GPS device suggestions

JMunaretto · Post by **JMunaretto** » Mon Mar 03, 2008 2:30 pm

I'm thinking of getting a GPS device, but I've heard many aren't that accurate in calculating elevation. Any personal experiences with measuring elevation gain/loss on a hike?

Thanks,

Joe

pilot · Post by **pilot** » Mon Mar 03, 2008 2:49 pm

GPS units determine elevation by either triangulation with four or more satellites (GPS altitude) or with a built-in barometric sensor. All GPS units can calculate GPS altitude but not all of them have a built-in barometric sensor. Generally, for hiking, a barometric sensor will give you more accurate elevation readings than GPS altitude but because altitude readings change with barometric pressure, you still have to calibrate it to the local barometric pressure or a known elevation. I use a Garmin 60CS which has a barometric sensor and have found it to be pretty accurate, at least accurate enough for hiking.

Rob · Post by **Rob** » Tue Mar 04, 2008 1:03 pm

Joe, when I'm hiking I check GPS altimeter as often as I check time. Using GPS altitude I know how close I am to trailhead or summit, and I know how my hiking speed varies at different elevations.

For me, the altimeter reading I get from my handheld GPS is as accurate as I need when hiking. My Garmin Vista CX includes barometric altimeter, and in the GPS menu there are several options for altimeter settings but I've never messed with them to experiment with accuracy. My summit altitudes are within a few feet of official elevation, and when I'm driving in the mountains, my GPS agrees with elevation posted on road signs.

I guess that GPS altitude readings are approximately as accurate as GPS horizontal location readings, which can vary from very precise to very poor. My worst GPS tracks have been in the folds of Momyer Trail (with trees & hillsides blocking satellites) and along narrow creeks north of Idyllwild (hiking under tree canopies next to rock formations). I guess also that accuracy depends on how many satellites you have line-of-sight contact with at any given moment, and that depends on always-moving satellite constellations, as well as rocks, trees, rain, flesh, and vehicle bodies.

Some GPS units can run in WAAS mode, and while the goal of perfect accuracy is noble, I don't need that for hiking, so I disabled WAAS in my handheld. Also, when I started researching why mountain elevations change and found that reference planes change occasionally (e.g., WGS84), I realized that precise navigation beyond a realistic level of accuracy was pointless.

Do land surveyors allow their GPS units extra time for more precision?

Rob · Post by **Rob** » Tue Mar 04, 2008 1:24 pm

My handheld GPS calculates total elevation gain continuously for a hike, but a trick to using that feature is remembering at trailhead to reset values to zero and doing so carefully to avoid deleting other stored data (such as waypoints). After a hike, after transferring track data to my computer, I view the elevation profile of my hike on my PC and tally elevation gain manually. My handheld has a window that shows me elevation profile, but I don't find that window useful during a hike.

Recently I started paying attention to a GPS reading that tells me how much time I accumulated "stopped" and compare it to time "moving." It's put pressure on me to waste less time on rest breaks or shooting photos.

AlanK · Post by **AlanK** » Tue Mar 04, 2008 2:05 pm

I'll look for a reference, but I recall that the vertical error for a typical GPS unit is about 1.5 times the horizontal error. Under good conditions, the GPS altitude will be that good with or without a barometric altimeter.

If a GPS has a barometric altimeter, it is used to improve the stability of the readings as the user moves. The barometric elevation readout is frequently re-calibrated internally by the GPS. In most cases, the user never resets the barometric altimeter in any way. In fact, it's easy to forget it's there.

Generally, the elevation given by a good GPS is accurate enough for hiking purposes, as mentioned by others already.

RichardK · Post by **RichardK** » Sun Mar 09, 2008 11:18 am

Here is a good discussion about GPS altitude accuracy:

http://gpsinformation.net/main/altitude.htm

I think that the altitude accuracy of GPS receivers is inherent in the system. Altitude accuracy is going to be 1.5 times horizontal no matter which receiver you use.

hillbasher · Post by **hillbasher** » Mon Mar 10, 2008 10:24 am

Was on top of Baldy yesterday and my Garmin Etrex Legend had it at 10,062 feet. Pretty damn close in my book.

JMunaretto · Post by **JMunaretto** » Mon Mar 10, 2008 10:41 am

thanks for the replies. I guess I'm just evaluating what I need. I'm more interested in getting decent altitude info so maybe an altimeter is good enough, as it doesn't seem a GPS unit will improve that.

Post by **Taco** » Mon Mar 10, 2008 11:08 am

FWIW Joseph, my Casio pathfinder watch has an altimeter. In most weather (unfortunately when I don't typically require it), and after "calibration", it's been accurate to within 80ft.

hillbasher · Post by **hillbasher** » Mon Mar 10, 2008 11:36 am

Avocet makes some nice watches with altimeters in them, but at close to $200 , I think I would rather have a good GPS, and I do have 2 of their watches which I used for years before getting my Etrex Legend.

JMunaretto · Post by **JMunaretto** » Tue Mar 11, 2008 6:35 pm

yeah, 80ft is good enough, I'm not that picky!