[Talk-GB] Incorrect use of OS VectorMap District when mapping?

[Michael Collinson]

Jason, Yes, I'll try to do this over the weekend. Give me an off-line 
poke if I forget.

Mike


At 14:45 09/03/2011, Jason Cunningham wrote:
>Hi Mike,
>
>Can you provide us with a grid ref(s) for a location where the OS 
>data is wrong
>
>Jason
>
>On 9 March 2011 13:33, Michael Collinson 
><<mailto:mike at ayeltd.biz>mike at ayeltd.biz> wrote:
>At 13:29 09/03/2011, Chris Hill wrote:
>On 09/03/11 11:57, Michael Collinson wrote:
>At 12:32 10/02/2011, Andy Robinson (blackadder-lists) wrote:
>Henry Gomersall [mailto:heng at cantab.net] wrote:
> >Sent: 10 February 2011 11:07 AM
> >To: Peter Miller
> >Cc: Talk GB
> >Subject: Re: [Talk-GB] Incorrect use of OS VectorMap District when mapping?
> >
> >On Thu, 2011-02-10 at 10:30 +0000, Peter Miller wrote:
> >> On reflection possibly we should use river-bank as that has more
> >> information in it, but recommend that anyone importing does a 'bridge
> >> cleanup' at the same time.
> >
> >This is an area I'm actually really interested in (for rural rivers) and
>keen to
> >contribute. So far I've been put off by exactly this problem. Is a
>reasonable
> >approach to use the OS data for river edges and then fill in the gaps
>(bridges
> >etc) with OSM data?
>
>+1
>
>If the OS vector data is only assumed to be the banks and the additional
>data for flow direction, bridges and other features are added from
>survey/BING etc then we should end up with a very functional dataset.
>
>
>A late response to this thread, but a word of caution. Comparing 
>Bing imagery recently for several Yorkshire rivers with folk's 
>riverbanks derived from OS data indicates that very frequently  the 
>OS are not tracing the riverbank as the dividing line between water 
>(clear river channel) and land (grass, scrub) but the top of the 
>riverbank or where the rough "verge" meets pasture land.
>
>A further word of caution: Bing and all other imagery only shows a 
>snapshot of the way things are, often many years ago, and in an 
>indeterminate state of water level. Some rivers have tidal 
>influences, some rivers have very different levels in flood or 
>drought. Sometimes where the rough "verge" meets pasture land is the 
>highest point the water reaches regularly, but still only occasionally.
>
>
>Certainly both Chris' and Phillip's cautions are certainly true but 
>I've paid particular attention to the River Wharfe mid-reaches, 
>which I know very well and flows in a well-defined channel with high 
>banks and  has not shifted markedly in the last 40 years. In places, 
>it is almost twice as wide as it should be. Chris may be right in 
>suggesting that the highest water mark is being mapped, but why map 
>the 10 - 25-year flood event level rather than the natural bank 
>line? I am tempted to think that automated software has been used 
>which like PGS coastlines occasionally gets confused by nearby 
>lineaments. I also recall comparing with digitised 25:000 maps 
>(vintage 1900 - 1960 surveying) and noticing that it correlates much 
>more closely with Bing than StreetView. Needs more analysis but be aware!
>
>Mike
>
>
>
>
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