23
Aug
2011
3

How do you measure sea level?

Have you ever looked at the height of an object above sea level on a map and wondered how that figure is worked out? With changing tides across the days and during the seasons, we get a higher tide or a lower tidal point.

The standard way to measure sea level is with an instrument called a tide gauge. These are used in ports and harbours the world over and record the heights of the falling and rising tides. Doing this over a period of time enables Mean Sea Level to be calculated and from this, the difference in height from this point to any other fixed location.

Ordnance Survey set up tide gauges in Felixstowe (1913), Newlyn (1915) and Dunbar (1917). Newlyn was chosen as the single reference datum, largely as it was situated in an area of stable granite rock and the gauge was perched on the end of a stone pier at the harbour entrance where it was exposed to the open Atlantic. This meant it wasn’t liable to be influenced by the silting up of the estuary or river tide delays.

Over the course of six years the Newlyn tide gauge took hourly readings. The gauge worked by recording the rise and fall of the sea with two floats attached by chains. The variations were recorded on paper attached to the rotating drum suspended from its centre.

From these readings the mean sea level was calculated and every height measurement in the country has been calculated based on the work of this machine.

Responsibility for the Newlyn Tide Gauge continued to rest with Ordnance Survey who provided a full time observer until 1983 when the station was handed over to the Institute of Oceanographic Sciences.

And of course, how we calculate the height above sea level has a direct impact on the heights of mountains and hills and the depth of valleys. Read about how we measure a mountain here.

Image courtesy of Ben Clark on flickr

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16 Responses

  1. Pingback : How do you measure sea level? | NCI Calshot Tower's Blog

  2. Simon

    HI I live in Southwark central London , 1km from the River Thames close to a spot height of about 3m
    The average tidal range at Tower Brige is 8m. Can you tell me if that means that we are 3m above the top of a high tide, 3m above the average of the mid points of the tides at Newlyn or 3m above the average lowest tide at Newlin. Are we 1m under the top of a high tide?

      1. Gemma

        Hi again Simon

        I’ve caught up with my colleagues in Customer Services and they say that the basic answer to this is that we don’t measure sea level at Ordnance Survey.

        We have sea level for datum, datum is used for orthometric height of land above sea level. This is then used for the foreshore which is between the Mean High Water and Mean Low Water.

        For sea levels and further advice on your query you would need to look at Admiralty Charts or contact the Institute of Oceanographic Services.

        Many thanks
        Gemma

        1. Jerry

          You mean you don’t know.? I think it would be half tide. When it’s half tide here. When it’s half tide on the Outher side of the see. Wen the see is flat. May be its just a guess .
          Jerry…

    1. Andrew

      You are 3m above Mean Sea Level (MSL) at Newlyn.

      MSL Newlyn will be more-or-less the same as MSL in London, but might be slightly different due to the effects of currents, prevailing wind, etc.

      Therefore (assuming all the measurements are accurate, and haven’t changed since they were made) you are about 1m under the top of a high tide.

    1. Hi Paul

      There no direct equivalent because for you are modelling earth surface the height will be dependent on the position/location within that model.

      Traditionally for Ordnance Survey Newlyn Datum, the Airy ellipsoid model is to determine height. WGS84 has its own ellipsoid model.

      To enable us to convert between these two ellipsoid models Ordnance Survey has it’s own transformation model (OSGN02).

      Further information about our GPS and position services can be found on our website here: http://www.ordnancesurvey.co.uk/business-and-government/products/os-net/index.html

      And FAQ’s here: http://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/gnss-positioning-services.html?q=afaq9#faq9

      Thanks, Gemma

  3. Paul

    Hi Gemma

    Thanks for your reply.
    This isn’t a re-projection issue. 😉

    To rephrase my question –
    If a GPS receiver was placed on the Ordnance Survey Newlyn Datum mark,
    what height would the GPS receiver read in WGS84 mode ?

    Reason for asking –
    I’ve some DEM data, which was originally supplied with ODN as the base elevation.
    Its being used in a WGS84 environment.
    All the elevations are lower than they should be, (by approx 5m).
    I suspect an inappropriate ODN/WGS84 vertical offset had been applied.

    Many thanks
    ATB
    Paul

    1. Hi Paul

      I’ve had a chat with the experts in our Geodesy and Positioning team and they’ve offered some further advice for you. They think you’re asking one of two things – “how to transform from ODN heights to WGS84 heights?” or “how to check an ODN to WGS84 height transformation?” and in both cases the answer is “use the OSGM02 model”.

      For transformation of individual or a few points they suggest using the online transformation at http://www.ordnancesurvey.co.uk/gps/transformation. For larger numbers of points they suggest downloading the Grid InQuest program from:
      http://www.ordnancesurvey.co.uk/business-and-government/help-and-support/navigation-technology/os-net/grid-inquest.html.
      OSTN02/OSGM02 is also incorporated into many popular GIS packages and since you already have your data as a DEM this might be the best way to transform from ODN to WGS84 height.

      The team also thought it was possible your data had been through some sort of transform already and you’re not happy with the result, perhaps when compared against other WGS84 points you have. So, the ODN-WGS84 offset AT NEWLYN is 53m. The reason for the caps lock is that ODN-WGS84 varies across the country. The team advise that VERY ROUGHLY the variation is from approx. 45m in the east to 55m in the west of GB. However it is not a smooth variation, hence the advice to use a model such as OSGM02 to get the best answer.

      Many thanks
      Gemma

  4. Romeo Dretcanu

    Hi Gemma,

    And thanks for this informative blog!

    My question: is is true that on nautical charts the shore line is not at OD but at MHWS level?

  5. Laurence Shelley

    If the Ordnance Survey still use the datum from the 1915-21 Newlyn readings to determine the sea level, would it not be incredibly enlightening for this research to be repeated now to give a very good indication of how sea levels have risen over the last hundred years? I’d be very interested to know if the Ordnance Survey or any other organisation have any plans to do so.

  6. Graham Morfoot

    Hi Gemma,

    Is ODN the datum used worldwide, if not, does that mean land outside UK above sea level is only relevant to local MSL, and therefore Mt Everest height above sea level would be different if it was in UK.

    Graham

    1. Hi Graham

      Apologies for the delay, I needed to wait for our Geodetics expert Mark to be back in the office to double-check on this. ODN is not worldwide – it is limited to GB. ODN like all other “classic” mean sea level (MSL) based height datums in use across the globe can only be realised by measurement of height differences (levelling) between fixed reference points on land (benchmarks). Most MSL based datums will be similar (i.e. it is not expected that they will be different by many metres) but different given that the MSL is observed at different locations and times. For example, measurements through the Channel Tunnel showed that there is an approximate 40cm difference between ODN and the French equivalent (ODN being higher).

      So, the mean sea level above which the height of Everest is measured is not the same mean sea level realised through ODN.

      Many thanks, Gemma

    1. Hi Tony

      You can use our benchmark database to get a rough height here: https://www.ordnancesurvey.co.uk/benchmarks/. If you enter the co-ordinates for the spot height (either eastings and nothings or National Grid KM square) you will get results of all benchmarks in the area and the height of when they were last leveled. If you don’t know the location, you can use the search facility within OS Maps (www.os.uk/osmaps).

      Please be aware that these heights have not been updated since the 1960s, as we use GNSS technology and our OS Net stations for surverying today.

      I hope that helps, but if not, do contact our Customer Services team for a chat. They’re on 03456 050505, Monday to Friday, 8.30am to 5.30pm.

      Thanks, Gemma

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