GPS Trouble in Kanangra

[HitchHiking]

We were walking around Mt Bulgin and Colboyd up at Kanangra. We hoped to follow Colboyd ridge down to the Christys. We were perfectly happy untill the ridge started playing out when we expected it to continue foranother few hundred meters at very least.The GPS was putting us exactly where we wanted and expected to be on the map(CMA edition 3). Now I of course then quadruple checked and tryed 3 datums, the accuracy and all other this and thats. Still nothing. We even got out a second gps (same model Etrex H) and had the same resuly. Accuracy down to 3m and we were clearly off by a good 230 Meters!

Made for some great route finding as we decided to cut across the gully that now blocked us from our wanted ridge.

I was curious to see if anyone else has had this sort of problem up there?

[Tony]

Hi Hitchhiking,

I had similar problems in the Monolith Valley last week, while deep in the valley coming down from Mt Owen, my GPS told me that I was very close to the Nibelug Pass Monolith Valley Track when we where at least 100-200 meters from it, most of the time the GPS showed we were right on the track.

Tony

[HitchHiking]

yeah it was very strange, we were using two different units (same model) at different times and only when the topography didnt match what we should have been seeing did we start to figure something was fishy.
It continued the whole trip.


Ill be back in the near future and will be able to see if it occurs again.

I now have some good curse words written on a random spur off of Mt Colboyd! haha

[phan_TOM]

I know that the yanks use to have the ability to mess with the accuracy of GPS, something about having an 'edge' on the competition (if you'd call it that) during one of their many wars, I wonder if it's something like that? Even though, I do recall that they had given up their control of GPS accuracy for the sake of land and maritime based safety some time ago... maybe it was just a glitch? another satellite spearing into the Pacific? disconcerting for GPS users regardless & it certainly highlights the need for competency with a map and compass.

[HitchHiking]

I know that the yanks use to have the ability to mess with the accuracy of GPS, something about having an 'edge' on the competition (if you'd call it that) during one of their many wars, I wonder if it's something like that? Even though, I do recall that they had given up their control of GPS accuracy for the sake of land and maritime based safety some time ago... maybe it was just a glitch? another satellite spearing into the Pacific? disconcerting for GPS users regardless & it certainly highlights the need for competency with a map and compass.

Yeah all possible from the rumors that float around.

No doubt have to have a grasp on the compass up the blue mtns

[MJD]

Seems more likely to be a problem with the datum used on the gps not matching the map datum. Or the cartographer made a mistake. How does a Google Earth image match up with the maps?

[taswegian]

yes datum could be the issue here, but also solar flares are a known issue with radio, phone, gps all affected.
They have been known to destroy radio communciations in past.
They really do play up with GPS and its not an uncommon phenomena.

But I would imagine its an AGD66 to GDA94 map datum problem and 200 metres is about the shift between the 2.

Here's a Datum conversion program if you want to see what the difference is in real figures.
http://www.binaryearth.net/
I haven't used it and so can't comment on it.

[HitchHiking]

Now I of course then quadruple checked and tryed 3 datums

When switching from 66 to 94 we went out from 400ish meters to 200ish.


The solar flares crossed my mind. BUt id didnt follow up on them. I am heading back soon and will see if the problem continues.

[MJD]

I did see the bit about checking the datums but it still sounds like that was the problem. Do you have (1) a gps track of your walk and (2a) a copy of OzTopo or (2b) know someone with a copy of OzTopo. If so, how does this match with everything?