Re: none

[Greg Troxel]

Anton Strydom <agstrydom@gmail.com> writes:

> The reason for all my questions is that it is easier for the survey
> community to work with ECEF than lat lon for a number of reasons.

Transforming ECEF (XYZ) to geodetic (latitude, longitude and ellipsoidal
height, usually written LLh) is straightforward.  Check out proj if you
are not familiar with it.  Note that this is just
repeatable/invertible/exact math with no datum transformation involved.


I would like to caution you that questions of accuracy and datum
surrounding GNSS data are complex and quite likely more so than you
realize.  Specifically:

  WGS84 is an ensemble, and the earliest member differs at the meter
  level.  Saying your data is in WGS84 means that you are saying it's in
  some member of the ensemble but you don't know which.   Don't do this.

  Recent WGS84 are equivalent to recent ITRF.  Agreement and intrinsic
  datum accuracy seem to be at the mm level.

  There is no way to precisely access WGS84, unless you are NGA/USAF.
  So the idea of even 10 cm level coordinates in WGS84 doesn't really
  make sense.

  The other 3 constellations have their own frame.  It's nearly
  impossible to find out what receivers do when combining them in
  producing navigation solutions.

  When using SBAS, coordinates are in the SBAS frame, not in WGS84.

  Post-processed coordinates with OPUS, CSRS-PPP and so on are typically
  in ITRF or national frames, not WGS84.

  With RTK the datum of the output is that of the reference station.  In
  the US reference networks typically are in NAD83(2011) epoch 2010.0.

  In nearly all places, ground points have significant velocity
  (multiple cm/year) when expressed in ITRF/WGS84.  Hence many use
  regional/national more-or-less-crust-fixed frames instead.  Strictly,
  there are still velocities, but they are much smaller in most places.
  As you probably know, California is unstable :-)

  To get to cm level you need to pay attention to antenna calibration.
  At this level you should be placing prism pole tips in punch marks and
  leveling the pole vs gravity carefully, and using survey-grade
  antennas mounted to the pole.  And you need to use a crust-fixed frame
  or deal with plate motion.

signature.asc
Description: PGP signature