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Ranz

I think the Datum is wrong.

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I did a personal look for two bench marks in my area (utah) today. In both cases the area in the description and the posted coords were about 150' off. The posted coords were East/SouthEast. So when I got home I started checking a few with mapping software.

 

I brought up the topo maps and satellite photos for a couple near where I live. Same result of the actual location being over 150' West of the waypoint.

 

I had seen this before when using NAD-27 maps so I took the coords and assumed they were NAD-27 and converted them to WGS84. Once converted the waypoint landed in the described area. I confirmed the calculation with a known (found) marker in my area. (Designation C 173) When I took the posted coords as NAD-27 and converted to WGS84 the waypoint hit the mark on the dot. With the coords as-is it ends up on the other side of the street (East).

 

Has anyone else run into this problem? Is NAD-83 off of WGS84 that much in my area?

 

geosign.gif

 

[This message was edited by Ranz on June 10, 2002 at 04:59 AM.]

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I don't think any of the benchamrks are WGS84. Best to find out what they really are (most probably the same thing as USGS topo maps) and convert or work in those coordinates.

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A majority of them are Nad 83 and are documented on each page in parenthesis pulled from the data sheet (though when the original data sheets are available on this site you can double check). With that said, when many of these benchmarks were placed they were using basic surveying equipment and as a result may be off a bit. But I'm just guestimating.

 

I'll add lat/lon to logs so folks can add updated coordinates if they like to. One thought would be to compile the logs from everyone and submit it to the NGS in a batch.

 

Jeremy

 

Jeremy Irish

Groundspeak - The Language of Location

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The benchmark position I tried was off by way too much to be either a surveying error or a NAD83/WGS84 difference. But its position was consistent with the posted coordinates being NAD27.

 

I actually tried two today; the first one appeared to be in the middle of a field, which I tromped through without success, and I didn't have time to pursue the matter, but its description looked to be off by about 50-100 m as well.

 

So I have the funny feeling that the coordinates are actually NAD27.

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Thanks for the link, Dave. I went to the NOAA site and retrieved the datasheet for the bechmark I tried yesterday. It said:

 

quote:
The horizontal coordinates were scaled from a topographic map and have an estimated accuracy of +/- 6 seconds.

 

Yow. 6 seconds is a long ways!

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I checked the one nearest my house in Berkeley using both the indicated NAD-83 datum and the usual WGS-84. Both times I was off by about 120 feet, in the same direction. I don't think it's a datum issue. The rounding problem that Jeremy posted about sounds more likely.

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Check out my post here for some possible reasons the coordinates may be way off.

 

-Elias

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Right, that's the one I meant, Elias not Jeremy.

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Do me a favor and take the coords you used and treat them like they were NAD-27. Covert them to WGS84 and see if they end up in the right area. That's the only reason I question it. It seems like quite a coincidence that they line up for me after the conversion.

 

quote:
Originally posted by jef:

I checked the one nearest my house in Berkeley using both the indicated NAD-83 datum and the usual WGS-84. Both times I was off by about 120 feet, in the same direction. I don't think it's a datum issue. The rounding problem that Jeremy posted about sounds more likely.


 

geosign.gif

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quote:
jef wrote:

I checked the one nearest my house in Berkeley using both the indicated NAD-83 datum and the usual WGS-84. Both times I was off by about 120 feet, in the same direction. I don't think it's a datum issue. The rounding problem that Jeremy posted about sounds more likely.


 

The problem is that NAD83 and WGS84 are basically the same; one will be off by no more than a meter or so. NAD27, on the other hand, is off from both NAD83 and WGS84 by a lot in our area (Northern CA).

 

I'll check the one I tried yesterday after converting the listed coordinates from NAD27 to WGS84, to see if it matches.

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I rechecked HT0968, the one near my house, using NAD-27 CONUS. This time my GPS was indicating a spot 150 feet to the west, instead of 120 feet to the SE. So, I don't think it's a datum issue.

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Hey guys - if they scaled it on a topo map, you can bet it was done in NAD27. ALL USGS topo maps are NAD27! Did they convert to put it back in NAD83? Maybe not!

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http://www.ngs.noaa.gov/datasheet.html#NGSmap

 

This link will allow you to look up the complete NGS datasheet. It will have all of the datum history including coords for each control point that were established with each survey datum.

 

WGS84 typically is not included on the NGS. I guess the ellipsoidal shape of the earth has not changed much since 1983 so the datasheets haven't had a new datum established.

 

WGS84 was introduced as a "New World Order" so to speak to globally calibrate datums so that GPS systems would work properly no matter where you are. I guess since the US owns most of the GPS technology in public, private and military sectors they started with NAD83 as the base and converted the rest of the world to that datum to produce WGS84.

 

But enough rambling.... the page link has all the datums for each point. Go check it out and play with your gps. Switch datums on your GPS and plug in all the numbers and see which ones work the best. Could add an interesting twist to the game. icon_wink.gif

 

Mtn_Bkng_Dave

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http://www.ngs.noaa.gov/datasheet.html#NGSmap

 

This link will allow you to look up the complete NGS datasheet. It will have all of the datum history including coords for each control point that were established with each survey datum.

 

WGS84 typically is not included on the NGS. I guess the ellipsoidal shape of the earth has not changed much since 1983 so the datasheets haven't had a new datum established.

 

WGS84 was introduced as a "New World Order" so to speak to globally calibrate datums so that GPS systems would work properly no matter where you are. I guess since the US owns most of the GPS technology in public, private and military sectors they started with NAD83 as the base and converted the rest of the world to that datum to produce WGS84.

 

But enough rambling.... the page link has all the datums for each point. Go check it out and play with your gps. Switch datums on your GPS and plug in all the numbers and see which ones work the best. Could add an interesting twist to the game. icon_wink.gif

 

Mtn_Bkng_Dave

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The original data sheet is now linked to from the PID page on geocaching.com as well.

 

Jeremy

 

Jeremy Irish

Groundspeak - The Language of Location

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Yesterday evening I looked through the local marks and picked out the ones which were "surveyed by classical geodetic methods", as opposed to the ones taken off a topo map and subject to the theorized +- 6 seconds roundoff error. I looked for five of the classical marks, and found three. In each case the GPS coordinates were dead on, within ten feet or less. I think this establishes that the roundoff theory is correct and the bad datum theory is not correct.

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Hello everyone, Im here to promote, and to thank you for, your interest in Land Surveying.

 

To clear up some of the confusion about the accuracy or inaccuracy of the coordinates provided for the markers you seek:

The most precisely surveyed markers in the United States are known as Triangulation Stations. They are typically described by names, rather than numbers. The names are usually related to some prominent mapping feature in the area. The coordinates of these points are extremely precise, since their purpose is horizontal control on the earths surface. It is not possible to obtain accuracy, using a handheld GPS device, that can surpass, or even equal, that of the methods used to establish the original coordinates for these points, so their coordinates must not be changed. Benchmarks, however, are a different class of control points. These markers are set to establish vertical control, rather than horizontal control. They provide a reference elevation for a particular area and are typically described by number codes, rather than names. Their exact location is of little importance, so their coordinates are often established by inferior methods, such as scaling off a map. Therefore, coordinates you obtain on these points may prove to be more accurate than those supplied in the data sheets. Nonetheless, any coordinates determined by handheld devices will not be sufficiently precise for use in most surveying applications.

 

For further information on Land Surveying, please feel free to visit rpls.com, where you can get answers to your questions about surveying from thousands of professionals practicing in all 50 states.

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One of the benchmarks in my area appears to have the coordinates a little over 2 miles from where the description places it.

 

I had a look around the coordinates today. They were suspiciously close to another benchamrk that I actually recovered. By suspiciously I mean less than 300 ft! After comparing the description with some maps I think I have a good idea where it should be and I'm going to check on it tomorrow.

 

I'm not lost!

I just don't know where I am.

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Originally posted by Ranz:

"I did a personal look for two bench marks in my area (utah) today. waypoint."

 

There are many fine points made in this thread. FWIW . . . .

 

The original data sheet must be consulted at the URL posted in this thread Or, as Jeremy notes, by clicking on the URL in the listing).

 

First issue is to convert any NAD27 data to NAD83.

Next issue: Many elevation markers are sited only to +/- six degrees. That's a 1200 foot diameter circle! If you see the elevation listed to three decimal places, it is probably an elevation bench mark and the horizontal coordinates will be to a lesser precision than if it were a horizontal mark.

Next Issue: For +/- 6 degree marks, carry a 25' tape measure and following the location directions in the station description.

Next Issue: For horizontal bench marks (purists would refer to these a monuments, not bench marks) you will find the station's bench mark stamped with a triangle while its reference markers will have an arrow pointing to the station. Some of these reference markers are listed separately -- for example, the Tri-State b ench mark here in Port Jervis, NY is actually RM2 for the Laurel BM but has its own listing and data sheet. This may account for the obvservation that some marks occur in clusters.

 

Read the data sheet from the bottom up. That way you will not have to plough through a lot of superceded data.

 

A very informative thread. Thanks!

...patrick

 

patrick & shirley

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