see http://www.wightpaths.co.uk/coast/  and hosting requirements for 
this are therefore minimal, if it was felt a coastline error map was useful.

The number of the second type of error (geometry issues) is still quite 
high, and probably the only way to show these on a slippy map would be to go 
down the transparent tile route, though I guess some form of clustering of 
marker points would be possible.  I do wonder though what the benefit of 
showing these on a slippy map would be.  Over the last few months I've been 
going round clearing these errors up, and will continue to do so.  I think I 
have reduced the number by about 2 / 3 rds so quantity wise it is not an 
impossible task.

If at the end there are any remaining errors they are likely to be two 

a) these are in Canada - Paul you are probably aware of my posting to the 
talk-ca list asking if it was worth my effort in clearing these up due to 
the possibility of wholesale replacement of the current coastline data with 
Canvec data

b) clearing up the error is too complex.  I know this is may sound like an 
excuse, but there was at least one area in the USA where a latter import of 
data with ways tagged as natural = water over the top of PGS ways tagged as 
natural = coastline, made a quick resolution of the problem difficult, and I 
gave up and moved to another area.  Without wishing to sound arrogant, if 
someone needs a slippy map to identify the errors, then they are probably 
not the right person to go about sorting out the issue in this particular 
circumstance. ( BTW that's not a comment aimed at you Paul -  if you're 
running the coastline error files and can get GeoBase NHN into OSM then I 
realise you are technically competent ; I'm just saying that crowdsourcing 
coastline error issues may in fact lead to more errors being created than 
are solved)

Anyway to summarise.  My belief is that producing a slippy map showing the 
coastline errors is not particularly necessary

Daily production, and upload to somewhere accessible, of the error points 
shapefilefile ,and to a lesser extent the processed_p shapefile, would be 
very useful


> 4. Given that it only took an hour to generate, is there any way to get 
> more
> frequent updates to the coastline files?
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