Barometric altimeter for determining high point

[DSunwall]

The most common method of determining which of several bumps on a ridge is the highest point is using a hand level. In cases where trees interfere with line of site, are altimeters an accepted method of verifing the highest point?

I am not suggesting we use them to determine the actual elevation just the difference between two points. Generally it only takes a few minutes to hike between the two points so a change in barometric pressures is not likely to make much difference. I do realize there is certainly possibilities of error, but, better than hunches.

I use the barometric altimeter on my GPS which does read to 1' intervals. I believe some wrist units have 10' intervals so they may not work in some cases.

I have looked at the GPS data track for Tarayall Pt. 11395, we did have good hand level site between two points that are close to the same elevation. Apparently someone else has decided that the north pt. is 4' higher than the spot elevation of 11391 given to the south point. My GPS data is pretty much in line with that, giving me further confidence in using these data tracks.

The yellow dot on the map corresponds with the blue highlighted data row.

[DSunwall]

This leads to me suggesting that the South East rise of Pt. 11060A in Park county is higher than the NW point as shown on LOJ.

http://listsofjohn.com/PeakStats/Climbers.php?Id=2363

Gerry Roach has this comment on his Retirement page.

Point 11,060 on the Topaz Mtn Quad has three summits with the 11,040-foot contour. With a field visit, I determined that the central summit is lower than both the western and eastern summits. It is hard to tell if the western or eastern summit is the highest, but I give the nod to the western summit with the extrapolated elevation of 11,060 feet.

[John Kirk]

The last two GPS elevation results illustrate my lack of trust - the elevation difference between those two contours is a bit much - 10965 vs. 11034 for the same contour height (theoretical maximum delta would be 39', whereas the difference is 69' in the example).

[DSunwall]

Notice that there was a 10995' reading for the NW point, just before the 10965, I didn't use that one because it showed just outside the 11K contour. that would put the difference right at 39'.

I understand the distrust, but in this case we really have nothing else to base it on.

With lack of hard evidence, leave it where it is, not that important.

[JoeGrim]

One thing I know for sure is that barometric altimeters can be quite off when it's windy. The sensor is so sensitive that when the wind blows over the device, it creates a small change in pressure inside; this is reflected in seemingly large changes in elevation. In calm conditions with a GPS that has had a lock on a lot of satellites, I would trust the elevation changes to a couple tens of feet over a period of less than an hour.

Of course, over the course of a hike, my eTrex Vista HCx can get quite off, primarily because of the change in atmospheric pressure. For example, if a high pressure system is moving into an area during the day, my elevation (which was calibrated right when I started) can be off by several hundred feet by the end of the hike. A rule of thumb for how this affects your GPS is 10 meters for every 1 mb in pressure change.

As for calibrating a GPS, the best way is to calibrate it at the beginning of your hike and places along the way where you know the elevation. If you calibrate it at home, the elevation can be way off by the time you get to your destination because the way pressure varies as you go up depends on several factors, including the vertical profile of temperature and moisture.

Probably more than you want to know, but here it is.

[DSunwall]

thats fine, I don't mind hearing about other experiences with altimeters. Some trips mine stays pretty close, I like to check the data track against the topo map when I get back, I will be watching all this closer from now on.

I do believe if you have autocalibrate set on it does a fairly decent job, even then it is good to calibrate at the TH. There probably are too many variables to rely on it for any point elevation checks, it may work at certain times but not always.

It was a nice calm day on 11060A though and no storms moved in later, so I am convinced the SE point is higher.

[DSunwall]

I have decided to give up on the idea of altimeters for point elevation differences.

[RyanSchilling]

The sensor is so sensitive that when the wind blows over the device, it creates a small change in pressure inside; this is reflected in seemingly large changes in elevation. In calm conditions with a GPS that has had a lock on a lot of satellites, I would trust the elevation changes to a couple tens of feet over a period of less than an hour.

Just curious, does leaving your GPS in your pack make a difference with the wind?

[JoeGrim]

Leaving it in your pack takes away the oscillations, so that's what I often do with my current eTrex Vista HCx when it's windy. It can get a pretty good lock on the satellites even in the pack. This might not work with older ones with the poorer antennae. (It doesn't help to know your altitude if you don't know your horizontal location )