I've read/heard somewhere that there may be certain compatibility or functioning issues with GPS devices in the Southern hemisphere, particularly S. America.
Is there any truth to this? What do I need to watch out for?
I mainly carry a GPS to use as a backup compass, to track waypoints in the event of a whiteout, and as an altimeter.

I would just ditch the GPS & just do it old school style. The old-timers didn't need them, & you can do it without too! Sack up and do it right. Problem solved.

I've used GPS in Peru, no problems. The only compatiblity issue is that the WAAS corrections will not be available to it, so it will only be as accurate as GPS was before WAAS came along (my unit is not WAAS compatible anyway) - not a real big deal. Also, I think the baseline for altitude (reference geoid?) that some GPS units use doesn't accurately take into account the additional tectonic mass of the Andes, so the elevation numbers tend to be a bit high in the Andes - this is probably true in other major mountain ranges as well.

Day Hiker used his 60csx on Aconcagua. Bring Li batteries. WAAS coverage is really not needed, given the new sensitive chipsets.

I would guess Buz is right about the geoid correction; maybe Day Hiker can tell us his experiences on Aconcagua. The GPS ellipsoid-geoid correction is proprietary, and differs among companies. It is thought that most (newer) units take the calculated ellipsoid, and use interpolation in a look-up table to generate the correction; I'll bet a lot more care has gone into the correction in NA, than in SA.

I would expect that the USAF needs to bomb stuff in the southern hemisphere as well, so I'd be shocked if GPS doesn't work there...

MoapaPk wrote:Day Hiker used his 60csx on Aconcagua. Bring Li batteries. WAAS coverage is really not needed, given the new sensitive chipsets.

I would guess Buz is right about the geoid correction; maybe Day Hiker can tell us his experiences on Aconcagua. The GPS ellipsoid-geoid correction is proprietary, and differs among companies. It is thought that most (newer) units take the calculated ellipsoid, and use interpolation in a look-up table to generate the correction; I'll bet a lot more care has gone into the correction in NA, than in SA.

It worked just fine, of course. My understanding is that it will work anywhere on the planet, unless you're shielding it from receiving satellite signals.

I took time-averaged readings on Aconcagua's summit with two GPSs, and the elevations were 22850 and 22855 feet. Considering the fact that peaks in the contiguous U.S. are not even surveyed within 5 feet of precision, I have little more belief in the "official" value of 22841 in Argentina than I do the readings I got. But even a time-averaged GPS reading on a summit (excellent, unobstructed view of the sky) cannot be trusted to within 10 feet, from my experience.

In other words, the readings I got were within the technology's precision of measurement of the "official" value, so I have no reason to suspect geoid or other error there.

I never use WAAS. Never needed it anywhere.

kheegster wrote:I would expect that the USAF needs to bomb stuff in the southern hemisphere as well, so I'd be shocked if GPS doesn't work there...

Yes, excellent point!

kheegster wrote:I would expect that the USAF needs to bomb stuff in the southern hemisphere as well, so I'd be shocked if GPS doesn't work there...

SA, not southern hemisphere. Australia probably has good corrections. Hugo Chavez tells me Venezuela's geoid corrections are excellent.

Geoid corrections affect altitude only. The general x-y corrections are all the same, regardless of GPS manufacturer. There are other ways to do the "correction" for military uses -- like just have measured GPS altitudes for places of interest.

As the others have said, GPS will work fine. But your compass may not. Most compasses are either Northern or Southern hemisphere versions. It has to do with the way the needle is balanced.

See this for more information:

http://www.suunto.com/suunto/main/produ ... ID=il8uqxU

Day Hiker wrote:I took time-averaged readings on Aconcagua's summit with two GPSs, and the elevations were 22850 and 22855 feet. Considering the fact that peaks in the contiguous U.S. are not even surveyed within 5 feet of precision, I have little more belief in the "official" value of 22841 in Argentina than I do the readings I got. But even a time-averaged GPS reading on a summit (excellent, unobstructed view of the sky) cannot be trusted to within 10 feet, from my experience.

My GPS is older and only does time-averaged readings for position, not altitude. The reading that I photographed on Pisco was 23 feet higher than the "official" value - still not bad.

edl wrote:As the others have said, GPS will work fine. But your compass may not. Most compasses are either Northern or Southern hemisphere versions. It has to do with the way the needle is balanced.

Thanks for that info! It makes sense, but I wouldn't have thought of it beforehand.

Not that I'll ever go to SA south of the equator...

edl wrote:As the others have said, GPS will work fine. But your compass may not. Most compasses are either Northern or Southern hemisphere versions. It has to do with the way the needle is balanced.

True but if you buy a compass with a global needle, like the Suunto MC-2G, you will not have this problem.

Day Hiker wrote: Considering the fact that peaks in the contiguous U.S. are not even surveyed within 5 feet of precision,

Actually, optical surveys (or newer GPS surveys with multi-frequency or differential against a surveyed base-station) can be quite accurate relative to the reference point -- down to cm accuracy.

Trouble #1: the reference geoid changes a lot. The USGS and geodetic survey recently changed the reference geoid, and that change added ~5' to the elevation of many NV peaks. Whitney is now over 14500' with this new reference.

Trouble #2: Many of the newer USGS maps are "provisional" and I somehow doubt they will ever be finalized. The elevations were largely determined by photogrammetry (pretty much an average of +/-5m), and many of the "T" checkpoints were never confirmed by optical surveys.

MoapaPk wrote:

Day Hiker wrote: Considering the fact that peaks in the contiguous U.S. are not even surveyed within 5 feet of precision,

Actually, optical surveys (or newer GPS surveys with multi-frequency or differential against a surveyed base-station) can be quite accurate relative to the reference point -- down to cm accuracy.

Trouble #1: the reference geoid changes a lot. The USGS and geodetic survey recently changed the reference geoid, and that change added ~5' to the elevation of many NV peaks. Whitney is now over 14500' with this new reference.

This is part of what I was referring to. If the official elevation of a peak (a whole assload of them, actually) is X one year and X+5 another, at least one of those numbers is wrong. So, they can optically survey to 1cm accuracy, in relation to . . . some other place, like their base station, which itself has a +/- few feet elevation measurement error.

If it's possible to accurately measure a summit to a precision of 1cm, that summit elevation could be published, down to the nearest cm, and it would not change from measurement to measurement, nor would it change from year to year, except for actual geologic causes obviously.

How many different numbers have we seen for Mount Whitney? 14494, 14495, 14497, and "over 14500." WTF? And that peak I am sure has had more precise efforts on it than most others.

is there a GPS that calculates global warming induced sea level rise as a zero point in measuring summit elevation?

sixfingers wrote:is there a GPS that calculates global warming induced sea level rise as a zero point in measuring summit elevation?

Ha ha! Has it even risen a foot? Only the Everglades would notice that.

Elevations are based on a mathematical model of Earth and not on the actual day-to-day or year-to-year level of the ocean.

(Yes, I know you were joking.)