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

[Jason Cunningham]

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 <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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