[Talk-GB] UK Quarterly Projects

SK53 sk53.osm at gmail.com
Mon Mar 14 21:21:19 UTC 2016


A while back I played with trying to create watersheds
<https://flic.kr/p/AXxSfm> using available OSM data. Each group of
waterways which interconnect (note I used st_intersects() on osm2pgsql
data, not the true topological relations in way_nodes) is given a different
colour. The map gives a fairly good synoptic overview of density and
completeness of waterway mapping. Of course it doesn't show accuracy, and
much of the dense coverage in Wales was done by Steve Chilton many years
ago from NPE maps.

Also I didnt try and handle rivers which pass through a lake or
rivers/streams only mapped as areas (this is really obvious on the
equivalent map for Ireland as the Shannon appears as several disconnected
watersheds). For this reason, and concave hull throwing errors on PostGis I
have not followed through with the aim of creating watershed polygons.

Jerry

On 14 March 2016 at 21:09, Andy Townsend <ajt1047 at gmail.com> wrote:

> On 14/03/2016 20:50, Rob Nickerson wrote:
>
>> My concern with rivers is that we don't have tools to measure progress -
>> taginfo gives a count of nodes/ways/relations whereas we'd want total
>> length of features per region (in miles/km).
>>
>
> I'd agree that tracking progress with that one would be tricky. I've
> mapped lots of rivers and streams from a combination of survey, imagery and
> OS OpenData, and while we've got something approaching the "length" of
> major rivers and streams, what we've got in some areas is largely ex-NPE,
> and in many cases some distance from the actual waterway on the ground.
>
> GPS traces (unless there are lots) and Bing imagery can of course be
> misplaced, but OS OpenData is normally pretty good (actually better than
> the OS vector data that people have imported in a couple of areas) although
> it can be wrong when watercourses have changed, and the top end of small
> Welsh streams is often a bit "wishful thinking" in OSSV - where in reality
> there's just a boggy mess the OS sometimes has well-defined streams.
>
> The other problem with the waterways we've got in OSM is that many are
> just either "stream" or "river" - with things that I'd normally map as
> drains and ditches just in as "stream".  Obviously changing a tag
> post-survey is pretty straightforward, but it's something else to bear in
> mind.
>
> There is a real benefit of having OSSV streams, ditches and drains in
> though - it's often clear that a watercourse hasn't moved for years, and it
> can then be used to help align imagery and GPS traces.
>
> Cheers,
>
> Andy (SomeoneElse)
>
>
>
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