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Looking For A True "benchmark"


Karma Hunter
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I would like to go to a physical location that has "official" latitude/longitude info so I can mess around with my GPSr and test accuracy. I want to walk around, stand still, play with settings, and generally be a geek to test effects on accuracy with an exact point to use for reference. I think other people have made reference to USGS or some other type of benchmark type things (measured benchmarks, not the older style map-based estimate benchmarks), but I can't remember what they are called, which is driving me crazy. Any info on how to find one of these places-I-can't-remember-the-name-of locally would be helpful too. Thanks!

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You want the NGS DATASHEET RETRIEVAL PAGE. Click on DATASHEETS. I find [county] to be the easiest link to use. Choose your county; Choose GPS sites Only; Pick a stability - B or better works well in my large urban area. Stability A includes lots of inaccessible marks in guarded buildings, behind fences, etc. You'll have to read individual data sheets to decide whether the site is useful. Look for recent recovery notes. Probably the best bets are those located close to bridges, rivers, waterfronts, etc.

 

Make sure the coordinates were established by GPS. The sheet will say that explicitly. The datum used will NAD83, which for our purposes is identical to WGS84.

Edited by blindleader
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:huh: Hello.

 

You should be able to do a Benchmark search for your area and find a few that you think are easy to get to. Go to the info pages of those and click on the "view original datasheet" link near the top.

 

From the data sheet you can check and see when the position of the benchmark was last checked, what the coordinates are, and how those coordinates were determined. There should be some sort of text that says something like, "The horizontal coordinates were established by classical geodetic methods and adjusted by the National Geodetic Survey in May 1991" or that the coordinates were determined from a map.

 

Hope this provides some help. - Trailkat :D

 

http://www.geocaching.com/mark/bench.asp?PID=AG1462

 

...

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I would like to go to a physical location that has "official" latitude/longitude info so I can mess around with my GPSr and test accuracy.

You can search for them right here on GC here. But make sure you pick one that is a "disk" and not a landmark. Also make sure it's location is ADJUSTED, not SCALED. (Side note: In a strict sense, a Bench Mark is a spot where the altitude is accurately known. A triangulation station is one where it's location is accurately known. However here on CG we tend to use "benchmark" to refer to both.)

 

Ideally, you'll want to find one that says it's location is suitable for GPS observation. That will make sure you have a good view of the sky, but don't discount any that don't have that since most marks haven't been updated or logged since the GPS system became active.

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Hello again, and thanks for all the extremely helpful info! Now I have more questions. <_<

I looked on GC and found a triangulation station disc near where I live. It is described as:

Coordinates may not be exact. Altitude is ADJUSTED and location is ADJUSTED.
Finding an adjusted triangulation disc seemed pretty promising, but then I read further and under Control Data it said this:
Depending on the time and method that the coordinates were acquired, the latitude/longitude (horizontal) and altitude may be off by quite a bit (or even be in the incorrect datum).
and:
The North Carolina/South Carolina HARNS have been completed but, due to contractual restrictions, coordinates for these stations will NOT be published in the near future. In the interim, the published coordinates in North and South Carolina will not be consistent with the Continuously Operating Reference Stations (CORS). The HARN coordinates for these stations are available upon request.
So now I'm wondering how accurate the coords are. It seems like they're saying that modern readings have been taken, but that for now they are top secret. Very cloak and dagger. I looked at several other benchmarks in this region, and they all had the same "contractual restrictions." Is this normal, or am I living in a weird place? What are the odds that the coords are good (last NGS entry was 1983)? Thanks again!
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It seems like they're saying that modern readings have been taken, but that for now they are top secret.

I haven't heard of this before, but I'll take a WAG.

 

Surveyors work with accuracy in a half a foot or less (frequently a lot less). Consumer GPS units are not capable of displaying accuracy any closer then about 6 feet in either direction. So what is of concerns to a surveyor may not mean diddily squat to you or me.

 

To see what I mean, click on the "view original datasheet" link and notice the location up near the top that the datasheet has 5 digits to the right of the decimal point instead of three like your GPS unit.

 

But to get a more authoritative answer, you might want to repost this question in the benchmarking forum where some pro surveyors hang out and may be able to shed more light on this.

Edited by GeckoGeek
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I don't mean to rain on anybody's parade but... Just go back to the first reply and follow my links. You'd never have had to read the data sheet you describe. Geocaching.com might be ok for finding brass caps to log, but as you've already discovered, it doesn't separate the wheat from the chaff as far as good coordinates are concerned. All you'll have to do is read the most recent recovery notes to decide whether the point is accessible.

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Blindleader, I checked the link in your first reply shortly after you posted it yesterday. It was helpful, but all of the stations, even the, Stability-A stations, include the same disclaimer as found in the GC listings:

The North Carolina/South Carolina HARNS have been completed but, due to contractual restrictions, coordinates for these stations will NOT be published in the near future...

They do however, give better detail about the data that IS provided for the stations:
The horizontal coordinates were established by GPS observations and adjusted by the National Geodetic Survey in April 1996.
For my purposes I assume the 1996 data is more than accurate enough, because like Gekogeek said
Surveyors work with accuracy in a half a foot or less ... so what is of concerns to a surveyor may not mean diddily squat to you or me.
However, I am curious how they could get better than thirty feet or so of accuracy on their GPS in 1996 since SA was enabled at that time. Did they have access to non-civilian signals? Did they use traditional mapping methods, then just confirm that they weren't off by more than 30 feet by using GPS? I'm also still curious about the reason for the contractually restricted data, not because I think any of this will change my world, but just because I like a good mystery. Thanks for the info, and the link to NGS. It is an excellent resource. Edited by Karma Hunter
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You'd never have had to read the data sheet you describe...

 

...All you'll have to do is read the most recent recovery notes to decide whether the point is accessible.

 

:D Aren't the recovery notes located in the datasheets?

 

- Trailkat <_<

Edited by trailkat
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I am curious how they could get better than thirty feet or so of accuracy on their GPS in 1996 since SA was enabled at that time.
The same way they get better than the two to four meter accuracy inherent in the system now. In a word, Differential GPS. Well ok, two words, or is that four? Never mind. Surveying using GPS bears no resemblance to what we do with hand held units, beyond the fact that both use timing signals from the same system of satellites. Measuring the location of a control point can take all day in the field plus lots of computer number crunching afterwards. That's about the extent of my firm knowledge of GPS surveying, as my own land surveying experience ended before the age of GPS.

 

Actually, SA could be quite handily negated even with consumer units. Approximately two hours of averaging pretty much removes the errors injected by SA.

 

Aren't the recovery notes located in the datasheets?
We both know that. I was referring to filtering of uninteresting data sheets from the search. I assumed that the sheet Karma Hunter quoted from was one for which accurate GPS coordinates were not published. Karma Hunter seems to imply that. I'd like to know the PID of that sheet so I can see for myself. Since the search should turn up only points with good coordinates, the only reading the user needs to do is of the recovery notes, which will give a fair indication of whether or not the point is accessible.
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I assumed that the sheet Karma Hunter quoted from was one for which accurate GPS coordinates were not published. Karma Hunter seems to imply that. I'd like to know the PID of that sheet so I can see for myself.
I don't know much of anything about this subject, but am interested in learning. If I imply anything about benchmarks it is more likely due to ignorance than intention. I think what I'm trying to figure out is how to know whether the published coords of a given benchmark are accurate enough to be worth going to visit as a reference point. I'd be excited if accuracy at the station is one foot or less (I know my own GPS is only good to six feet). The PID of the specific station I mentioned is FB2512.
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However, I am curious how they could get better than thirty feet or so of accuracy on their GPS in 1996 since SA was enabled at that time.

They same way they got that accuracy before pocket calculators, much less GPS - careful measurements from known points.

 

Did they have access to non-civilian signals?

Yes. That's why pro units take up a backpack and cost thousands of $.

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I looked up BLUE RIDGE. The coordinates are good enough for your purposes. The key statement is:

"FB2512.The horizontal coordinates were established by GPS observations

FB2512.and adjusted by the National Geodetic Survey in April 1996."

The narrative suggests that the HARN coordinates (unpublished) might be as much as a foot and a half ( 5 decimeters) different from the published ones. If you're really geeky, you could call one of the two phone numbers given and get them. If you're as geeky as I am you could also try to engage the person on the other end of the phone in a conversation about how all this relates to "true" coordinates. :D

 

Here's a really exciting :Darticle about the subject.

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