Topo/ GPS Anomaly

Wentworth

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Hi All,
back from three days offtrack walking in Kanangra National Park. Beautiful scenery, the thickest scrub I've ever encountered. The Hakeas were growing so close together, I literally couldn't push through them. I had to lie ontop of them and kind of half walk/ slither my way across. Terrible!
We didn't end up doing everything planned as I was feeling rotten, couldn't coordinate my feet as required when doing offtrack, felt like a newborn.

We experienced some issues with our topo maps and GPS. We both carry GPS units not so much for navigation, more as a means to occasionally double check the grid reference to see that we are where we think we are. We're both competent navigators with just a topo and baseplate. But on a few occasions on this trip we encountered something strange.

When we were at an obvious feature (Pindari Tops) as indicated on the map, we double checked our position with the GPS, which indicated that we were where we thought we were.
When we arrived at another obvious feature, a huge stone outcrop, the highest point and unmistakable for anything else, the GPS indicated that we were 200m down the ridge.
Again, the next day when we were on the flat section of a peak called "Mount Great Groaner" (we were definitely on the peak, a long flat section a few hundred meters long), it indicated we were about 200m down that ridge.
On the return trip, the GPS indicated that we were where we thought we were. It just seemed to be in this one area, a few km wide.

We're trying to work out if it was a mapping error or a GPS error. We were both nav'ing indipenantly with our own topos, compasses and GPS and arrived at the same conclusions, also, the terrain was so obvious that it wasn't an operator error or nav problem. We were also definitely on the right map datum setting on the GPS for the maps.

Anyone experienced anything like this before?
 

Corin

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Only when the datum was not set correctly. Oh and it was really common back in the early 90's when GPS first came out.
 

Aussie123

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Could there be a magnetic anomolie in that area which may either throw out the gps, or have thrown out the original survey of the map ?

Probably not enough detail, but have a look here, I think all the meta data is available somewhere, but I can't actually locate it ! The meta data is approx 27 million line-kilometres of survey data (one point 1 km for every km fro the continent) (10GB of raw data) !

https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&catno=70282

EDIT - sorry this is the 2010 edition. The 2010 edition has only 19 million line-km of survey :triste:
 
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Aussie123

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Just thinking ...
Given that other features were "correct", and one or two areae "wrong"

When you were at one of the "wrong" features, you could have taken some bearings for a triangulation to correct points, then compared that to some map readings. Then may have shown an issue if the problem was with the map itself ?
 

Bartnmax

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Remember that although were generally not using Mil' satalite signals the satelites are still operated by the US & controlled primarilly by their military.
If their military are 'up to anything' they can & do degrade the civilian signs so they cant be used against them by enemy forces.
Sounds like the US mil might be 'up to something' ???
At least that's the way I understand things. Maybe some of the guys here with Mil background might like to correct me.

Bill A.
 

Aussie123

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Not sure about that. In the past - yes, the mil would and could deviate the satellite signal, although usually only regionally "as required".
I think that Pres Clinton passed legislation to ensure that for signals would not be deviated because of the growing civilian dependancy and use of the sats; I believe the military then launched their own, new sats for their purposes.

Many planes and ships now rely on civilian GPS, not to mention cars, surveyors and bushwalkers - so I don't believe that there is likely to have been any funny business with the signal.
 

bobmouse

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i had a similar thing happen in cradle mountain Tasmania although i was using a basic car gps. i was looking to find my accommodation for the night and was following the gps directions which didn't make sense. to cut a long story short a parks officer set me straight adding that "gps doesn't work here". i remember it because it didnt sound right.still doesnt. the gps had gotten to within 5km of the motel then became unreliable.
 

Dusty Miller

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Reflection from large objects or canyon walls can produce position errors (called multipath errors).
Also if all visible satellites are in a small section of sky, significant errors in position can occur.

The US can turn Selective Availability on at any time by changing the satellite clock data in the signal and changing the sat correction data. This reduces accuracy to +- 100 m. It was turned off May 1, 2000
There are two codes in GPS (any receiver you are likely to take into the bush is a code observable GPS system)P and C/A
P code is high res and used by military +- 1.5 m. C/A code is the civilian one. OVersmapling can improve this accuracy, which is why we get better results usually. ( ~15 m)

There are new tramissions planned on different frequency to give channels, don't know if they have gone through yet, hadn't in 2009.
 
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Bartnmax

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Not entirely sure of the details but I have also been led to believe that Russia also employ their own GSP sat system.
 

Wentworth

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Thanks for the responses all.
It was just in this one area, the areas either side were fine. We could see exactly where we were and the GPS confirmed it. We could have triangulated to confirm our position, but it was unnecessary as the nav was so clear as to make it impossible to mistake one feature for another.
Very odd... I'm inclined to think that the GPS was working and that it may have been a mapping error.
 
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