[PROJ] Tie breakers used when ordering candidate operations

[Brian Shaw]

All

There have been many discussions here and at NGS about WGS84 and how it 
should or should not be used.  While it is not published yet there is a 
paper that is close to being published about WGS84 that is coauthored by 
an NGS Geodesist Michael Dennis and Esri Geodesist Kevin Kelly.  It 
discusses all of the versions of WGS84 and their relationships to ITRF 
that NGA says are roughly equivalent.

One reason for recommending people use ITRF over WGS84 is that WGS84 
only uses 16 tracking stations for the global solution where ITRF2014 
used 884 GNSS sites, 124 VLBI sites, 96 SLR sites and 71 DORIS sites 
(according to these slides 
https://www.iau.org/static/science/scientific_bodies/divisions/a/2018/Altamimi.pdf).

When I was talking with our Chief Geodesist about WGS84 he summed it up 
( to the best of my memory for 2-3 years ago ) that WGS84 should/can be 
used for navigation but should not be used for positioning (unless you 
have military receivers).  WGS84's 2-3 meters of uncertainty is fine 
while you are moving in a vehicle but not for determining coordinates at 
a specific location unless that uncertainty works for your needs.

Ill make sure to email the PROJ list when this is published.

Cheers
Brian


On 2/12/2021 2:03 AM, Lesparre, Jochem via PROJ wrote:
> Greg wrote:
>>   People talk about WGS84
> Everyone should stop doing that and avoid EPSG:4326 at all times. ITRS is the one official international system, recognised by the UN.
>
>
> Greg wrote:
>> ITRF2008 is pretty much the same thing as WGS84(G1762) ), and ITRF2014
> The major difference is that WGS84(G1762) is really messy at centimetre-level. It ignores the tracking station velocities during the year and uses an annual step-wise update (at a random date in January) of tracking station coordinates used to generate ephemeris broadcast message.
> https://earth-info.nga.mil/GandG/publications/NGA_STND_0036_1_0_0_WGS84/NGA.STND.0036_1.0.0_WGS84.pdf
>
>
> Greg wrote:
>> So, I think people who ask for transforms to WGS84 should be told "That doesn't really make sense any more; I will assume you mean WGS84(G1762) which is basically the same as ITRF2014" and then you should use those transforms instead.
> That is what I am doing all the time, except that I first ask them "Do you really want time-dependent international coordinates? Otherwise, I will assume you mean ETRF2000".
>
>
> Regards, Jochem
>
>
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-- 
*************************************
Brian Shaw
Rocky Mountain Regional Advisor (CO, MT, WY)
NOAA's National Geodetic Survey (NGS)
Cell Phone # 240-988-6363