Dear All
Although I do not answer for the Ordnance Survey I think you can assume
that rises in sea-level will not effect the contours on maps in the U.K.
The OS relates all altitudes to Ordnance Datum, which is mean tide level
(for a set period of time in the past) in Newlyn, Cornwall. In effect a
zero line has been defined for the country. Since this line relates to mean
tide level, the water surface tends to reach higher altitudes. Thus for
most sites around the U.K. the highest tides perhaps reach 2-3m O.D and in
parts of the Severn estuary highest tides may reach 5-8m O.D.
The contours thus have no relation to local sea levels, parts of the U.K.
are already below sea-level with altitudes of 1-3 m O.D. I thus would have
thought that witha predicted rise of 30cm over the next 100 years the OS is
not going to alter the contours on its maps.
Callum Firth (Brunel University)
> To: [log in to unmask], [log in to unmask]
> Subject: Re: O/S contour lines
>
> Hi Hillary,
>
> You made a fascinating observation about the impact that a rise in sea
> level would have on the location of contour lines portrayed on large
> scaled maps. It would seem that with a rise in sea level these lines
> would indeed shift noticeably over time on maps scaled at 1:50,000 or
> 1:25,000, particularly where the topography of the land is characterized
> by gentle slopes with a mild gradient.
>
> Unfortunately, I don't really know what the practice of the UK Ordnance
> Survey is with respect to updating contour lines. I contacted the
> Ordnance Survey a few days ago, and posed the question directly to them.
> I am currently waiting for their response, and will gladly forward it to
> you as soon as I receive it. If you wish, you can also try to contact
> them by visiting their site at: "www.ordsvy.gov.uk/".
>
> I have looked at a rather different aspect of the problem, and focused
> exclusively on the impact that a rise in sea level would have on the
> limits of maritime jurisdictional zones seaward of the low-water line,
> as opposed to contour lines which are situated on land, or bathymetric
> lines which basically portray the depth of the water column. The
> problems that a rise in sea level creates for the limits of maritime
> zones is somewhat different than the problems it would generate for
> contour lines or bathymetric lines, and I am not sure how helpful my
> comments would be.
>
> Hopefully, the media services of the Ordnance Survey will be able to
> shed some light on the question you asked.
>
> I am very interested in learning more about your work, so feel free to
> contact me, and keep in touch.
>
> Dr. Alain K. Khadem
> 17 Gatelot Avenue,
> Lake Ronkonkoma
> NY, 11779-2304 USA
>
> Tel: (516) 981-6808
> (516) 981-6807
> Fax: (516) 981-6805
>
>
> Hillary Shaw asked:
> >
> >Does the Ordnance Survey ever revise the contour lines on its
> >1:50,000 or 1:25,000 maps. In the South - East of England, where the
> >land is sinking relative to sea level by about 50cm a century, these
> >contour lines would over a hundred years or so be subject to
> >noticeable lateral "migration". For example, take the 10 metre
> >contour, in an area where the gradient of the slope is 1 in 200. Over
> >100 years this contour would migrate uphill by 100 metres, which is
> >2mm on the 1:50,000 map and 4mm on the 1:25,000 maps. In a few
> >hundred years time our contours could be out by a metre or two
> >vertically, and hundreds of metres horizontally. Will the O/S
> >respond?
>
>
>
> Get Your Private, Free Email at http://www.hotmail.com
>
Callum Firth
Brunel University, UK
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