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Mag stations and meridian stones


Bill93

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In cases where magnetic stations were set as pairs they provided both the determination of magnetic declination and the difference with a geodetic azimuth (bearing) that surveyors could use to compare their instruments. Where they were set as a single station then their primary purpose was to determine declination.

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I'm guessing that declination was found by comparing a precise compass reading at the point with astronomical sights (e.g. Polaris).

 

The station on this list in my home town (which now I have to go look for!) was a single station that I see no evidence was part of the horizontal network, but they recorded azimuth to a church spire and courthouse spire (both gone now), so without establishing a second station it could still be used to check compasses.

Edited by Bill93
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Bill93, you are correct that the typical process was to determine an astronomic azimuth (either Solar or Polaris) and then observe the magnetic bearing of the line. In quite a number of cases the local county courthouse had a record book where the surveyors could log in their results each time they did a comparison.

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There isn't a magnetic station listed for the county I'm now in, but I visited my home town over the past weekend and found the one there. Unfortunately neither of the building spires sighted from it now exist so I can't use it for practice.

 

I precalculated the position of the post from the data sheet coordinates of the former courthouse (intersection station) and guessed location of the church spire from Google Earth. A spot reading on my handheld GPS said I was 4.5 ft off line to the courthouse and well within the tolerance set by locating the former church.

 

201108152214484e499a58109fe.jpg

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Late last month two friends and I came across a benchmark/Mag station combo.

 

Wide shot of the BM and Magnetic Station

 

Close up of Mag Station set in 1905. We debated over a long time if that was 1905 or 1965, but I finally found a report that clearly stated it was part of a 1904-1906 set of stations put across the country. This holds the record of oldest mark I've found to date of any kind.

 

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Edited by Ashallond
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Why not open the link? :unsure: :unsure:

 

Because it's an e-book from google's e-book store. You can't simply link into the middle of it. Plus it's more than 100 pages......

 

Nice work on coming up with that book Ashallond. I'll save folks a little time reading through the data in the book. The stone in Ashallond's post was monumented December 4, 1905, and it's one of twenty in Arkansas. It's a sandstone post "30 x 12 x 12 inches, projecting 1 inch above the ground and lettered U. S. C. & G. S., 1905". The coordinates in the book are N 34° 34.7' W 093° 37.2', with a declination of 64° 27.5' East. For comparison the coordinates of EJ1473, which is a few feet away, measured by GPS, are N 34° 33.454' W 093° 37.939'. I'll be honest, I don't know exactly what to do with that declination value, and it may alter my thinking on things. Maybe somebody here can explain it to me. The straight line distance between those two coordinate pairs is 2.5km.

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Limited success in SD.

 

Filtering GSAK for 'MAG' and 'MAGNETIC STATION' returns a PID in 15 counties in South Dakota.

I targeted five (CLARK, CODINGTON, FAULK, HAMLIN AND SPINK) of these when returning to

Montana from Lake Superior last month. Just one was found and it was a 1935 triangulation

disk replacement for the original six pointed star disk.

 

464102b0-c619-43fe-ba43-6e2c4be9fe5c.jpg

 

FAULKTON MAGNETIC 1935

 

QR0679'---------------. THE ORIGINAL MARK, SET IN A 6-INCH SQUARE

QR0679'CONCRETE POST, WAS REPLACED BY A STANDARD TRIANGULATION

QR0679'DISK AND COMPLETELY REINFORCED. MARK IS STAMPED

QR0679'FAULKTON MAGNETIC 1935. IT PROJECTS 6 INCHES.

 

CASTLEWOOD MAGNETIC 1935 PR0844 is a hard luck story. A new school was to be built at its

location so PR0843 CASTLEWOOD was monumented in 1951. At sometime after 1991 and prior to

9 July 2004 a new gymnasium was built at this location. AZIMUTH MARK NO 2 set in 1983, but

not listed in the box score was found at the northwest corner of Pearl St and N 6th Ave.

 

CLAY Co NM0757 has been logged as FOUND (no photo) and SANBORN Co PS0121 logged as NOT FOUND.

Several others have NOT FOUND NGS recoveries, but I would hope that someone will someday log,

and post a photo here, of an original MAGNETIC STATION disk in SD.

 

A sample star from near Ryegate Mont

 

cc975d26-63ba-4f6c-8cce-08d6030b19c9.jpg

 

Got one map scouted for when I snowbird later this fall. kayakbird

Edited by kayakbird
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Another MAGNETIC STATION strike out.

 

The stock pens in Loa, Utah have been removed but the call from the loading chute didn't help previous loggers; and the 87 feet from the T road east intersection has to be wrong.

 

Viewing the west face of the snow covered Henry Mountains made the extra miles from Salt Lake City to Green River well worth it. kayakbird

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Another MAGNETIC STATION strike out.

 

The stock pens in Loa, Utah have been removed but the call from the loading chute didn't help previous loggers; and the 87 feet from the T road east intersection has to be wrong.

 

Viewing the west face of the snow covered Henry Mountains made the extra miles from Salt Lake City to Green River well worth it. kayakbird

 

For what it’s worth, using Google Earth and throwing out the 87 foot measurement I get a lat/lon of 38°24'15.60"N 111°38'58.19"W.

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