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This is the response I got from the Ordance Survey concerning contour lines: Dear Mr Khadem - Thank you for your enquiry. Ordnance Survey has measured mean (average) sea level around Britain over extended periods three times over the past 150 years - first from a point in Liverpool between 1842 and 1859, then at Newlyn in Cornwall between 1915 and 1921 and most recently in Newlyn again between 1951 and 1956. You will see from the approximate periods between these dates that before too long we are likely to be embarking on a further round of measuring. One aspect of this will be a highly detailed aerial survey of high and low watermarks around the entire coast, a project which is being substantially funded by the Government as part of a new seven-year agreement on services required in the national interest.. We have not made wholesale amendments to our contour lines this century. Obviously there have been many local changes, particularly as land has been reshaped by Man as opposed to nature. But generally speaking any changes in mean sea level (as opposed to land erosion or sea incursions) have not warranted changing contours (which are at 5 metre or 10 metre intervals depending on the map scale) but they have necessitated us changing the height values of many of our official Bench Marks around the country, which are much more precise in their height information. Having said that, the sea level change is only one consideration in when any recalculation is being considered - inland land movements are also an important factor. The new coastal surveys which will be taken over the next few years as part of the national agreement will indicate whether further consideration needs to be given to changing any of our height information on a more general basis. I hope this helps. Philip Round Senior Press Officer ---------- From: [log in to unmask] To: Round, Philip PO Subject: Request for information Date: 12 March 1999 16:50 Dear Mr.Round, I would appreciate if you would let me know whether the Ordnance Survey adjusts the contour lines on maps. I am particularly interested in knowing whether, and if so how often, these contour lines are adjusted, and whether the gradual rise in sea level along the southern and eastern coast of England are taken into account. I thank you in advance for your assistance. Alain K. Khadem >From: Callum Firth <[log in to unmask]> >Reply-To: Callum Firth <[log in to unmask]> >To: [log in to unmask], [log in to unmask],[log in to unmask] >Subject: Re: O/S contour lines (fwd) >Date: Tue, 16 Mar 1999 15:00:51 +0000 (GMT) > >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 >[log in to unmask] > > Get Your Private, Free Email at http://www.hotmail.com %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%