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Richard Imrie
(@richard-imrie)
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Joined: 3 years ago
Posts: 936
December 2, 2018 8:09 pm  

We've found a record statement for some control stations established around year 2011 and as an example the statement says:

Geodetic Coordinates

ITRF2005@2008.0 GRS80

Latitude XX XX XX.xxxxxx S

Longitude XXX XX XX.xxxxxx E

Ellip. Height(m) XX.xxx

I sort of understand the ITRF2005 bit and the GRS80 (as the reference ellipsoid to go from ITRF to geographical coordinates, but what does the @2008.0 signify (should they be the other way around)? Ultimately we are intending to transform this to WGS 84 (? which one ? this an airport related project).


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Jeran Hopfe
(@jeran-hopfe)
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December 2, 2018 8:44 pm  

Hi Richard, this would refer to the 2008.0 epoch of ITRF2005. This should be the calculated reference frame as of January 1, 2008.  ITRF2005 is designed to be updated for the shift of earth’s tectonic plates based on GPS measurements, if I’m not mistaken.


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Richard Imrie
(@richard-imrie)
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Joined: 3 years ago
Posts: 936
December 2, 2018 9:04 pm  

I sort of get that too, but I found this on the internet, which is where I'm loosing it on the order of things:

Capture 1

Capture 2

Capture 3

 

 


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Jeran Hopfe
(@jeran-hopfe)
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Posts: 4
December 2, 2018 9:54 pm  

So far as I can tell, ITRF2005 was designed to have no transformations applied at epoch 2000.0, and every year afterward, there would be a set of translations applied to adjust for the movement of the plates. ITRF2008 was designed to have zero transformations at 2005.0, and I think this is where the confusion is coming from. What you’re looking for is ITRF2005, with transformations applied to epoch 2008.0.  I would suggest using a conversion utility to convert your coordinates to WGS84, if that’s what you intend on using for the project.  Definitely document it, and I would certainly be willing to do a double check for you, but that’s the route I’d suggest.


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Richard Imrie
(@richard-imrie)
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Joined: 3 years ago
Posts: 936
December 2, 2018 10:10 pm  

Thanks for that. It is what we intend doing.


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GeeOddMike
(@geeoddmike)
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Joined: 8 years ago
Posts: 877
December 2, 2018 11:06 pm  

DEFF83A4 77E6 451B A72F DF03FBC8A97A
BC927A79 235F 4D23 AF10 6B8598A5E7B0

This topic has a lot of past posts. You might want to do a site search.

An important point is that ITRF/IGS and WGS84 are separately developed and maintained. WGS 84 has been updated periodically to keep it within 5cm of the current IGS. IGS references their updates with a year and epoch date; NGA uses the GPS week.

It is important to know which versions of each you are using.

As to issue of how coordinates are updated and transformation parameters, see images below.

 

 

This post was modified 7 days ago by GeeOddMike

“To be surprised, to wonder, is to begin to understand,” - Ortega y Gasset


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GeeOddMike
(@geeoddmike)
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Joined: 8 years ago
Posts: 877
December 5, 2018 10:00 am  

While pruning my photo directory I came across this tabulation of the changes to WGS84. Not sure if it is up-to-date, this is not an NGA site. The policy of NGA ,as I understand it, is to periodically update coordinates of their sites in order to maintain agreement with the ITRF/IGS at the <5cm level. This is done by recomputing their site positions using IGS orbits. There was an interesting technical paper generated after their first “validation.”  ( lousy spell check keeps altering my intended wording #$%^&&!!!)

2AB7D0F0 6E6D 4B36 AA26 898C31E5BF0C

This post was modified 4 days ago 4 times by GeeOddMike

“To be surprised, to wonder, is to begin to understand,” - Ortega y Gasset


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