GPS accuracy/Summit measurements/Four Corners

GPS accuracy/Summit measurements/Four Corners

Joined: January 29th, 2004, 3:33 am

February 13th, 2008, 6:30 pm #1


I have three related outdoor inquiries — 1. Relating to GPS accuracy, 2. Where summit elevations are measured from; and 3. The location of the true "Four Corners."

1. When I climbed Humphrey's Peak last September, my Garmin GPS was off by about 15 feet from the posted 12,633 elevation of Arizona's highest summit. I can't recall if it was high or low, but it was off.
My GPS readings often also seem to oscillate back and forth by 10 or so feet on its readings almost continually.
Have others found similar discrepancies? Are higher end GPS devices more accurate?
My GPS saves me from any disappointments regarding false summits when I hike, but it's never exact on its readings it seems.
I believe the display on my GPS even says accuracy to within so many feet at times.
Obviously when I'm holding my GPS at chest level, it is 3 or so feet higher than the ground.

2. Hikers always pile up rocks on peaks like Kings and Humphrey's for shelters or to make it higher, but I've always assumed the peak's listed elevation is from the ground, not piled up rocks. Is that right? (Of course all of Kings Peak seems to be loose rocks...)

3. When I visited the Four Corners monument (Utah-Arizona-Colorado-New Mexico common point) 3 years ago, I didn't have my GPS along. Since then I see occasional references to the actual Four corners being hundreds of feet or yards away to the west.
Has anyone ever checked the accuracy of the Four Corners monument? It may be where it is because of access and flat ground and not true accuracy..
Also, when I checked my GPS against the location of the Utah-Arizona border near Littlefield a few years ago, my GPS said the actual stateline was about 80 feet south of where the border's sign and marker were. So, it's not just elevation my GPS varies on, but coordinates too.
Maybe these are more geocaching questions, but I still wonder.
Jack Parsell in his Tri State Corners book never seemed to question the location of markers, like at Four Corners .......
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Joined: August 2nd, 2001, 8:13 pm

February 13th, 2008, 6:59 pm #2

Lynn, GPS units are inherently more accurate in X/Y location than in Z (altitude). The GPS system works by timing of signals from satellites. The most accurate X/Y fix will be when you have satellites spread out near the horizon, giving your GPS direct data from which to compute your lat/lon. The corresponding measurement for Z would be to have one satellite straight overhead (easy) and another one directly under your feet. Now, that's a bit of a problem...something like 8,000 miles of rock and iron between you and the satellite "down" there make that an impossibility. So, the altitude measurements are based, at best, on satellites near the zenith and the horizon, which are only 90° apart, not 180°, so the differential measurement is going to be less precise. Indeed, I find my GPS often thinks I'm floating about 50 feet above a summit...makes me look for my parachute...

In X/Y, I find I can almost always trust my GPS to about +/- 50', or .01 miles, which is how I have the display set. When I set a waypoint and return to the same spot later, I am almost always at zero or .01 miles indicated. Having a poor satellite "constellation" due to vegetation or landforms (say at the bottom of a canyon) will make the repeatability worse but for spots where there is a reasonably clear view out to the horizon, the GPS will get me back to where I marked a spot.

The "summit" by Highpointers' definition is the highest natural landform, so "creative" efforts like the 12' tall cairn atop Mount Katahdin (Maine), raising its 5268' elevation to an even mile high, don't count. Indeed, there is an ongoing debate about the Delaware highpoint, listed as 448', where there is a closed 450' contour adjacent to the benchmark. It is not clear, but it is assumed that the 450' contour is the result of grading in the trailer park and is therefore not "natural." Then there could also be mobile summits, like McKinley and Rainier where current snowfall and drifting patterns might raise or lower the summit as well as move it several feet one way or t'other from year to year. Not much we can do about those...

Corner markers are only as accurate as their original survey. For instance, my GPS recorded the CO/WY/NE tri-corner point at 41° 0.101'N, 104° 3.153'W. I wonder about that 0.101'...I thought the border was at 41° even. There certainly isn't any want of flat space around that marker, so it might be surveying error or just a difference between the datum the surveyors used and the one my GPS uses.

You also asked about whether new units are more accurate than old. Part of the improved accuracy has to do with the government turning off "selective availability." This was a military security feature that scrambled the timestamps from the satellites by enough that you would end up with greater than 100' of uncertainty no matter how good your reception was and how well-placed the satellites were. This was turned off during the Clinton administration, so accuracy now is better than it was 10 or 12 years ago, regardless of the GPS receiver. Better receivers that lock onto weaker satellite signals will, of course, give you a better chance of a good constellation, and WAAS-enabled receivers are more accurate, as well. All-in-all, though, my original Garmin GPS II+ that I have been using since '97 or '98 works just fine...
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Joined: November 25th, 2000, 10:31 pm

February 13th, 2008, 7:45 pm #3

Besides the Russian GLONASS, the Europeans are supposed to be building a completely superfluous Galileo system. Dunno what Glonass is worth.

I read someplace that that 4-Corners monument is misplaced. It's on the Navajo Nation reservation and there's a toll gate, off US-160, as well as a lot of booths selling Indian handicrafts. The brass strip running through Flamsteed House, the Old Royal Observatory, in Greenwich by London, that shows the Prime Meridian, Long. 0, is also supposed to be out by 100 m. or so. Odd. The 180 Meridian, opposite, runs through Taveuni in Fiji; I haven't been there so don't know if it's marked. Can't walk from Su. to Mon. as they cheated and bent the International Date Line around the islands. I wonder how accurate the "Mitado del Mundo" monument in Ecuador and a sign for the Equator in Kenya are; I've been past the latter on a bus. Also one for the Tropic of Capricorn just S. of Rockhampton Qld. 4700.

Finally, I wonder how long. is determined on other bodies. You have to pick a prime or 0 meridian, so I wonder what this is. Latitude, of course, is determined by the object's spin.
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Joined: January 20th, 2004, 9:10 pm

February 13th, 2008, 11:02 pm #4

I have three related outdoor inquiries — 1. Relating to GPS accuracy, 2. Where summit elevations are measured from; and 3. The location of the true "Four Corners."

1. When I climbed Humphrey's Peak last September, my Garmin GPS was off by about 15 feet from the posted 12,633 elevation of Arizona's highest summit. I can't recall if it was high or low, but it was off.
My GPS readings often also seem to oscillate back and forth by 10 or so feet on its readings almost continually.
Have others found similar discrepancies? Are higher end GPS devices more accurate?
My GPS saves me from any disappointments regarding false summits when I hike, but it's never exact on its readings it seems.
I believe the display on my GPS even says accuracy to within so many feet at times.
Obviously when I'm holding my GPS at chest level, it is 3 or so feet higher than the ground.

2. Hikers always pile up rocks on peaks like Kings and Humphrey's for shelters or to make it higher, but I've always assumed the peak's listed elevation is from the ground, not piled up rocks. Is that right? (Of course all of Kings Peak seems to be loose rocks...)

3. When I visited the Four Corners monument (Utah-Arizona-Colorado-New Mexico common point) 3 years ago, I didn't have my GPS along. Since then I see occasional references to the actual Four corners being hundreds of feet or yards away to the west.
Has anyone ever checked the accuracy of the Four Corners monument? It may be where it is because of access and flat ground and not true accuracy..
Also, when I checked my GPS against the location of the Utah-Arizona border near Littlefield a few years ago, my GPS said the actual stateline was about 80 feet south of where the border's sign and marker were. So, it's not just elevation my GPS varies on, but coordinates too.
Maybe these are more geocaching questions, but I still wonder.
Jack Parsell in his Tri State Corners book never seemed to question the location of markers, like at Four Corners .......
The monument was placed long before detailed and accurate GPS devices were on the market. It was probably placed with the best intentions to be at the actual four corners, but it's not surprising to see it off be a hundred or so feet in any particular direction. Even switching between the two common datums (NAD27(CONUS) and WGS84) will give two different locations for the "true" four corners. Given the development there, I doubt they'll bother moving everything 100 or so feet just to agree with the GPS. Most people won't ever know the difference.

I've experimented with my GPS at summits and state lines and they're always off a bit - one or the other. I'm more apt to believe in the lat/long readings than in altimeter readings myself. At the degree confluence site (www.confluence.org) there's an interesting discussion about the various US/Canadian boundary markers that are supposedly set on the 49th parallel N, but in truth (again, GPS truth), lie variously tens or hundreds of feet inside the US or Canada.

If a new datum is ever adopted, the whole issue will re-appear again, as all borders will likely scoot some distance one way or another as a result.
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