Outstanding In A Field

[Photobuff]

This should be easy. It's smack dab in the center of a farmers field, probably two feet below the surface. Even with a good WAAS GPS signal, that only gets me within a few feet. I'm not really up to digging a two foot hole over a wide area. It's too deep for my metal detector, though somebody else was there recently and must have gotten a hit on an old plow part- no benchmark in their hole either! Enough rocks and deep enough that my 3/8" probe rod wasn't much help. What else to try?

 

OF1455

 

I might also add that I don't have a lot of confidence in the GPS readings, since there are microwave and radio station towers at the perimeter of the field, just a few hundred yards away. The level of RF is probably tremendous, even though the GPSr seemed to work ok.

Edited May 1, 2006 by Photobuff

[+Klemmer]

Ground penetrating radar. But there is a little budget issue.... Unless you can sweet talk someone who owns one. Many archeology digs are using them these days....

Edited May 1, 2006 by Klemmer & TeddyBearMama

[2oldfarts (the rockhounders)]

The local True Value / Ace Hardware used to rent metal detectors that had 18" & 24" coils. Those should reach deep enough to find a disk. Or may be a local surveyor would be will to assist you with their detector.

 

John

[+bicknell]

There are a number of measurements. You may need several long tape measures, I finally bought a 300' one myself and that only gets you part of the way on the one measurement.

 

Anyway, if you can find enough of those other objects and measure, along with the GPS position I would think you could get it down to an area perhaps 3 feet in diameter....

[Photobuff]

I'll bet I'm within 3' of it now, but a 6' diameter circle, 2' deep, is a heck of a lot of dirt! It isn't possible to measure to any of the objects, as the radio transmitter building, and some other stuff, is in the way. Because it has adjusted coordinates, anybody with a Trimble could zero me in, but I don't know any local surveyors. The giant coil metal detector sounds like the most accessible option, and I may be able to wind one for the detector I've got. I know they sold a larger coil as an option, but I'm too cheap In theory, I just have to measure the inductance of the existing coil, then wind a larger pair to the same value. And if I could do that, I could also find the rather deep 1875 mark nearby

 

BTW, that 66.3' distance in the data sheet is in error. It's really 663'!

Edited May 1, 2006 by Photobuff

[mloser]

By the way, if you read the description of the 1875 mark you saw that a newer one was set in 1939 because the old one was in a garden. If the aerial photos are correct that location is no longer a garden so you might get permission to search there. I would be willing to bet the owner would love to see some 1875 history!

[+PFF]

Set your GPSr to the decimal degrees mode (ddd.ddddd).

 

Move around in the field until you get the following reading:

N43.03457

W077.42312

 

(You will note that the GPSr is very sensitive to horizontal motion. A very small movement of the GPS unit produces a change in the reading.)

 

Come at the target from several directions. Mark spots with a coin or other object. They should average to the same place, over several passes, and this technique will put you right on top of the monument, plus or minus six inches.

 

By coincidence, this mark is referred to in the description of OF1438, although no new info is added. It serves, however, to confirm the distance from the county line:

 

USC AND GS AND SS 1178 IS A BRONZE DISK SET IN TOP OF A

6 X 6 INCH PRECAST MONUMENT SET 2 FEET BELOW THE SURFACE. IT IS

53 FEET SOUTH OF COUNTY LINE AND 640 FEET WEST OF THAYER ROAD.

 

Paul

Edited May 1, 2006 by PFF

[+dirtfinder]

I have found that if the field is cultivated it is more than likely destroyed as many farmers (in my area that is) have plowed over them and have ripped them out. I have searched for a few untill the farmer comes and tells me that his grand dad had ripped it out of the ground 30-40 years ago. So they have told me where they placed them but again no sign of the destroyed bm.

[mloser]

On the flip side, I have discovered marks at ground level that remained undisturbed in cultivated fields. One farmer told me he practiced "no till" methods that didn't disturb the marks. Of course it wasn't the benchmarks that caused him to farm like that, it was environmental concern, but still, he hadn't damaged the covers on the rod style benchmarks at all. I am not up on my farming techniques but I would imagine that using disks would be less damaging than some other kinds of equipment.

 

So anything is possible.

[holograph]

Come at the target from several directions. Mark spots with a coin or other object. They should average to the same place, over several passes, and this technique will put you right on top of the monument, plus or minus six inches.

 

I don't think it is quite that easy. People who've done the experiment by averaging out repeated readings usually need hundreds of readings over many different days and times of day and night to obtain an average location that is accurate to sub-meter precision.

 

Even if you did the trick on one afternoon and did see your readings converge to a single location, it might still be considerably off the mark because of the ionospheric refraction at that particular time. It might be possible to get a cluster of readings within 6 inches of each other, but it still could be many meters off the mark.

[Photobuff]

I agree that good averaging takes a long time. I've played with the SA Watch program and can see that systematic errors will give you erroneous positions for long stretches of time, that simply average to the wrong spot. This mark was placed quite deep, and the farmer is certain it's still there, and about two feet down. He tills to 10" max. My GPSr is usually right on if WAAS is active, but I didn't have WAAS signals until recently, and they might not be helping things all that much. I'm also concerned with all the high power radio towers messing things up. The readings just wander more than usual in that field (pun intended). The metal detector still seems like the most deterministic solution. Today I went back and recovered the azimuth mark for the 1939 benchmark. It was easy because I just drew the vector using USA Photomaps, and pulled the coordinates where it intersected the hedgerow. Got me to within 15' or so, then it was just a matter of seeing the witness post. I was surprised to see a relatively recent witness post there, as I thought nobody used azimuth marks anymore. Today I didn't have WAAS, yet the readings were more stable than before. Can anybody tell me what a "ridgeline" is?

[+Kewaneh & Shark]

...Can anybody tell me what a "ridgeline" is?

A ridgeline is the continuous top or peak of a line of hills or mountains. Think of the roof on a house: where the slopes meet is the peak, the line of that peak is the ridge, or ridgeline.

[+PFF]

Even if you did the trick on one afternoon and did see your readings converge to a single location, it might still be considerably off the mark because of the ionospheric refraction at that particular time. It might be possible to get a cluster of readings within 6 inches of each other, but it still could be many meters off the mark.

 

 

Hi, Folks:

 

I was passing along actual results that I have obtained, using a "consumer-grade" GPS receiver. The key to the deal is putting it into the decimal degree mode. Once you do, the results are amazing.

 

Granted, the accuracy of the unit does not increase. However, the fact that the display changes with just a small amount of horizontal movement is a great advantage. At station after station, I have moved around in circles until I see the readings are exact, and then found myself standing on the mark.

 

It has been more tedious since they began messing with my area's WAAS satellite. But I recently found two long-missing reference marks by using the NGS FORWARD program to compute the latitude and longitude, and then using the technique I described above.

 

I admit that, although I have done this dozens of times, I've only searched at 36 degrees Latitude. I can't speak to what happens up farther north, at 43 degrees and above. The original poster might try using the technique on a known mark in the vicinity, to see how well it works--before he starts digging at the target.

 

-Paul-

[+Harry Dolphin]

Not claiming any great accurace for my Garmin Etrex, at 41º north, but I usually seem to be within 30' on well placed caches. I checked the photos for 24 benchmarks found during the last month, and found the average accuracy to be 131 feet. Low of 9 feet, high of 259 feet. These are for scaled benchmarks.

I do find it curious that KU4050 which was monumented in 1930, but never recovered until now has five decimal points of accuracy for the seconds, but our GPSes logged it as 55.6 feet off.

[+jwahl]

There are these situations where if you don't have the metal detecting capability, and you don't want to or can't dig everywhere, the only answer is to survey a position in accurately. There are a couple of ways to do that. One would be to use surveyinstruments from nearby stations. In this case it appears there are a few. This takes the proper equipment and surveying expertise to do. The other is to use some form of survey grade GPS (sub centimeter), or resource grade differentially corrected, probably to a foot or so. Both of those would require enlisting the help of someone who has that equipment and knowledge who might be willing to help on occasion.

 

Otherwise you get close with the WAAS corrections. Do some averaging for say 10-20 minutes, then see if you can pick up anything with a good metal locator.

 

- jlw

 

Not claiming any great accurace for my Garmin Etrex, at 41º north, but I usually seem to be within 30' on well placed caches. I checked the photos for 24 benchmarks found during the last month, and found the average accuracy to be 131 feet. Low of 9 feet, high of 259 feet. These are for scaled benchmarks.

I do find it curious that KU4050 which was monumented in 1930, but never recovered until now has five decimal points of accuracy for the seconds, but our GPSes logged it as 55.6 feet off.

[Photobuff]

Well, for the mark in question, I don't have line of sight for optical stuff. The field will probably be plowed and planted shortly, so I may not pursue this one this year. OTOH, the 1875 mark has a bearing and direction from the TURKS HILL 2 mark. A friend of mine has an antique transit that he's been bugging me to repair (needs crosshairs). Yes, I do that sort of thing. It occurs to me that if I repair his transit, I can get the angle between one of the reference marks, and the 1875 mark. Then I just go 70' and dig a 30" hole to reach the mark. These plans always sound so fool proof on paper. Then I discover I'm a more clever fool than anticipated

[2oldfarts (the rockhounders)]

Photobuff, before calling it quits for this year there is still a way to get within just a few feet of the mark.

 

According to the description, there is a radio tower to the north. This should be easily seen from the mark. There will be a barn in line with the tower and a double trunked walnut tree. With these lined up the mark will be 5 1/2' west of that line.

 

There are 2 houses on the east side of Thayer Rd. Start with the one that looks the oldest. Where the center of the driveway for that house and the center of Thayer Rd cross, set a waypoint with your GPSr., walk away a bit and do a go to to that point you just set.

 

Now head out to that line of sight with the tower, barn and tree. When you are 5 1/2 feet west of that line walk along it until your GPSr says the driveway is 422 feet away. That will give you 2 of the 3 references needed for this mark. The 3rd is the county line. At this point you should be 53 feet south of the county-line.

 

You are now standing on the benchmark. Compare this spot with the GPSr set for the benchmark and you should be reading on the money.

 

John

[+Team_Talisman]

when hunting for benchmarks under the ground I use two copper rods shapped like an upside down L

 

But it seems that not all people can do this, its like witching for water...

 

I am find caches under the snow this way...

 

email me if you have questions no charge....

 

Dave from Team Talisman

Edited May 5, 2006 by Team_Talisman

[Photobuff]

Alas, I think the double walnut tree is long gone, and the cell tower transmitter building blocks the view of the barn. The driveway intersection is still there, and USA Photomaps gives the correct distance, so I know it's the right one. The problem comes back to the GPS uncertainty. Any location, line, or distance I establish with the GPS could be off by a dozen feet. There is a way, by establishing a distant bearing, then sighting to it, I think the error can be reduced if one end point is known. If both end points were off by ten feet, then the line could be off by ten feet. Darn the lack of WAAS satellites right now, and darn the surrounding radio towers for messing up what signal I do have. If I can fix my friends transit, that's probably the best way. Just start at the easy benchmark, and work my way around to the buried ones.

 

As for dowsing, I'm too much of a skeptic for it to ever work for me!