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I have dGPS positions collected using a Leica 1200 system (base station and rover) recorded at the same site on multiple occasions over two years (being used to georeference terrestrial laser scanner data).

The positions were collected and post processed in WGS84 and converted to OSGB36 using OSTN15/OSGM15 as if they were ETRS89 using Grid InQuest.

As I understand it, OSTN15/OSGM15 should take ETRS89 lat/long as input and ETRS89 coordinates are slowly diverging from WGS84 coordinates.

Questions: 1) Will my positions be accurate relative to eachother, i.e. if I had surveyed the same mark each time, would the easting, northing and ODN elevation be the same or would it gradually shift due to tectonic plate movement? (I am interested in cm scale changes so the 2.5 cm annual change would affect me)

2) If not, can I correct them using a time dependent transformation?

3) Although I understand any transformation will introduce error, will any offset introduced by OSTN be consistent at a particular position?

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  1. Yes; The OSTN15 transform is computed using constant values. That is, transforms for given points between ETRS89 and OSGB36 using OSTN15 will always result in the same values. At some point, OSTN15 will presumably be superseded in the same way that OSTN02 was. I don't believe that OSTN15 accounted for plate movement – it wasn't mentioned in any of the accompanying guidance, and OS literature describes the UK as "tectonically stable". Are you referring to the difference between ITRS (a precise version of WGS84) and ETRS89, which is growing by 0.025m / year?

  2. I don't have experience with time-dependent transformations, so I can't directly address this, but:

  3. So long as the positions you capture use the same realization/epoch, your OSTN15 offsets will be the same. That is, transformations between ETRS89 and OSGB36 (using OSTN15) will not change, but measuring the same physical point using dGPS in subsequent years may yield different ETRS89 coordinates if the ITRS epoch (known as both ITRS annual realizations in the form ITRSyy, and as WGS84 epochs) used at the time of measurement differs. Thus, if you've made your initial measurements using ITRS16, you'll have to check whether it's been superseded (it doesn't happen every year AFAIK; there's no ITRS15), and if it has, either make the new measurements using the old realizations, or re-process the old measurements using the new realizations, before converting those values to ETRS89 and OSGB36.

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