Never trust your GPS AEG

[joebartels]

Never trust your GPS units AEG reading. Here's the proof. It's from a modern Garmin.

Code: Select all

Point	LAT	     LON	      Elev	    +/-	AEG	AEL	TEC
1 	34.33991 	-111.0961 	5392 		0 	0 	0
2 	34.33991 	-111.0961 	5379 	-13 	0 	-13 	13
3 	34.33991 	-111.0961 	5392 	13 	13 	-13 	26
4 	34.33991 	-111.0961 	5397 	5 	18 	-13 	31
5 	34.33991 	-111.0961 	5376 	-21 	18 	-34 	52
6 	34.33991 	-111.0961 	5395 	19 	37 	-34 	71
7 	34.33991 	-111.0961 	5379 	-16 	37 	-50 	87
8 	34.33991 	-111.0961 	5392 	13 	50 	-50 	100
9 	34.33991 	-111.0961 	5391 	-1 	50 	-51 	101
10 	34.33991 	-111.0961 	5373 	-18 	50 	-69 	119
11 	34.33991 	-111.0961 	5400 	27 	77 	-69 	146
12 	34.33991 	-111.0961 	5376 	-24 	77 	-93 	170
13 	34.33991 	-111.0961 	5392 	16 	93 	-93 	186
14 	34.33996 	-111.0962 	5391 	-1 	93 	-94 	187

As you can see by the thirteenth point of data there hadn't been any movement. Yet 93 feet of Accumulated Elevation Gain was recorded. To make things even worse none of those elevation marks are near correct... the elevation for 34.33991 -111.0961 is 5,458 ft

To compound the issue the more data you have the worse it gets.

If you want accurate data you need to upload your route as a line track(definitely not as waypoints) to a mapping program such as TOPO! Then plot the route using between 100 to 200 points of data per mile.

[dysfunction]

Yes, I've had the exact same track post different AEG's every time.. over 20 times.. (interesting what you notice when you run the same trail) GPS is notoriously bad at elevation.

[Sredfield]

GPSJoe, what say you?

[fricknaley]

well, considering my GPS routinely tells me my AEG for Mt. Wrightson is about 500 feet, I'd have to agree with ya.

[azbackpackr]

Huh, I was going to ask you guys about this very topic, having read that 2009 Hiking and Climbing totals thread. I knew my GPS is not accurate on altitude, and maybe I should have bought that one that has a barometric altimeter in it as well, which would be more accurate than the triagulated satellite data. But anyway, I am just not going to worry about it too much, I guess.

Just hike, that's the main thing.

[Sredfield]

. . . Just hike, that's the main thing.

There's some distilled wisdom! Bravo!

[azbackpackr]

[nonot]

Barametric altitude will be even worse in accuracy if the weather changes while you hike!

[azbackpackr]

Huh, now I hadn't thought of that! Best to go by your map contours, I guess.

[johnr1]

Barametric altitude will be even worse in accuracy if the weather changes while you hike!

The more expensive GPS blends baro altitude with GPS altitude in order to try to get the best long/short term estimate. I find that you need to calibrate the altimeter when you start so that the initial filter stae is correct and then the altitude estimate will be a lot better. Accurate altitude estimating is very difficult, however, and it is pretty amazing to me that the handhelds do as well as they do.

[gpsjoe]

Just saw this thread. I never trusted the AEG number at any time. In addition to the problems Joe mentioned the barometric altimeter can't tell the difference between changes in weather and changes in elevation. It assumes all pressure changes are due to elevation change. Also if you hike in high winds the GPS will sense the pressure change and interpret that as an elevation change.

I always use a gut check to decide if I agree with the GPS. On most hikes you usually know what the high and low elevation is. So the difference between these sets a minimum AEG. To that you want to add the elevation change due to ups and downs that you hike. I have no precise way of determining that but I use my best judgement to get a ballpark figure for that. AEG is an illusive number but a reasonable ballpark figure is useful I think.

[hippiepunkpirate]

I don't have a GPS, so I use contours lines a lot for my estimates. With most peak climbs, I tend not to worry a whole lot because they usually consistently uphill will very little additional gain.

Here's a good example from Elden Lookout: http://hikearizona.com/location_g.php?GPS=6534
The AEG is 2424 feet, with the additional gain being about 40 feet, which sounds about right to me.

Now check out this recent post on Humphrey's : http://hikearizona.com/location_g.php?GPS=7189
The additional gain is about 6500 feet! The first 3.5 miles to the saddle is very consistently uphill. The 1 mile from the saddle to the summit has some ups and downs on the ridgeline, but I would guess 500 feet of additional gain at most, not 6500.

[Al_HikesAZ]

Now check out this recent post on Humphrey's : http://hikearizona.com/location_g.php?GPS=7189
The additional gain is about 6500 feet! The first 3.5 miles to the saddle is very consistently uphill. The 1 mile from the saddle to the summit has some ups and downs on the ridgeline, but I would guess 500 feet of additional gain at most, not 6500.

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[Vaporman]

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