How accurate? Montana elevation data

[The Bigfella]

This is a screen grab from a section of the Sangkhlaburi - Umphang trip that I did recently with KTMPhil and BSABob

I know it was steep in places... but this steep?

 

[LivinLOS]

Guessing its GPS inaccuracy showing up..

Does the montana have a barometric altimeter like the 60csx ones ??

 

[The Bigfella]

I wasn't aware of that.... so I just checked, and yes... its barometric. I wasn't breathing on it.... honest.

Incidentally, back in the early days of GPS... about 24 years ago, IIRC, I had my height measured by GPS. It was pretty damn accurate too, but it took a while. I was doing some work for the then Land Information Centre in NSW and they were doing some displays of emerging technology.... laptops with maps that moved.... revolutionary stuff. They had the access to the full military spec setup, so, with a bit of computing power, they got things very accurate.

Back in those days they were doing some trial mapping up where you are too... using ultralight aircraft.

 

[LivinLOS]

Yeah mil spec GPS goes to a lot more degrees of accuracy than the consumer stuff.. I dont think we have WAAS or EGNOS options here in asia ??

 

[KTMphil]

This is the elevation data recording from a Garmin Montana of a long long link up we did a few days ago in the Omkoi region, quite interesting

 

[Loop]

Makes you wonder how much traction you'd be getting @ 74%? that's getting right up near vertical.

Sounds a little out to me but of course some small section can be quite steep.

 

[Tom]

Yeah mil spec GPS goes to a lot more degrees of accuracy than the consumer stuff.. I dont think we have WAAS or EGNOS options here in asia ??

As of fairly recently, there is actually an SBAS (Satellite Based Augmentation System - like WAAS/EGNOS) available here in Northern Thailand, the Indian GAGAN-system (GPS Aided Geo Augmented Navigation, built by Raytheon). If you switch on WAAS/EGNOS on your unit, you should see satellite 41 (or rather, NMEA ID 41) show up on your satellite status page. This number, although not it's real PRN, corresponds to GSAT-10, one of the GAGAN-satellites (http://garminmontanagpsr.wikispaces.com/Satellites). I have not been able to find any information on where its base-stations are located, and normally you would want to be no more than 500 km from the nearest one (WAAS, for instance, has 38 stations scattered all over the US to achieve this). Looking at the coverage area for GAGAN it looks like there must be some base stations not too far away from Thailand, but some actual measurements in the field with and without SBAS enabled are needed to be able to tell how effective it actually is here.

As for the OP's question about accuracy of the (GPS-based) elevation-data, well that is always the weakest part of any GPS-position, simply due to the geometry of the satellites. Normally the error on the elevation will be 1.5-2 times the horizontal error. This can of course be exacerbated by both periods of high DOPs (DOP = Dilution Of Precision) due to poor satellite geometry, poor reception (and thus again high DOPs) due to topography as well as inclement space weather (solar flares).