gps problem can anyone help

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peterau

Peter
Joined
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Location
Adelaide, SA
Hi, I have a gps problem? and was wondering if there is any electronics whizzes that might have an idea on how to help me remedy it. It is a Garmin Oregon 600t. I save a way point directly off the oregon units gps map, and when I put the way points in google earth the plotted point is a half a k different . I also have an earlier garmin Oregon mdel and the results match up as they should perfectly. So I don't know if it might be a incorrect settings issue or if it will need to go somewhere for a fix. Also if it does need fixing does anyone know the best way to proceed. Hope someone can help. Peter
 
I think S&D is right, if the GPS and Google Maps aren't both set to the same datum coordinate system, the locations won't match up. Here is the relevant section from the Garmin manual:

1623647752_gps_datum.jpg
 
peterau said:
Hi, I have a gps problem? and was wondering if there is any electronics whizzes that might have an idea on how to help me remedy it. It is a Garmin Oregon 600t. I save a way point directly off the oregon units gps map, and when I put the way points in google earth the plotted point is a half a k different . I also have an earlier garmin Oregon mdel and the results match up as they should perfectly. So I don't know if it might be a incorrect settings issue or if it will need to go somewhere for a fix. Also if it does need fixing does anyone know the best way to proceed. Hope someone can help. Peter

If that is happening you need to set your Garmin to the same Grid system that Goole is using, In the setup page it has a list of about 30 or 40+ Grid/Map systems that are used across the globe and Some countries like Australia and the US change their measuring systems from time to time so you need to go in to that list and pick the right one,

Also If the US have some Military Skullduggery going on they oftem mess with the satelites making them read inaccurate by as much as a couple of Ks which would explain why google maps are 1/2 a K out, But most likely it might just be set to the wrong Grid Lat Longs.
 
As above.

Google uses, or at least used to use, WGS84, lat/long.

Your GPS Unit should use the same.

Even if you're using gda94 or gda20 you should not have a difference of more than a few metres.

SA, selective availability, used to be an issue when the US was worried about ICBMs etc during conflict but with most gps units able to track other satellite positioning systems from Russia, China, Europe etc simultaneously then selective availability doesn't really apply so much these days.

A 500m difference would point to a projection mismatch between GE, Google Earth, and your Garmin.

HOWEVER its possible that your Garmin simply did not have a good satellite positiin fix when you saved the waypoint, particularly if you had jist switched it on.

I would refer to the manual and garmin forums. I've never used one so apart from the above I cant add much else.

Good luck with it, it can be frustrating :)
 
Simmo said:

Well I'm Sorry but my manual says otherwise,

QUOTE:- *Subject to Accuracy degradation to 100m under the U.S. DOD-Imposed Selective Availability Program.

When they wrote that Info that was back in May 2000 but it was not done untill 2006,, At times even with 8 to 10 Sat's I have had mine tell me to take the next right turn to get back on the Highway and it was showing me I was about 3 or 400m driving through a paddock

They can also change the time on the Clocks which will put them out and I found some info about GPS Jamming,

This is an interesting Read,
https://www.intechopen.com/books/multifunctional-operation-and-application-of-gps/gnss-error-sources
 
I Googled the following

The American GPS system use WGS84
Google maps use WGS84
Microsoft Virtual Earth use WGS84

All are on the same datum system

I use Oziexplorer (laptop) and in turn can load Google maps into it. The PC based version allows me to check map calibration and on doing so I confirmed that the map datum is indeed WGS84. I can mark a track on Google Map on laptop, download it to my Hema or Tablet, show it on a Topo map and the tracks lines up.

I then loaded my Vicmap 1:25,000map and did the same, it's Map Datum is set to Australian Geocentric 1994 or GDA94 acronym. Auslig 1:250,000 is set to WGS84. If I mark a track or waypoint on either, they show the same location on either map.

My Hema navigator is also set to WGS84 (default) and I regularly transfer tracks and waypoints between it and the laptop 2 as well as my Garmin E20 handheld both ways. When I show them on the GDA94 maps, they are in the expected and correct position.

From my experience, transferring data form WGS84 to GDA94 shows no positional error within Australia.
 
Thanks very much for all your replies, I'm pretty sure you's have fixed the problem. It was on wgs84 but the format was wrong I had it on something like mm, mmmm I reset on the same one as my other gps, I didn't know this existed ,so once again thanks for your help . Peter
 
Copied from Geoproject.com.au

What is the difference between GDA94 and WGS84?
WGS84 is the datum used by the GPS system, and also for many small scale maps (e.g. maritime charts).


When GDA94 was defined, it was a requirement that coordinates of points in these 2 systems should be within a metre of each other. This was achieved, and is generally still the case in 2000. The spheroids used for WGS84 and GDA84 are also almost identical, and both systems are geocentric.

Thus for most mapping, exploration and GIS uses, WGS84 and GDA94 coordinates will be the same. If you use a GPS receiver (alone, or with a differential service) and the GDA datum is not a built-in option, you can select WGS84.

For precise surveys, however, the difference between WGS84 and GDA94 may be significant, and changes slowly over time. There are two reasons:

WGS84 is recalculated on a world-wide basis every few years for GPS control
Tectonic movements of the Australian plate affect WGS coordinates for Australian stations, but do not affect the GDA coordinates (see answer below "How will continental drift affect GDA94?").
The difference between GDA94 and WGS84 is approximately 45cms in 2000.


I also read some data on this site that states the Australian Tectonic plate moves at around 7 cm per year so at this point in time that adds a further 147 cm = 192cm total or less than 2 metres. Many handheld or vehicle GPS systems only give an accuracy of "Garmin GPS receivers are accurate to within 15 meters (49 feet) 95% of the time. Generally, users will see accuracy within 5 to 10 meters (16 to 33 feet) under normal conditions." (Copied from Garmin Support online)

Given the error v GPS accuracy the average punter won't see the difference. :) I would rule out this as a reason for a 500 M discrepancy.
 
FYI we buy GPS receivers for a few $100s. "Precise" GPS devices run into 10s of thousand $. Where I worked in Defence had one and back in around 2000 it cost over $30k and believe me, it was a lot more accurate than 5 M.
 
condor22 said:
FYI we buy GPS receivers for a few $100s. "Precise" GPS devices run into 10s of thousand $. Where I worked in Defence had one and back in around 2000 it cost over $30k.

I have had mine read an accuracy of 5ft/1.5m but it was within 6 inches or less from marker stones I placed there a week before.
 
condor22 said:
FYI we buy GPS receivers for a few $100s. "Precise" GPS devices run into 10s of thousand $. Where I worked in Defence had one and back in around 2000 it cost over $30k and believe me, it was a lot more accurate than 5 M.

The ones we use, with the marker in place are to the centimeter.

And the US don't play with GPS under the ISA system!

Covid never happened either! ;)
 
Simmo said:
condor22 said:
FYI we buy GPS receivers for a few $100s. "Precise" GPS devices run into 10s of thousand $. Where I worked in Defence had one and back in around 2000 it cost over $30k and believe me, it was a lot more accurate than 5 M.

The ones we use, with the marker in place are to the centimeter.

And the US don't play with GPS under the ISA system!

Covid never happened either! ;)

Simmo I think your joke about them not playing with the SATS might be real, My older GPS has the warning in the book but they said about not doing it anymore in 2000 and then in around 2006/2007 they agreed not to mess with them anymore, I bought another Garmin handheld about 4 or 5 years ago and sure enough it says this which is 8 years after the fact, So something is not right Ay.

You will notice that they have mentioned the US DOD info twice on 2 pages.

* Subject to accuracy degradation to
100m 2DRMS under the US DOD-
imposed Selective Availability Program.

DGPS: 10 ft. (3 m) 95% typical*
*Wide Area Augmentation System
(WAAS) accuracy in North America.
Velocity: 0.1 meter/sec steady state

AccuracyGPS:
<33 ft. (10 m) 95% typical*

* Subject to accuracy degradation to
100m 2DRMS under the U.S. DoD
imposed Selective Availability (SA)
Program when activated.
 

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