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I recently read an Ordnance Survey Blog article that seemed to suggest that it was necessary to leave the GPS device in use for four hours to gather requisite data. I am aware that "domestic" GPS units benefit from being left in one position to get better positional accuracy but had assumed that survey quality equipment was "accurate" immediately. What am I missing please?

Nige

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Because there are lots of random errors in the GPS signal (water content of the athmosphere,ground bounces,...) which affect positioning accuracy, collecting hours of GPS info and then using some mean value (median,...) will get you a more true position value.

When you have a known point and are using a DGPS base station on this point those random errors can be modeled subtracted and so your "rovers" which are connected to the base station via wireless link (or using post-processing) can obtain reasonable positional accuracy using only a few measurements.

See whubers comment below for clarifications!

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    +1 Some clarifications: (1) the random errors you are talking about are the unsystematic kind that eventually balance out to zero. If there are any substantial systematic errors, then the average of GPS readings over time, no matter how long, will be wrong. (2) The whole idea of DGPS (differential GPS) is not to "model" the errors. Instead, the hope is that because the GPS and and ground station are physically close, they will be subject simultaneously to essentially the same errors. Merely subtracting one reading from the other cancels all such errors. – whuber Apr 5 '13 at 13:10
  • Thanks Guys. My mistake was in forgetting what I used to know about GPS and thinking that survey grade kit was somehow magically more accurate; I forgot that the errors are not for the most part in the kit but in the rest of the 'system'. The blog that @Mapperz has managed to post a link to above is the one I was reading. It does state that post processing was required and I understand why now :) – nigellaw Apr 5 '13 at 14:38
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Nigel,

Check out the data sheet for the unit you mention.
What accuracy do you hope to achieve? Are you planning on using an RTK network solution? That solution, depending on the RTK network being used, can likely achieve a very accurate solution in a short period of time. If not RTK then you will may need a long occupation and a post-processing solution (of course how accurate?.

The GS15 seems to be able to looking at its spec sheet and I would call a knowledgeable Leica vendor or post a GPS forum such as RPLS.com with more specifics.

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The accuracy of GPS position always depends of lots of factors. You can look at this site it should resolve your problem. :)

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