Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

GPS Question

Old 08-08-05, 08:09 PM
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Stjtoday
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GPS Question

Maybe someone who is familiar with GPS units can help me out here. I have a Garmin GPSMap 60CS. Use it mainly for mapping routes and providing elevation data. I reset the trip odometer and elevation log before each ride. When I return from a ride and download the "track" file off the unit, the data on the track never matches the data on the trip or elevation "page". If my elevation monitor records 2000 ft. typically my "track" file will show 2200 ft., with all the supporting data hills ect. ect. (I am certain the "track" file I am reading only contains the data recorded from that ride.)

Which is the more acurate read? The actual trip elevation indicator, ("page"), or the "track" file downloaded upon completion of the ride?

Any help would be greatly appreciated.
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Old 08-08-05, 08:36 PM
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I have a Garmin Forerunner 301, but have a coworker who loves his GPSMap 60CS. I would think that with three or more satellites (creating a 3D perspective of your location) locked onto you, that calculation should be very accurate. However, my friend recently activated his 60CS during a cross-country airline flight and got weird numbers. His elevation was always shown as 3-4000 feet when he was actually at 30,000 feet.
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Old 08-08-05, 08:59 PM
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I think GPS is still "ify" until you get to the level of GPS survey eguipment or commercial quality aviation units.

The question of SA (selective availability) is supossedly answered, but I wonder if the Air Force has truelly "opened up" the GPS signals....a few nanoseconds would be just enough to alter a single point reading slightly, but cumulative figures would be affected far more.

Another factor to consider is that the advent of very accurate GPS is rewriting the maps of the world. Your base data map is probably based on WGS 84 data but your GPS is, possibly, recording accurate real world 2005 data.

Remember too, that your "cumulative" reading might only be an inch or two off, added over how many thousands of points?

Last edited by RhumbRunner; 08-08-05 at 09:06 PM.
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Old 08-08-05, 09:01 PM
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I am very very skeptical of the elevation data I get from Garmin Etrex Legend. I can at 20 feet or better accuracy and watch the elevation reading scroll up 20 feet, down 15 feet, down 8 more feet and then back up 17 feet, all in the course of a few minutes. I think the hand held GPS units handle X and Y coordinates (lat and long) much better then Z (elevation). It may be due to satelite position, but I have rarely gotten elevation data that I trust from my GPS.
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Old 08-08-05, 09:07 PM
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I don't know about the units discussed at the start of the thread, but the eTrex vista gives a barometric altitude reading, with a calibration feature that, eventually, brings the barometric reading in line with the GPS satellite-calculated elevation. But that calibration can take a while, and in the meantime you're off to somewhere else.

Both the barometric and the satellite elevation are subject to a lot of fluctuation. I have sat in one spot and watched the satellite-calculated elevation change more than thirty feet, in just a few minutes. Altitude readings are obviously harder for the satellite-based system to calculate than latitude or longitude.

You just can't expect a handheld altimeter to be accurate within such a small distance, particularly not if you move. Even the altimeters they put in airplanes are not that accurate. So if you want to know the true elevation change, your best bet is a topographical map.
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Old 08-08-05, 09:43 PM
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Originally Posted by steveh2
I don't know about the units discussed at the start of the thread, but the eTrex vista gives a barometric altitude reading, with a calibration feature that, eventually, brings the barometric reading in line with the GPS satellite-calculated elevation.
The GPSMap 60CS combines both barametric with GPS satellite to improve it's elevation readings. I'm not all that concerned with the pinpoint accuracy of my elevation readings. I'm very comfortable with the degree of error in the units, weakest signal I have seen is within 40 ft accuracy.

The think that's bustin my nut is the unit has two means of recording, a data file called track and a so called trip recorder.. real time display of your elevation gain/descent. Why the heck can't the two of them match up, and if not is one the measurements more accurate than the other.
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Old 08-08-05, 10:54 PM
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Just out of curiousity, where does one find one of these topographical maps on the internet? I have searched and have found none that are free. I was trying to figure out the grade of the climb on this trail in Kent, WA.

-Jason Keller
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Old 08-08-05, 11:08 PM
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Originally Posted by RhumbRunner
Your base data map is probably based on WGS 84 data but your GPS is, possibly, recording accurate real world 2005 data.
Like RhumbRunner said, could it be the "track" calculations are based on legacy GPS elevation data whereas the trip recorder is showing you data based on up-to-date elevation data?
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Old 08-09-05, 01:43 AM
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Originally Posted by Stjtoday
...When I return from a ride and download the "track" file off the unit, the data on the track never matches the data on the trip or elevation "page". If my elevation monitor records 2000 ft. typically my "track" file will show 2200 ft. ... Which is the more acurate read?
Hi,

I've experimented with both a Garmin GPSMAP 60CS and a Garmin eTrex Vista. The details here pertain to the Vista, which I prefer for cycling. I don't know which of the readings is more accurate, but that won't stop me from rambling a bit:

In my experience, the total ascent/descent figures I calculate from the track log are a bit higher than those indicated directly by the unit. I suspect that the unit is performing some sort of data smoothing on the barometric altimeter readings before adding them to the ascent or descent totals, and as a side-effect they miss some shorter/slower sections of vertical travel. I think the track log altitudes recorded are read from the baro-altimeter, versus the satellites, so it comes down to how those are post-processed. (If track log altitudes came from the satellite "altimeter", I'd consider that the larger ascent/descent numbers could be caused by the unit interpolating positions between satellite calculations and over-shooting at slope changes.)

After playing with the units for a bit, I've settled on turning the barometric altimeter auto-calibration off, and calibrating the units manually with the GPS altitude at the start of each trip (with a good satellite lock). If I'm at a place I've seen many altitude reading of (e.g., my driveway), I'll just manually enter the altitude. I ditched auto-calibration upon noticing that, after climbing the same steep hill repeatedly along the same course (in the name of science!), the altitude profiles of the same road in the track log drifted with each successive pass. Eliminating auto-calibration got rid of this, as far as I could tell. So, in my experience, auto-calibration introduced more systematic error than arose from barometric drift (which is AFAIK the whole reason for auto-calibration to exist). My test rides were over periods of 1-2hr, in elevations of about 0-800ft as reported by the GPS, with no significant storm systems moving through (this being San Diego and all...).

When collecting cycling track logs for analysis, I set the the track log mode to "auto" and "more often", and I always download the "ACTIVE LOG" directly to the computer instead of having the unit "save" it first. Saving the track log within the unit seems to drastically reduce the resolution of the data, and also discards timing info. I run the GPS receiver in "normal" mode, without WAAS and without "battery saver".

Hope this helps,

-JAB
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Old 08-09-05, 02:03 AM
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Originally Posted by killahkosha
Just out of curiousity, where does one find one of these topographical maps on the internet? I have searched and have found none that are free. I was trying to figure out the grade of the climb on this trail in Kent, WA.

-Jason Keller
Try this:

http://www.topozone.com/

or this:

http://terraserver.microsoft.com/
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Old 08-09-05, 05:13 AM
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I have the Garmin 60CS also...though never tried what you are doing.

I would think that the "page" is set to update on intervals....so it may miss some points. The track data would be continually recording (also at intervals, but much faster ones) thus more accurate.

However....keep in mind that elevation is the hardest position feature to compute for GPS and not always that reliable.
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Old 08-09-05, 05:28 AM
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Great info Jab, makes good sense. Thanks all for the input.
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Old 08-09-05, 07:49 AM
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Originally Posted by sunninho
I have a Garmin Forerunner 301, but have a coworker who loves his GPSMap 60CS. I would think that with three or more satellites (creating a 3D perspective of your location) locked onto you, that calculation should be very accurate. However, my friend recently activated his 60CS during a cross-country airline flight and got weird numbers. His elevation was always shown as 3-4000 feet when he was actually at 30,000 feet.
The 60CS uses barometric pressure to determine altitude.
The airline cabin air pressurization would mess up any reading he was getting.
I think they typically pressurize to a higher altitude than 3-4000 feet though.
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Old 08-11-05, 07:08 PM
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Originally Posted by zippyh
The 60CS uses barometric pressure to determine altitude.
The airline cabin air pressurization would mess up any reading he was getting.
I think they typically pressurize to a higher altitude than 3-4000 feet though.
Most commercial airline flights are pressurized to around 8,000 feet altitude inside the plane regardless of actual altitude.
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Old 08-11-05, 07:13 PM
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My Garmin GPS V is dead nuts accurate in regards to altitude. It targets up to 12 satellites to compute altitude rather than using barometric pressure. I've found the softwre Garmin supplies with thier equipment sometimes does not accurately reflect actual street and landmark locations.
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Old 08-11-05, 07:58 PM
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On an airplane youre better off looking for the "Satellite altitude" reading if you GPSr has one.

I have a eTrex Vista and love it (except for the small screen when you're lost in your car)
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