2
Aug
2011
5

How do you measure a mountain?

In the past week of so, you may have read about plans by Nepal to re-measure the height of Everest. Apparently being the world’s highest peak at 8,848m just isn’t enough – they want to use the latest satellite technology to get the most accurate measurement possible.

Everest

It’ll also an opportunity to settle a long standing disagreement between China and Nepal. According to the BBC, the Chinese argue Everest should be measured to its rock height, while Nepal maintains that any figure should include the snow on the summit – which would add about four metres.

This in itself is an interesting debate – what do you think is fairer?

Anyway, it’ll be two years before we hear the results, so in the meantime I thought it would be worth explaining how you actually go about measuring something as huge as a mountain.

Obviously Britain doesn’t have any peaks of Himalayan proportions, or any with 4 metres of standing snow, but whether it’s Everest or Ben Nevis, it’s not necessarily an easy task.

The most accurate method, which the Nepalese are using, involves using GPS satellite technology, or more precisely GNSS (Global Navigation Satellite Systems) technology, of which the American Global Positioning System is just one example.

Satellite technology will give you an instant position to a few metres, but the height will not be very accurate, so the GPS unit has to remain stationary and record data for a period of time – preferably several hours. Easier said than done in some conditions!

Photogrammetry

That data is then “post processed” where it is combined with data from other stationary receivers (what we call OS Net in Britain) to obtain a more accurate height measurement. The more data collected the more accurate the resulting height.

Using this method, accuracy of just a few centimetres can be achieved and I’m sure this is what the Nepalese government are hoping for.

So that’s the answer right? Well not quite. Satellite technology has transformed map making, however because of the time it takes to get an accurate height, it remains impractical to measure every mountain in this way. That’s why we use an alternative method for measuring most of Britain’s hills and peaks.

It called photogrammetry. It involves flying the area to be measured taking overlapping, high resolution photos from which a 3D representation is then created. Measurements can then be taken that translate directly to positions and heights in the real world.

This allows large areas to be surveyed quickly and safely and to an accuracy of 3-4 metres – more than enough for most uses in what are usually remote areas.

Having said that, when a mountain is just above or below a special threshold, that margin for error is just too tantalising for some

I hope that’s interesting, any questions just let me know.

[Everest image by claude_florin via Flickr]

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15 Responses

  1. Pingback : Ordnance Survey Blog » How do you measure a mountain? | Today Headlines

  2. Kevin Perrina

    Hi, this is an interesting topic, thank you for sharing it. However it’s all very well knowing what height the summit is in regards to sea level or whatever datum the Nepalese use but how do you determine the actuall height of a mountain, what definition gives to base height?

    Regards
    Kevin Perrina

  3. Hi Kevin, I’m glad you enjoyed the post.

    It’s a good question, and honestly I’m not sure what they will be using for Everest, but the ‘standard’ way, and certainly the way on nearly all maps, is to measure the height of the summit above ‘Mean Sea Level’.

    At Ordnance Survey this is the method we use, where Mean Sea Level has been determined by measuring the tide over many years.

    There is a bit more information on that here – http://www.ordnancesurvey.co.uk/blog/2010/07/surveying-our-antiques/

    Paul

  4. Pingback : Links In Darkness: Tuesday 2nd August – Thursday 11th August | Set In Darkness

  5. Ted Fowler

    we take many ‘measurements’ for granted these days. When was the ‘mean sea level” mark chosen to measure the height of land? In fact when did it become possible to measure the height of land/mountains.

    1. Hi Ted

      Over the centuries, there have been many different methods of measuring the mountains, although the geodetic method is generally considered the most accurate, and is the means Ordnance Survey followed when starting the principal triangulation of Britain back in the late 1700s. There’s a great deal of detail about this in this document on our website: https://www.ordnancesurvey.co.uk/docs/ebooks/history-retriangulation-great-britain-1935-1962.pdf

      You may also find this article of interest, it traces the measurements of mountains back to the Ancient Greeks and discusses the different methods for measuring: http://penelope.uchicago.edu/Thayer/E/Journals/ISIS/12/3/Determinations_of_Heights_of_Mountains*.html

      Many thanks
      Gemma

  6. raul baral

    This has been a very fascinating topic for me since the day i saw the Gaurisankhar Himal range on my bike ride from ktm to dolakha and wondered how on earth did people knew what it’s height was..ok we knw wat several of the world’s tallest mountain’s height was.but how was it measured?..obviously now we have several methods like gps and so on but wat about before the age of satellites?..did people like tenzing norgay and edmund hillary knew wat everest’s height was before they became the first persons to reach the summit and returned???

  7. Andy

    Can you tell how much of a mountain is below ground, if so how do you measure it? How do you know, where the mountain stops and the rest of the earth begins, if you see what I mean?

  8. Hi, I am just picking up on this thread from 2011 about ‘How do you measure a mountain?’. This is of interest to me as I am planning to write an article about the measurement and surveying of mountains. Would it be ok if I included a quote or two from this blog, concerning GNSS and photogrammetry? I would of course acknowledge the OS – or should I acknowledge a specific individual? Thanks in anticipation.

    1. Rob

      Hi, we are happy for you to quote the blog and use any of the content. If you do have any questions then please call Andy at OS on 02380 055565

  9. David Lynch

    Given that sea level is rising with warming seas and glaciers melting etc. how does the OS take this in to account? Will the rise in sea level reduce the height of Ben Nevis again in a few years time?

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