Author Topic: Did the Earth move for you?  (Read 444 times)

TheGUYuk

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Did the Earth move for you?
« on: 12:35:15, 17/04/13 »
Did the Earth move for you?
 
The Ordnance Survey posts some myths and then tells the truth. So next time someone asks you did the Earth move for you, you can reply yes  ;) ;D

Myth 3: ‘The true coordinates of a ground point do not change’
They certainly do, due to the continuous deforming motions of the Earth. Relative to the centre of the Earth, a point on the ground can move as much as a metre up and down every day just because of the tidal influences of the sun and moon. The relative motion of two continents can be 10 centimetres a year, which is significant for mapping because it is constant year after year – after 50 years a region of the earth may have moved by 5 metres relative to a neighbouring continent. Many other small effects can be observed – the sinking of Britain when the tide comes in over the continental shelf (a few centimetres), the sinking of inland areas under a weather system high (about 5 millimetres), and the rising of the land in response to the melting of the last Ice Age (about 2 millimetres per year in Scotland, up to 1 centimetre per year in Scandinavia). Generally, as the size of the region of the Earth over which we want to use a single coordinate system increases, the more these dynamic Earth effects are significant.
The modern trend is to use global coordinate systems even for local applications. Therefore, it is important to realise that in a global coordinate system, the ground on which we stand is constantly moving. This leads to subtleties in coordinate system definition and use.
http://www.ordnancesurvey.co.uk/oswebsite/gps/information/coordinatesystemsinfo/guidecontents/guide1.html
 
Now how old is that map you are using?
Or have you used a trig point to set your GPS height?
 
Now, " small amounts " , some will cry, and small errors snow ball, if  you are not careful.  :) ;)

 

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