What's typical for coordinate accuracy?

[mchsbr]

I just got a GPS unit today. Found 1 cache and 1 benchmark. The benchmark was 3 blocks from my house so I knew right where it was without the GPS. When I created a point using the coordinates from the website, it was 5 blocks off. Is this typical? I have a Mag. Sportrak pro and also the ToPO CD. I have switched between the WGS84 and the NAD 83. That didn't make any difference. The benchmark is LZ0125 in Pennsylvania. Hopefully someone can help me. I'll be discouraged if they are all off this bad.

[+GatoRx]

I'll put it this way - after looking for my first benchmark with my GPSr, I learned to use the descriptions listed with the benchmark very quickly; the arrow was pointing me several hundred feet out into a river. Now the GPSr stays in the car, while the benchmark listing and a compass go with me.

[+GatoRx]

I'll put it this way - after looking for my first benchmark with my GPSr, I learned to use the descriptions listed with the benchmark very quickly; the arrow was pointing me several hundred feet out into a river. Now the GPSr stays in the car, while the benchmark listing and a compass go with me.

[umc]

Yup, unless it was last updated with a survey crew using GPS or the like it can be quite a ways off. I have come a cross a Benchmark that was about 200' off. Like I said the ones that were on were updated recently. As long as your using the correct format you should be fine.

 

[ArtMan]

... not just the geocaching.com page.

 

Note below that this is a true "bench mark disk." Despite the loose use of that term on geocaching.com, benchmarks in surveying parlance refer to marks used for elevation purpose, and you will notice the very precise elevation of 570.56 feet. It is a "second class" vertical station, which has certain technical specifications, but for our purposes note the absence of any horizontal class. In fact, read further, and you'll see that the horizontal position (lat/long) was just estimated form a topo map. The +/- 6 seconds is equivalent to a possible error of roughly 600 feet in any direction.

 

I didn't know any of this stuff a month ago, and one of the attractions of benchmark hunting is that there is a lot to learn. (Not much of any likely practical value, I'll concede, but still learning stuff is fun and rewarding.)

 

I always use the official NGS datasheet, rather than the geocaching page, and you may find it useful to do so, also.

 

LZ0125 *CURRENT SURVEY CONTROL

LZ0125 ___________________________________________________________________

LZ0125* NAD 83(1986)- 41 03 38. (N) 076 14 18. (W) SCALED

LZ0125* NAVD 88 - 173.906 (meters) 570.56 (feet) ADJUSTED

LZ0125 ___________________________________________________________________

LZ0125 GEOID HEIGHT- -32.52 (meters) GEOID99

LZ0125 DYNAMIC HT - 173.834 (meters) 570.32 (feet) COMP

LZ0125 MODELED GRAV- 980,206.0 (mgal) NAVD 88

LZ0125

LZ0125 VERT ORDER - SECOND CLASS 0

LZ0125

LZ0125.The horizontal coordinates were scaled from a topographic map and have

LZ0125.an estimated accuracy of +/- 6 seconds.

...

LZ0125

LZ0125_MARKER: DB = BENCH MARK DISK

 

[This message was edited by ArtMan on September 20, 2002 at 02:45 PM.]

[+Black Dog Trackers]

The location of benchmark LZ0125 is scaled. This means that the posted Lat-Lon coordinates are only approximate. Some benchmarks' scaled coordinates are better than others, and those of LZ0125 are apparently among the worst. There may even have been a typo in the coordinates too.

 

For your next quest, pick a benchmark page that says "adjusted" instead of "scaled" and you should have a better experience. It will be right on, and your GPS should lead you to within 10 feet of the spot. I always give my gps a bit of time to 'relax' as well. Sometimes it has a hard time keeping up with my spinning around, blocking its views of its favorite satellites.

 

The scaled benchmarks I've found aren't usually so bad, but I've run across one that was over 200 feet off. But then, those just make the benchmark hunting game a bit more challenging!

[+Web-ling]

Unless the disk is a horizontal control disk, the coords are very approximate. They're often off by 100 yards or more. They'll get you to the right part of town, but that's about it.

 

What I've found very useful are USGS maps. Many (but not all ) benchmarks are marked on the maps. When printing a benchmark page, I usually click the Topozone link to see if the mark is shown on the map. If it is, I print the map (or go to LostOutdoors.com to get a close-up version) and use the map and the description instead of my GPSr.

 

[+Web-ling]

Unless the disk is a horizontal control disk, the coords are very approximate. They're often off by 100 yards or more. They'll get you to the right part of town, but that's about it.

 

What I've found very useful are USGS maps. Many (but not all ) benchmarks are marked on the maps. When printing a benchmark page, I usually click the Topozone link to see if the mark is shown on the map. If it is, I print the map (or go to LostOutdoors.com to get a close-up version) and use the map and the description instead of my GPSr.

 

[mchsbr]

I went out a found another benchmark, it was also a scaled one. It was only off by 90 feet. At least I know what to expect now. Thank you all again!

[rusty0101]

If you find a scaled benchmark, you may wish to consider recovering the benchmark to the NGS. As part of your recovery print off a copy of the web page used to log/recover the benchmark specifically this form.

 

Fill out what information you can, while you let a GPS settle on the benchmark. From what I have been reading, that can take 20 min or so, so sit down, celebrate by eating that sandwich you brought along, fill in the form, Take a few measurements from anthing that looks like it is new and will stick around for a while, notice that the tree they said was a 20" oak in 1965 is now a 30" oak, etc. Log the Lat and Lon in hddd mm ss.s form (note the datum your GPS is reporting as well)

 

This is perhaps more important on benchmarks that you find that were reported lost, are estimated from an earlier datum, or that are scalled off of a topographic map rather than derived from other benchmarks.

 

Then again, I could be wrong.

 

-Rusty

[+infosponge]

I probably wouldn't report any coordinates off a consumer-grade GPS unit to the NGS. We have nifty toys, but I wouldn't pretend for a moment that I can get numbers as good as a professional with the right equipment and experience.

[+raouljan]

I noticed that this was a vertical control.

If you can walk up to the control using the description on the datasheet, I would probably not include the GPSr co-ords. However, if the "gray" area for finding this benchmark requires you to poke under bushes, climb over rocks or the description is off due to new building, then the addition of the GPS co-ords would probably be doing the next person that goes out there to find it a big favor.

 

>Personally Responsible for the Recovery of .00217% of the Benchmark Database!<--watch this number!

[jscotti]

Most of the benchmarks in my area that I've hunted for have been within about 1 arcsecond of latitude or longitude of the coordinates given on their data sheets. 1 arcsecond is about 100 feet. The data sheets themselves suggest an accuracy of only 6 arcseconds, though (assuming it isn't one of the higher precision benchmarks). I use the GPS, the descriptions on the data sheets and the topo map which often shows which side of a road or other topographic feature the benchmark is on in my search. I've often found the descriptions to be out of date due to road construction and other site changes, so don't take any of the sources as exact.

 

Jim.

 

Jim, Team Scotti

http://pirlwww.lpl.arizona.edu/~jscotti/geocache.html

[survey tech]

I imagine a lot of people will be supplying coordinates in their recovery notes in the future as the technology becomes ever more ubiquitous. Its fine to provide them if it helps in finding the point, but professionals will never use them to precisely define the point, because in so doing they would inherit the liability for any inaccuracy in those coordinates. In fact, many surveyors will not even use coordinates provided by another surveyor without checking them.

[damfyno]

Although very new to this activity, I have used my GPSr extensively in the past 8 years for various reasons. In benchmarking, the GPSr gets me close to the mark and from there I might as well shut it off unless I want it in the picture. The provided information, a good compass, distance judging, and imagining what the area looked like 60 or so years ago takes over from there. I also have a local advantage, being over 50 years old I remember things that are long gone.