Correcting GPS height against known height?

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Gillian

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I need to obtain accurate vertical heights of dipwells in a peatland for a PhD project. Unfortunately peat has a habit of expanding anc contracting dpendign how much rain has fallen, so I need to re-take these heights regularly. My budget doesn't run to a RTK system, so I was wondering if it might be possible to reference heights taken on my Trimble GeoXt 2005 against a known fixed (non-peat) height taken with a RTK system? The fixed height would consist of a pole driven throught the peat into the underlying mineral base which has been marked. If I were to check the height on the GeoXT could I use the height difference to correct the GeoXt's heights?

Hope this isn't a really dumb question!

Thanks

Gillian
 
We have to visit the dipwells every couple of weeks, so initially we would take the vertical measurements then. With the GeoXT I need to spend 10 minutes at each location to obtain enough carrier to post-process, so it would probably take a few hours to get all the dipwell heights. As we are trying to map ground water flow we would need accuracy to within a few centimetres.

I propose to check the height against the RTK height mark each time before we start measuring the dipwells. It seems to me to be workable - the question is how long the measured height difference would be good for?

thanks
Gillian

ps. I hope to pursuade a colleague to hump his RTK kit accross 1km of bog to do the initial survey!
 
It would be nice if your colleague lent you his RTK system each time you wanted to go out and take the measurements!
Instantaneous standalone GPS accuracy (particularly vertical height) changes quite quickly (perhaps over a meter in 30 minutes).
Your idea of post-processing data from your RINEX observables will give you the best accuracy (maybe as good as +/- 2.5 cm) without an RTK system.
 
Hi Mark. Thanks for your response. I tried taking 4 logs over 10 minutes each in exactly the same place but found, even with post processing, that the vertical height wandered too much - by approx. 20 cm max. Hiwever, the horizontal accuracy was excellent. Plan B is (funding willing) is to purchase a Trimble geo 7 Centimetre edition, which claims to be accurate to a centimetre or better. We need to use post processing rather than real-time as some of the sites don't have mobile phone reception.
 
Hi Gillian,
The height variation is no surprise :(.
Make sure you purchase the Zephyr 3 antenna to go along with the cm version of the Geo7!
Good luck with the funding, I'm not sure how much this is going to cost.
An alternative could be an Swift Nav's RTK base and rover Duro system with a radio system at less than £5500 (commercial price - your university may be able to get a reasonable discount?)
Regards
Mark
 

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