base9geodesy, post: 363286, member: 7189 wrote: The only way to get real WGS 84 values is to perform an autonomous position just with respect to the satellite constellation, in which case your positional accuracy will be in the range of 2-5 m so a tool like HTDP is useless. I've seen far too many users quote WGS 84 with no realization (of the 6 that there have been) and no epoch and say they have a few tenths or a cm or two. As a civilian you can't get coordinates in WGS 84 that accurate - you need terrestrial control in the datum which the National Geospatial-Intelligence Agency (NGA) does not provide. In almost all cases that I've dealt with the user was actually connecting to the NSRS (HARN or CORS) which is some iteration of NAD 83 and called it WGS 84. As Bill93 pointed out the manufactors most often use the old military document transformation which calls for 0 in X,Y,Z. Regrettably the military doesn't seem to have learned much, in the 2014 edition of the WGS 84 documentation NGA still publishes the original 1986 0 transformation to NAD 83.
I've wondered about that many times. Is my Garmin giving me WGS 84 to whatever accuracy it can?
And when I see something like this, which claims WGS 84 in conjunction with a localization, I really wonder. What are these guys really getting?
https://xfer.services.ncdot.gov/gpsdata/u2579b/u2579b_ls_gpscalib_071217.pdf
mkennedy, post: 363508, member: 7183 wrote: Probably an ignorant question, but could those users who were reporting WGS 84 actually have been recording ITRFxx coordinates?
Good question Melita,
Could be!
BUT, inasmuch as many software packages "report" WGS84 as somewhat of a "catch all," default thing, it's anybody's guess.
I have personally seen GPS networks observed, adjusted, and "published," that STATE in no uncertain terms that all of the coordinates are WGS84, where in fact, the only "fixed" position was NAD27...YES NAD27! Basically the entire network was constrained to an old USGS Triangulation Station, and that's the only Lat/Lon they could get their hands on.
ANYTIME I see "WGS84" on a Plat, Record of Survey, or in a Report, the BS-a-meter goes off, and I start making calls, and digging deeper.
There are any number of reasons for the confusion on many folks part, but the bottom line is ignorance.
YES...a fugarwe IS a WGS84 position (+/- a meter or MORE).
YES...any vector solved using the Broadcast Ephemeris is expressed in WGS84 (including "classical" Base/Rover RTK).
And yes, IGS08 and WGS84(G1674) is pretty much the same thing.
BUT...
Sometimes the Metadata is more like metadudu.
Loyal
The old WGS84 the ellipsoid and the WGS84 datum mixup. I'll never forgive them for naming them the same.
WGS84 the ellipsoid is a correct designator. And just about the same as GRS80 ellipsoid. The difference is small enough to ignore. And you never see GRS80 on a map as a datum reference because it's not a datum. There is a WGS84 datum, but ITRFxx is widely used, NAD83(xx) gets closer to it with every major epoch revision.
The NGA (or is it NGIA) has an interesting policy with respect to their WGS 84 coordinates. Since it does not include a provision for velocities, they periodically "validate" and "improve" the coordinates of their network points.
Basically, they take IGS orbit data and compute new values. They assign a GPS week to their data products.
A 1996 report, TR96201, is linked below. The first link is to some articles used, evidently, for in-house training.
http://earth-info.nga.mil/GandG/coordsys/geoarticles/geoarticles.html
http://earth-info.nga.mil/GandG/publications/tr96201/tr96201.html
I guess that as part of their membership in the intelligence community they have to make information hard to find.
A little more searching revealed the following notice: http://earth-info.nga.mil/GandG/sathtml/IMPORTANT_NOTICE.html
which describes the close linkage between NGA and IGS products. Still haven't seen the GPS week tag for the most recent version of WGS.
Gotta keep up. 20 years of revisions, miss a few memos and then ...
"Consequently, rigorously speaking, a transformation between NAD83 and WGS84 should be interpreted as a transformation between NAD83 (CORS96) and WGS84 (G1150). Transformations between older realizations of NAD83 and WGS84 are now outdated and will not be discussed here."
"... Although on November 5, 2006, the GPS satellite orbits provided by the International GNSS Service switch to frame ITRF2005, everything mentioned above is still valid. Nothing practically will change considering that the differences between ITRF2000 and ITRF2005 are smaller than the differences between WGS84 (G1150) and ITRF2000. Except for accurate geodetic investigations these differences could be assumed negligible."
http://www.ngs.noaa.gov/CORS/Articles/WGS84NAD83.pdf
Larry Scott, post: 365881, member: 8766 wrote: Gotta keep up. 20 years of revisions, miss a few memos and then ...
"Consequently, rigorously speaking, a transformation between NAD83 and WGS84 should be interpreted as a transformation between NAD83 (CORS96) and WGS84 (G1150). Transformations between older realizations of NAD83 and WGS84 are now outdated and will not be discussed here.""... Although on November 5, 2006, the GPS satellite orbits provided by the International GNSS Service switch to frame ITRF2005, everything mentioned above is still valid. Nothing practically will change considering that the differences between ITRF2000 and ITRF2005 are smaller than the differences between WGS84 (G1150) and ITRF2000. Except for accurate geodetic investigations these differences could be assumed negligible." [/QUOTE]
Sorry. You must have missed the latest. Current version is WGS84(G1674) epoch 2005.0
See: https://confluence.qps.nl/plugins/servlet/mobile#content/view/29855173The magnitude of the changes in ITRF/IGS get smaller and smaller. As your posting indicates, for most board members the moot. The differences are now within the uncertainties.
I referred to the ellipsoids: GR80 and WGS84 as same.
Per NGS:
"...Please note that the GRS80 and WGS84 [ellipsoids] are considered to be the same. Actually, there is a very small difference in the flattening which results in the semi-minor axis, b, being different by 0.0001 meters. There is no known application for which this difference is significant."
http://www.ngs.noaa.gov/PC_PROD/Inv_Fwd/readme.htm
Scott Zelenak, post: 363035, member: 327 wrote: What baked my brain was that northings were positive and eastings negative.
Completely opposite to the sign convention.
I couldn't figure that one out until I read the last sentence on page 382.
While this thread has meandered into the issue of what does a tag WGS84 mean, I was wondering whether anyone can provide any additional detail on the use of South as azimuth origin. My recollection of a long ago discussion was simply that it was what the Germans did and we adopted their convention.
That's just the way it was back in the day.
Azimuth from south. Maybe due to the tracking stars as they cross the meridian for longitude: due south, As the ecliptic is in the south for North America. I've come across a few old descriptions with south azimuths, I've seen it textbooks. But I can't find s reference online at the moment.
But the USC&GS, or maybe even the Coast Survey did it that way for a long time.
I expect someone has a textbook with a description of south azimuth, but probably not a justification.
Darn, the answer to my question about the rationale for using South azimuth was too easy to find. See:
http://www.ngs.noaa.gov/PUBS_LIB/Azimuths_From_South_W_Lambert_1945.pdf
This is a 1945 letter from Senior Mathematician Walter Lambert. I recollected the bit about our copying German practice. Hail to CF Gauss.
There you go. And that dates back to practices from 1851 thru the late 40s.
"reckoned from south ... as we do now", for no specific reason. And I had figured that the ecliptic being south might be as good a reason as any.
How fun to find such a specific letter answering your same question. And in the opening paragraphs, it says "speculative elements"
Also note, sidereal time is on the ecliptic, so due south would be the direction to time the transit of low declination stars. Just like Julian day calendar increments on noon Greenwich - the sun, which again, is south.
Good find.