Global GPS versus NZ map coordinates

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BruceHamilton

77 posts

Master Geek

#198979 30-Jul-2016 06:12

Maybe somebody can point me to a simple explanation on the Internet. The LINZ site was full of acronyms for different positioning systems, but didn't really answer my question. A recent news article noted that Australia's map system data was different to the GPS system by 1.6 metres, due to their continental drift north of 7 cm/year. They were initiating a correction now, but it would be based on where Australia would be in 2020, as the difference was greater than the accuracy of upcoming commercial GPS systems ( a few centimetres ) used for vehicles/drones etc.

NZ moves about 5 cm/year ( the LINZ site has a map showing the differing drift directions of the NZ landmass ), and our map coordinates seem to be based on 2000 with some form of date correction applied for drift and major earthquakes up to the current date. LINZ last month announced new vertical mapping of that was accurate to 3 cm.

My question is, why doesn't NZ just adopt the Latitude / Longitude / Height conventions of the GPS system as our map system to avoid current and future corrections?.

1eStar

1604 posts

Uber Geek

#1601178 30-Jul-2016 07:54

Your reasoning is a little simplistic. There are more variables than simply continental drift. In NZ there are many local variations, Mt Cook rising at mm/year. Places like Hanmer and Orewa sinking at mm/year. Some parts of canterbury were moved up to 3 metres relative to each other in the Darfield Quake. Etc. Then you can start the discussion on what projection your mapping system is based on, WGS vs NZMP. The world is not round, different models/geodetic shapes are used to represent the map information, it's not an xyz model. There is a calculator on the Linz site to convert coordinates as required.
But I'll leave further explanation to the surveyors who study this stuff for years before they get a degree in it.

Fred99

13684 posts

Uber Geek

#1601205 30-Jul-2016 09:19

I'd love to have it explained, as my (lack of) understanding leads me to believe it must be based on magic. Had surveyors here (next door actually) checking boundary positions, new pegs had been placed just before the Chch quake sequence, he checked them and determined that they were still correct (within a mm or two) but commented that locally they could be out by a few hundred mm or more if there was some mass ground movement, there were cases where that was a bit of a nightmare as it was found that looking at repairing / rebuilding damaged homes that the existing building had shifted outside the property boundary. Given that the whole place moved rather a lot - we went up about 1/2 metre and laterally a metre or so in a very uncomfortable few seconds, being able to nail our position as accurate to within a few mm must be magic.

GNS used to have GPS timeseries app where you could select a station and timescale, and it would produce a chart showing movement. You could see the movement - normal over time, then the big jump for each of the major Canterbury aftershocks. The app is still there kind of http://magma.geonet.org.nz/resources/gps/timeseries/ - but I can't get it to run with security settings (W10/IE or Chrome).

For an example of how complex it must be, here's an animated GIF showing movement in mm per day for a slow-slip event. Quite large differential movements are happening in NZ all the time - even when there aren't significant quakes:

daringpeter

49 posts

Geek

#1625130 7-Sep-2016 15:48

Just noticed this one. NO sovereign country would trust their land survey system to the weapons delivery system of a superpower....

tripper1000

1619 posts

Uber Geek

#1630577 14-Sep-2016 23:52

+1 the weapon comment above.

GPS position isn't the be all and end all.

Its accuracy varies depending on conditions in the atmosphere bending the signals and thereby altering the measured distance to the satellite. Your GPS comes up with a bunch of possible positions, depending on which combinations of satellites it is using and averages them out - that is what EPE or Estimated Position Error shows.

It is also possible for the US military to intentionally degrade the accuracy. They have not done so in a while, but GPS in NZ was near useless during the first gulf war - intentional errors of Km's.

WRT to the survey peg's - they may have all moved a meter or three relative to distant survey marks, however if all the pegs in the neighbourhood have move the same amount, in the same direction, it makes no difference to anyone in the neighbourhood - no one has lost or gained land. If the surveyor said "they've moved 3 meters", confusion would reign but relative to the neighbouring pegs they may still be "accurate" within mm's.