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eTrex 30 Elevation Issue

Hi all - newbie to the forum. Ihave an eTrex 30x and am having some issues with elevation.

I have noted that, during recent loop hikes, that the total descent always exceeds the total ascent. Not by much...25' - 75' but always the same pattern. Maybe that's within the margin of error but when I download the track, I can see that the elevation of the last point on the track is 25' - 75' lower than the 1st point (both points obviously recorded at exactly the same spot). The entire difference between ascent and descent is exactly the elevation difference of the 1st and last track points. So, the unit is accumulating ascent/descent properly but changing its elevation reading for a common spot.

I figure that this has something to do with calibration and/or settings.

Steps I normally take:
1. Turn the unit on 10-15 minutes before I start the hike
2. Calibrate the GPS to the elevation showing on two iPhone apps that are usually with 5' of each other.
3. Hike
4. Save Track

My altimeter settings:
* Auto-Calibration = On
* Barometer Mode = Variable
* Pressure Trending = Save When Power On
* Plot Type = Elevation/Distance

In addition, my track settings include:
* Record Method = Auto
* Recording Interval = Most Often

I don't mind if the elevation is wrong if it is "consistently wrong". In other words, give me same (or close) elevation reading at a common spot 3-4 hours apart.

I am wondering if anyone else has had this issue and, if so, what setting (or process) changes will fix this - thank you in advance.

John

Comments

  • nevw 38 Points
    Could this be due to diurnal pressure variation. Ingnoring the effect of any movement of different pressure systems through the area, there a daily cycle in pressure ever 6 hours reaching a maximum at 10am and falling to a minimum at 4pm. So if you set off at 10am and finish at 4pm there will be a drop in pressure (assuming other effects are neutral). From memory it is about 1.5 to 2 hPa drop and this will be reflected in your elevation reading as it is correlated to pressure.
    Again from memory each hPa equates to 30 feet.
    So if you regularly set out mid morning and finish mid afternoon you would have to take the variation into account.
  • ReachELS 0 Points
    If your memory is correct, that would imply a 45' - 60' altitude change. Assuming that we are talking about an altitude drop, that is darned close to the 25' - 75' elevation drop that I am seeing. We do regularly set out mid-morning and finish mid-to late afternoon.

    Thank you for your feedback - assuming this to be the case, what would (theoretically) be the "correct" ascent? Ascent per my GPS (too low), descent per my GPS (too high) or the average of the two? I am thinking average?
  • nevw 38 Points
    What if you climbed 500m in the first hour and there was neglibible diurnal pressure change. That would mean the asent was correct or very nearly so. Then you spent the next 5 hours slowly descending back to your start position during which all the diurnal pressure change occured. That would mean that the descent figure given by the gps is wrong and be too small. But if you go out and back and end at the same place you started ascent must equal descent so all the correction must be applied to the descent to get the correct figure for descent. So an average would not work in this case.
    But an average between the ascent and descent if the hike was spread evenly in ascent and descent during the day would often be ok.
    Probably best would be to work out the fraction of time of the hike spent ascending and descending and apply that fraction of a correction to each.
    You could probably apply corrections using that method during your hike and see how close you get to correct figure by days end.
    You could probably send an email to you local meteorological office and mention what you are up to and they are likely to be able to give you figures for the average diurnal pressure change in 3 or 6 hourly intervals during the day for each month of the year for the nearest airport or possibly another place near your hiking area.
    Then you could split those further to apply corrections during your hike.
  • ReachELS 0 Points
    Thank you for your suggestions. I could also quit being so darned anal retentive and simply treat these elevation figures as estimates :).

    But I did find the diurnal pressure change topic extremely interesting - did quite a few google searches. Quite a bit of research was based in Australia.

    I was not aware of the cyclical nature of the pressure peaks and valleys...I was an accountant, not a scientist. I guess it makes sense is you think about the ocean tides and all.

    Thank you for your input.

  • Boyd 1960 Points
    edited April 10
    You could also just use the elevation data from the map. I don't really know anything about the eTrex 30 so I'm not sure if that is possible or what settings you would use. Basically, there are three methods of determining elevation. The sensor, the satellites or the map.

    Satellite data is wildly inaccurate on most devices, GPS has a much higher horizontal accuracy than vertical accuracy.

    "Real" Garmin topo maps contain Digital Elevation Model (DEM) data. Garmin's 24k topo maps have fairly high resolution DEM and their 100k topo is lower resolution. Third party maps don't have DEM because Garmin has never disclosed their format. Within the past year third party mapmakers have finally "cracked" Garmin's system however, so this situation should change.

    Anyway, if you use DEM for elevation, it won't change with the atmospheric pressure. :)
  • truckinguy 115 Points
    edited April 15
    You'll never get the elevation to be that spot on.. it's just the nature of the beast with gpsr's. to be off by 75 ft isn't bad at all. A simple change in the baro pressure like a weather front coming it or leaving will change it a lot. Elevation is not like over the ground data.. over the ground can be fairly accurate. Elevation can't be very accurate on these units.. Google GPS Elevation,etc. and you'll learn alot.
    Accurate elevation is a very fine science a few know how to use and do.
  • Boyd 1960 Points
    edited April 15

    You'll never get the elevation to be that spot on.. it's just the nature of the beast with gpsr's.

    A blanket statement like this isn't quite true. I'll grant that what you have said is relevant to using the barometer or signals from satellites. But very accurate DEM data is now available for free from the USGS. As I wrote above, the problem has been that Garmin's DEM format was not understood in the past, since they have never published it. But with recent developments, I'm hopeful that we'll soon be able to use this data in our third party maps.

    The USGS is currently creating digital models of the earth using LIDAR at a 1 meter horizontal resolution. Imagine a grid of 1 meter (39 inch) squares covering the landscape. With one meter DEM, each of those squares has a bare earth elevation value accurate to within 19 centimeters (about 8 inches) with 95% confidence (although heavy vegetation could degrade accuracy).

    See this: https://pubs.usgs.gov/tm/11/b07/tm11-b7.pdf

    There is no technical reason why this data can't be used in a map on your GPS.
  • truckinguy 115 Points
    You can find accurate gps it in a map either paper or computer but the common Garmin gps having the ability to calculate and show in the data box the true elevation consistently will never happen.
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