May 15, 2011 at 6:16 pm #1273863
Greg MihalikBPL Member
First of all, I'm new to this. It started as an exercise to see whether or not a GPS will be useful for some canyon country route finding.
I go NG Topo for Utah, build a waypoint from a trip report UTM route table, that should pinpoint at specific point in a canyon, (which I have personally visited and know the topography) and find the waypoint is 100 meters south and 200 meters east of where it should be.
The difference is between a point on the canyon floor and a point on the rimrock. You could Not get to that UTM location from the bottom. Someone taking a fix from the site, is forced to being pretty darn close.
I checked a few other points, like the confluence of two streams, from other sources, and found similar discrepancies.
Is this a TOPO issue?
Operator (me) error?
Edit: I'm using NAD 27May 15, 2011 at 6:36 pm #1736979
Greg, NAD 27 versus the more standard 1983-1984 datums will build some problems. Typically, the original recorder might have done a good job, but then they used the wrong datum, or they reported no datum at all. That is about the amount of discrepancy that you get off mismatched datums. In some terrain, that is enough that you would recognize it and compensate. In some other terrain, you might not recognize it until you were already in trouble.
Also, if your GPS receiver is way down in the bottom of a canyon where view of the satellites is limited, it has an impact on position accuracy. If you were up on the rim where the view is virtually unlimited, it is likely much more accurate. Also, before I start into any serious navigation, I let my GPS receiver "soak up" fresh satellite ephemeris data immediately beforehand. That just lets the receiver do its very best. If you don't get fresh ephemeris data, it might have less position accuracy. But, you don't know what the original recorder did to get his data.
Around the western U.S., I have found plenty of places where the printed maps have position accuracy problems. I have found plenty of places in a GPS map database that have problems, and I have seen rare GPS receiver "blunders." Although TOPO! tends to be very good, it is not 100% perfect, either.
–B.G.–May 15, 2011 at 6:36 pm #1736981
drowning in spamMember
I've heard the reflections off of canyon walls can cause some pretty big errors.May 15, 2011 at 6:43 pm #1736984
Reflections can cause big errors from multipath interference. However, canyon walls tend to be very random, so the multipath interference tends to be very random, so the accuracy kind of comes and goes. A good GPS receiver will know that it is getting hit with this kind of degradation. It will still show you your position, but the estimated position error number increases.
Canyon randomness is because natural rocks have all sorts of shapes and materials.
Now, if you want to see a really bad multipath interference problem, go to the streets of New York City. That is what we call the concrete canyon. There, you have all sorts of smooth glass, smooth metal, and similar materials. That makes the reflections the worst possible, and that is why a New York City cab driver learns not to depend on GPS for 100% of the time.
–B.G.–May 15, 2011 at 7:03 pm #1736992
Greg MihalikBPL Member
I changed the Datum. Not much difference.
I looked at another dozen points and most come out close.
I believe, of the early examples I looked at, they are just wrong.
From two different sources, one on-line and one a published guide book.
So, not really operational/compatibility issues.
Just the quality of my sources.
Thanks.May 15, 2011 at 7:20 pm #1737001
Another thing you can do…
Somewhere USGS maintains a large file on standard benchmarks out in the field, and they maintain the most recent or most accurate position record for each. So, you can go find some of those that are close to you, and check your own GPS receiver against those published data points. Many of those standard benchmarks were done with a professional model averaging receiver, so they have very little error of anything.
Once you know how much you can trust your own GPS receiver, then you try to find another standard benchmark out near the area where you need to navigate. You compare your own GPS receiver's accuracy at home to what you think you see there in the field area. After that, you will know whether the errors are from the original recorder or if they are closer to you or your receiver.
If you go through this exercise in an area of seismic activity, the recentness of the benchmark data will be important. Some areas creep around by two inches per year. Other places like Sendai, Japan, get a big error in a hurry. But, those are nothing like 100-200 yards overnight.
I've been in buildings where we had six GPS antennas mounted on the roof and six GPS receivers a few floors down. We had enough relative accuracy that we could identify which antenna was which, and they were each about 18" away from the next.
–B.G.–May 26, 2011 at 11:07 pm #1741690
Ross P HemphillMember
Bob: that's a helpful tip. Thanks!
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