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Mystery Benchmark Disk


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I was out benchmark hunting the other day and found this little rarity. I was looking for benchmark NK0586. It is supposed to be a standard USGS tablet stamped BLACK and 1970, centered in 6inch field tile and filled with gravel up to within 1 inch of the top of the tile.


Instead what I found is a USCGS disk set in a 6 inch by 6 inch square cement post. It does not have a date stamped into it, but the designation stamped on it is either "7 6" or "7 6\". The coords I got with my Magellan Sportrak Map were N 42.19.681 by W 092.30.874. It was a clear sunny day and I was receiving 7 satellite signals and had WAAS enabled.


I have posted 2 pics on the geocaching site and would appreciate any suggestions/help.

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One possible answer is you were close but not right on the 'mark'. You're coordinates are off by About 15X20 feet not counting the signal error.


We have found benchmarks (both in the NGS database) within 30 feet of each other.


The pictured disk does not appear to be in the NGS database. I determined this by clicking on the 'nearest benchmarks' on the page for the mark you were seeking. There also appears another 7 to the left of the center line on the pictured disk.


It maybe worth checking the area again, perhaps in a radius of 30 feet from the found benchmark.


Good luck,




edited for speeeling.

Edited by 2oldfarts (the rockhounders)
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I don't find your mystery disk in either GeoCaching or the NGS databases. I suspect that you have already done a radial search from your coords but here is what I got:

from GeoCaching:


NK0586 0mi NW BLACK IA survey disk

NK0580 1.1mi SE REBAR 1202 IA metal rod

NK0115 2.4mi NW G 11 IA bench mark disk


from NGS: http://www.ngs.noaa.gov/cgi-bin/datasheet.prl


|Dist|PID...|H V|Vert_Source|Approx.|Approx..|Stab|Designation

|----|------|- -|-----------|-------|--------|----|-----------

| 0.0|NK0586|2 .|29/VERT ANG|N421941|W0923052|....|BLACK

| 2.4|NK0115|. 1|88/ADJUSTED|N422103|W0923258|C...|G 11

| 1.1|NK0580|2 .|29/VERT ANG|N421916|W0922941|D...|REBAR 1202


I do notice however that the disk that you seek is subsurface.




Maybe you woudl want to try to locate the reference marks first to get some idea where to start digging? Good Luck!

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I'm not surprised this is not in the NGS or Groundspeak data base. I have found 3 similar ones down here a couple counties away that were not listed.


Like yours, they are "US Coast & Geodetic Survey and State Survey" with the county number stamped near the outer edge of the clear space and a serial number in the middle. The digit 7 off to the left would be Black Hawk county, which it is within. The serial number is either 76 or 761 with a very slanted 1 This series must have never been worked up with the computations to put them in the data base.


The station BLACK you were originally seeking is set up like a triangulation station, although it is just listed as "survey disk". Tri stations typically have 2 reference marks nearby, and an azimuth/reference mark perhaps a quarter mile away (this one uses a landmark instead of an azimuth disk). The reference marks are most often along a road right of way and can often be found without too much trouble.


BLACK is a newer style mark, on a deep rod, and is buried. The "surface mark" for the older (1930's) tri stations around here is usually buried 12 to 20 inches in a cultivated field. The REAL mark, which even the NGS doesn't look at except for the most serious work, is about 4 feet deeper, with the surface mark carefully aligned over it.


To claim a find on a station, you need to find the surface mark, not just the reference marks, and I haven't been digging them up due to the bother of finding the right person to ask permission from. It is quite useful, however, to report on the condition and coordinates of the reference marks, and the sky visibility of the probable station location.


I read some of your other logs and see that Black Hawk county has quite a few more recently placed stations that aren't buried, although they do have reference marks. All the tri stations I've looked for are the older buried style. I expect the first one I'll actually see is MG0879 AVIATION RESET down by Iowa City as it isn't covered up by anything but a pool of muddy water.

Edited by Bill93
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