[Proj] Understanding deformation model usage

[Kristian Evers]

Steve,

I don't expect the EPN velocities to match the velocities in the NKG velocity grids.
They are not supposed to be the same. The NKG velocity grids are modelling
post-glacial uplift, whereas the EPN velocities are a record of how fast the various
stations are moving. The EPN velocities are, among other things, used to determine
a best fit for rigid plate motion of the Eurasian contintent (at least the European part).
This plate motion fit is absorbed into the Helmert transformation parameters for
ETRFxx transformations. So somewhat simplified you can say that the EPN velocities
are made up of two components: Rigid plate motion and non-rigid plate motion.
The NKG velocity model is only concerned with the latter. That is also why the
full transformation include steps going from ITRSxx to ETRF00.

I have tested the NKG transformations against results from two of the authors of
the paper that describes the transformations. All three of us has developed our
code independently of each other so I am quite confident that the end results
is correct. But it is entirely plausible that I have mixed up grid dimensions
somewhere but in a way that the errors cancels out. If you think that is the case
please provide some "proof" so I can investigate it further.

/Kristian

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Sendt: 7. marts 2018 08:45
Til: proj at lists.maptools.org
Emne: Re: [Proj] Understanding deformation model usage

Kristian,
Yes, that makes sense to me now. But that means those ETRF00 velocities
(with a slight adjustment of position for epoch 2010.00 to 2000.00 should
come close to the realigned NKG velocity grids you have?
 
But although the z is reasonable, the xy don't look so good. Is it possible
the velocity components have accidentally been transformed (ie. grid values
treated as heights in gdalwarp GDAL 2.2.1 and above)?

I've transformed the Cartesian EPN Stations ETRF00 to long lats at epoch
2000.00 here 
https://1drv.ms/u/s!AvBQ7Pt73oO6iS0Y_-aOIBqwxtEs

Steve


Kristian Evers-2 wrote
> Steve,
> 
>> Are the Cartesian deformation grids tied to a particular TRF ? I was not
>> expecting them to be.
> 
> At least for the use case that I developed this for, yes, the grids are
> tied to a
> particular frame.  In case of the Nordic ITRFyy(yy) -> ETRS89
> transformations
> that frame is ETRF00. That is, it is co-rotating with the Eurasian
> continental
> plate.
> 
> The EPN velocities you have linked to are relative to the various frames
> in the list. So the ITRF velocities includes the rotation velocity
> of the continent but the ETRF velocities doesn't (since the frame itself
> is defined
> as co-rotating with the continent).
> 
> Does that make sense to you?
> 
> /Kristian
> 
> -----Oprindelig meddelelse-----
> Fra: 

> proj-bounces at .maptools

>  [mailto:

> proj-bounces at .maptools

> ] På vegne af stevejking
> Sendt: 28. februar 2018 12:29
> Til: 

> proj at .maptools

> Emne: Re: [Proj] Understanding deformation model usage
> 
> Kristian,
> A follow up question, related to usage,
> 
> Are the Cartesian deformation grids tied to a particular TRF ? I was not
> expecting them to be. The reason I ask is that on this website:
> http://www.epncb.oma.be/_productsservices/coordinates/index.php
> Station positions (I assume Cartesian) and velocities are given using
> three
> different reference frames. Drop down list top right you can select
> individual stations. Class A have average velocities (to GPS week 1950).
> And
> although the Cartesian coordinates from each TRF don’t differ that much,
> the
> average velocities do?
> 
> In the proj documentation example I think it assumes the deformation grids
> are not tied to a particular TRF.
> 
> Thanks again
> 
> Steve
> 
> 
> 
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