You do know that difference between wgs84 and nad83 or grs80 to be exact is less than 1mm?
You do know that difference between wgs84 and nad83 or grs80 to be exact is less than 1mm?
BROUHAHA!!!!
That explains a lot!
🙂
You obviously have no idea what you are talking about.
I tried to nicely point out that you are comparing apples and oranges and you try to make it look like I am the one that is lacking in knowledge. Hardly
thanks for all the input, I have really learned a lot from your expertise.
thanks for all the input, I have really learned a lot from your expertise.
I don't think you have. Maybe someday.
When I taught, I looked for teachable moments; now I look for learning moments, and there are several within this thread.
Experimenting as you did is commendable. Sometimes an hour spent that way is worth a week spent in other ways. To make it worthwhile, though, you really must look at the data exhaustively. Let me show you what I mean.
You provided some output from OPUS, I think, that showed both NAD 83 (2011) coordinates and ITRF2014 coordinates. From that output, you chose the UTM coordinates based on NAD 83. That's fine, but look also at the ITRF2014 lat/lon and note that they are different from the NAD 83 lat/lon. That's evidence of an apples-to-oranges comparison and should be a warning that NAD 83 and ITRF2014 at the epochs chosen produce different coordinates, something that @John-Hamilton pointed out. Note that, if lat/lon are different, then any plane coordinates produced from the two will also differ.
To reinforce the point, consider the output below from NGS' HTDP that converts the NAD 83 position to ITRF2014, each at the requisite epoch. Note the close agreement with the OPUS output from your post and the migration.
HTDP Output
****************************************
HTDP (VERSION v3.2.7 ) OUTPUT
TRANSFORMING POSITIONS FROM NAD_83(2011/CORS96/2007) (EPOCH = 01-01-2010 (2010.0000))
TO ITRF2014 or IGS14 (EPOCH = 12-02-2016 (2016.9183))
LATITUDE 16 07 0.48511 N 16 07 0.50752 N 17.91 mm/yr north
LONGITUDE 239 19 5.97983 W 239 19 5.98541 W -77.08 mm/yr east
ELLIP. HT. 140.403 142.622 m -1.78 mm/yr up
X -3127522.495 -3127523.343 m 69.70 mm/yr
Y 5271188.424 5271190.178 m 33.59 mm/yr
Z 1759187.803 1759189.080 m 16.71 mm/yr
P
NGS HOME PAGE
Clearly, ellipsoids and epochs are important and any position comparisons should not mix them.
As to what to compare, the XYZ positions are a good choice; three dimensional and easy distance-off calculation.
Your AusPos output did not show distances and you did not take the time to compute them. But the correlation of variability with distance is the whole point of the discussion. AusPos did provide positional uncertainties. A graph with uncertainties on the y-axis and distances on the x-axis would have gone a long way to demonstrate which assertion is correct. It's not too late to do that and the learning value from doing it is very high.
The Eckl paper that @GeeOddMike provided is worth reading. Although the work was done in 1998, when you prepare the graph from your data, it's likely that the paper's results will prove still to be valid.
We all shoot from the hip sometimes, but careful and thoughtful analysis of the data before shooting can eliminate a lot of misunderstandings.
By the way, are you in the Philippines or at one of the sponsor organizations?
@jt50;
I think I might be able to shed some light on this misconception; some programs use WGS84 as a default description for the base GPS coordinate system. Even though the actual coordinate is (at least in the states) NAD83. This leads users to believe that they are surveying in WGS84 coordinates when they are actually in NAD83. Of course NAD83 was developed to model North America, hence it's name, it is not the same as WGS84, think 1000mm, not 1m.
Yes I am working in southern part of Luzon. We usually do not use online GPS processing services because we have heard since day 1 that accuracy of GPS positions is dependent on distance from base. You get that information from every GPS/survey equipment vendor. That is the first part of their product brochures.
You know what would be very useful? A detailed explanation from the vendors themselves. Does accuracy depends on distance? On observation time? Because if accuracy depends on time duration of observations then we could get all our positions from OPUS by just leaving all gps receivers overnight.
Accuracy vs time duration is only for static right? So RTK is covered by the 5mm +1ppm limit. What about NTRIP? Radio?
Anyone from a vendor here?
Why not calculate the distances between your AusPos positions and graph them against the uncertainties? You probably have the means to do that efficiently, but here's an alternate resource if you need it: https://www.ngs.noaa.gov/cgi-bin/Inv_Fwd/inverse3.prl
Not a lot of work for the value of the information.
