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

[SK53 on OSM]

On 09/03/2011 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.
>
> Mike
>
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Interesting point Mike.

There are similar issues about tracing from imagery or using Vector Map 
District when doing other waterbodies: reservoirs are the ones which 
immediately come to mind. Often the landward side of the splash zone is 
more obvious than the usual water level, and if that is used for mapping 
it gives a false impression. Patches of riparian scrub and marsh also 
seem to be treated inconsistently by the OS (perhaps aerial interpretation).

Most larger rivers will have flood-level gauges (right word?) which 
might be some kind of aid for choosing a 'natural' level to map, but 
sourcing then is not straightforward. I've only done a bit of mapping by 
wandering around in wellies with one foot on dry land and the other in 
the water.