# A "dog" Of A Bench Mark

Followers 1

## Recommended Posts

Ever have a bench mark that "gets under your skin"?? Well folks, I found one today.

Dog PB&PP is such a mark. It is located near Great Falls, Md and does not show up on any old maps of the area. Add to this a very heavy tree canopy.

I knew this just had to be there, but where?? There is no reference mark or witness post in sight.

Finally located this one by setting the gps on a stump and letting it settle for 30 minutes, then probing the ground with a steel rod. Gosh darn diddly ding dang thing was 2 inches below ground level.

Anyone ever go through this much trouble just to find a single mark??

Difficult Run -

Kudos on the find. I've been through the Great Falls area (as you have seen). It's beautiful, but I haven't had much success finding marks there.

Also, well done on finding the remains of HV1776; another one that stumped me.

Will

Congratulations for sticking with it through multiple attempts! Great job--especially with the coordinates being scaled!

I've used a similar technique on several marks, recently. Just a few days ago, I had to use the GPS because the Corps of Engineers moved this reference object.

Several months ago, I discovered that my GPS receiver reacts much faster to movement if it is in the DDD.DDD mode. It is easy to convert the benchmark's coordinates to this format, and moving only a foot or less produces a change in the displayed reading.

To convert the GEOCACHING format (DD MM.MMM), divide the minutes by 60. The resulting number is the decimal point for degrees. Try this, the next time you need to know where to dig or probe--especially for ADJUSTED coordinates. I believe you will be impressed with the precision!

One word of caution for those whose mathematical minds are racing to the next level: Yes, you can take a reading on a mark in the DDD.DDDD mode and convert backward to other formats. (You multiply by 60 instead of dividing.) With my GPS unit and two calculations, I can produce a reading to degrees, minutes, and seconds with three decimal places. But my tests on known locations have demonstrated that those extra digits after the decimal imply an accuracy that is not there.

In other words, math alone cannot improve the accuracy of a consumer-grade GPS. The good news is that you can take several readings--approaching from various directions--and average them. Then round off, using the extra digits.

As Casey from NGS says, the published coordinates will not be adjusted, but it makes a nice addition to the "how to find it" description. Especially for SCALED coordinates.

-Paul-

Anyone ever go through this much trouble just to find a single mark??

I have to chuckle with you here... as a for instance, I have dug them up under 1 foot of decomposed leaves and topsoil in the middle of blackberry bushes, and that is just one time. I work in the survey field and hunting these is often part of the job. I have looked for thousands of survey markers and continue to do so all the time.

I could go on, but you know Let me just say that for many, a challenge like you had is sort of what many here secretly hope for. That is part of the fun of it, and we hope that you likes it too.

Good Luck!

Rob

Difficult Run,

I visited KW1220 4 times, researched it online, and finally gave up on it and marked it as Not Found. That was an unsuccessful one. I have had better luck with other marks, including KW1316, where I got two property owners, their relatives, and a shovel involved. That one took about 2 hours start to finish. KW2992 Barry 1885 gave me some trouble as it was a stone marker with stone pile reference marks set on a hill made up totally of stones! I couldn't probe because it was all rocks, I couldn't metal detect because it wasn't metal (and oh yeah I didn't have a metal detector). I did something similar to what you did to make my GPSr behave. Instead of setting it down I would walk towards the location it pointed to, pass it, then stop, and see where pointed next. Doing that for about 15 minutes got me to a "suspect" area about 10 feet in diameter. Then I started looking hard and located the square monument amidst the oddly shaped rocks.

By using my GPSr this way I can increase the perceived accuracy. If it tells me the mark is 15 feet away to the north, I mentally mark a point near that location, then I head to the east. There it says I am 10 feet to the east of the mark, so I go north. By circling it this way I can mentally triangulate to where my GPSr is taking me. I discovered this by accident when searching for Barry 1885 and it has served me well a couple other times.

JV4592 SUGAR LOAF 1865 was another one I had to return to, and may return to again with some benchmarking friends. The first day I went was cloudy and wet and I climbed to the mark and looked without much success. The second day I returned (a 1.5 hour drive) on a cloudless 75 degree day and spent over 3 hours measuring, talking to people, reading the descriptions and remeasuring. That day alone on the mountain was worth the trip. It was gorgeous and I met a bunch of nice and interesting people (it is a popular public hiking spot).

Matt

Difficult Run -

Excellent work and a great success!!

I'm assuming that, like me, you don't use a metal detector.

On the subject of Sugar Loaf 1865 that mloser brings up, I have been thinking of proposing a second local benchmark get-together at this fascinating area of several marks on Sugar Loaf mountain. I believe that mloser and I both agree that there are possibly some as yet unsolved mysteries about the place and it might be an amusing site to meet soon and work on the marks there a bit more. In any case the old markings (surveyors' etched marks in stone) there are quite interesting to see and worth the trip for others to see and log, have a lunchtime picnic, and share some benchmarking stories.

BDT, Excellent suggestion. For those outside the DC area, he is referring to a lunch gathering some time back with about a dozen benchmark enthusiasts and an NGS delegation including Dave and Casey. I think when it was organized we talked about lunch + benchmarking, but in the event all we actually did was eat. (Not that there's anything wrong with that....) I like the idea of another gathering focused on actual benchmark hunting.

And Difficult, great work and perseverence!

Paul, why do you suppose the GPS would resolve more rapidly in one display mode than another. I would assume it captures the data and does its black-box calculations — that's the time-consuming part — then displays in the format selected by the user by simple arithmetic calculation, which I would have supposed would take only a trivial amount of time. (And good point in reminding us of the limits of precision.)

-ArtMan-

Thanks to everyone for the kind words. Each trip I made took at least 2 hours each, of which an hour was spent probing the ground in a random fashion. Yes, it would have been much easier with my metal detector, but the National Park Service has a very dim view on that. Last I heard, the use of a metal detector on park property carried a \$40,000 fine. I didn't even bring a shovel, chosing to dig with bare hands, as this area was home to several gold mines from 1861 to 1951, producing over 5000 ounces of gold.

SugarLoaf Mountain. I like the idea of an organized benchmarking party. It could be a collaborative effort with everyone responsible for thier section of the recovery documentation. One or two people to make measurements, another to photograph each mark/reference, another to render an overall diagram and someone to write a new description.

Who's bringing the beer??

- Mitch -

mloser -

Excellent description of a good technique. When the references are too far (or too few) to run a tape triangulation and I'm working under a clear sky, I've used your technique to find marks with adjusted coordinates with good success.

W

I recently spent 6 trips up a deserted railroad bed looking for OC0397. It hadn't been touched in 60 years and was buried under mounds of dirt and debris. I finally

used my metal detector and dug up the thing.

OC0397

Edited by osprey46
Several months ago, I discovered that my GPS receiver reacts much faster to movement if it is in the DDD.DDD mode. It is easy to convert the benchmark's coordinates to this format, and moving only a foot or less produces a change in the displayed reading.

To convert the GEOCACHING format (DD MM.MMM), divide the minutes by 60. The resulting number is the decimal point for degrees. Try this, the next time you need to know where to dig or probe--especially for ADJUSTED coordinates. I believe you will be impressed with the precision!

One word of caution for those whose mathematical minds are racing to the next level: Yes, you can take a reading on a mark in the DDD.DDDD mode and convert backward to other formats. (You multiply by 60 instead of dividing.) With my GPS unit and two calculations, I can produce a reading to degrees, minutes, and seconds with three decimal places. But my tests on known locations have demonstrated that those extra digits after the decimal imply an accuracy that is not there.

In other words, math alone cannot improve the accuracy of a consumer-grade GPS. The good news is that you can take several readings--approaching from various directions--and average them. Then round off, using the extra digits.

The old question of precision versus accuracy. I have the scientifical training to know the difference between the two. And I've given up arguing it at work. If we measure a box to the nearest inch, none of the figures after the decimal point are of any accuracy. Fictitious numbers with no meaning! Actually, the last digit before the desimal point is seldom accurate. Yet, our packing lists list the cubic feet to three decimal points. I find it embarrassing.

Which brings up (yes, I've mentioned this before) altitude on a disk to three decimal points. KV0491. We found this one today. You will note that this disk is located at exactly 21.586 feet. 21 feet 7.032 inches? In your dreams that such accuracy existed in 1956. In your dreams that, even if that were true, that some slight subsidence into the Hackensack River would not change that figure. Not true then, not true now. Precision with absolutely no accuracy. Embarrassing to see anyone make such a fool of themselves.. Oh, well.

But the question is dogged determination. How many times would I look for a benchmark? Generally only once. Usually I know that I'm not going to be able to find it. Probig and digging is another story. Do I really want to probe and dig in the traffic circle at Bergen County Community College? If the mark is there, it's under the grass. I was not equipped for digging and probing. On the other hand, I dug for the one at the old Sparta train station, because I noticed that the footings had been buried (comparing one foundation with the others.)

Here's one I'd put off looking for: KU1432

Three buildings south of the 86th Street Traverse. The eastern building and the middle one are part of the Central Park Precinct. The western building belongs to the Central Park Conservancy. The east and west building but up against the hill, so that the second floor does have doorways in the back. I wandered into the police building on the west, and was told that I shouldn't be there, but they helped me look. 30' south of the 86th Traverse. Climbed up to the north part of the Great Lawn, and searched the back of the buildings. I did not find the mark, though MOCO did. We will be back with a measuring tape/rope, to measure 570' from the 'tunnel', and see exactly which building it is on!

There is still the possibility that we may find this one yet! And therefore, we will go back to look.

We found this one today.  You will note that this disk is located at exactly 21.586 feet.  21 feet 7.032 inches?  In your dreams that such accuracy existed in 1956.  In your dreams that, even if that were true, that some slight subsidence into the Hackensack River would not change that figure.  Not true then, not true now.  Precision with absolutely no accuracy.  Embarrassing to see anyone make such a fool of themselves..  Oh, well.

I typically make road grader operators grade 5/8ths minus crushed rock to +/- 2/100ths of a foot accuracy, and I can quickly measure that accuracy with a simple hand level and a 25 foot lufkin tape measure. I can do it all day with a Construction Laser and a grade rod too. RTK GPS is really fast at it these days and a lot less work.

I commonly expect Bulldozers and Excavators to cut grade to 1/10th of a foot, some operators are able to exceed that. It depends on the soil conditions and the operator. Keeping this sort of accuracy is important. It ensures a lot of things like Storm drainage and sewer pipes maintaining a constant flow to the plant. Sewer pipes overall maintain a 2% grade in the flowline of the pipe, otherwise the design won't work well. The water will outrun the toilet paper, and that is not what you want.

What this means that if my construction plans call for me to have the subgrade for a road meet 21.58 feet, which is 21 feet 7 inches, I intend for my rock to be within 21.56 and 21.6 feet above sea level, and I can hit it right on the Money if the the Road grader is GPS equipped. or the operator is good, and some are that good. 5/8ths Minus rock is typically bigger sized than 2/100ths of a foot, but after compaction, it is real close. Close as we can typically get it.

There are 8.3 hundredths of a foot in one inch, (it is actually a repeating decimal which we typically round to the nearest hundredth, not thousandth, because we can balance a hundredth anywhere without it making a difference.) so I typically expect my subgrade to be accurate to +/- 1/4 inch or less all over my job site, repeatably. I have to because when I go from Grade work which is typically measured in Hundredths of a foot to concrete which is typically measured in inches, everything has to match, and the only thing adjustable is the grade. If we cannot do it, then water will puddle everywhere. So we do, and we do it in a production like manner. Heavy equipment operators have to be that good all day every day.

Concrete curbs and sidewalks are held that accurate as well. Even the ones made in the 50's. If the plans say a point on my finished grade F/G needs to be 21.59 feet, then it will be. You would love laying out super elevated curves with vertical curves set for intersections in the middle of them. Way more fun than you want, yet they happen to be everywhere, especially near freeway ramps and elevated structures. We have to hit the numbers that tight though or you will spill your coffee while driving, and we like to avoid it as much as possible.

If I take a road section which is typically stationed, from the plans, scale so many hundreds of feet from a point of known survey at finished grade, and calc the rise over run from the plan, I should be able to go out to the site and measure a grade which should nearly concur with my estimate. I do it all the time as a matter of fact, and I am rarely very disappointed. The Math will fit the planned terrain, and all roads are very highly planned and documented. Z15 can back that. You can fill the back of a Suburban with road plans, easy.

Geodetic Leveling is to the 1000th of a foot easily, and yes it is a repeatable measurement. We have rods that will read a thousandth, Optics that can see it, even in the 1930's and GPS today that accurate as well.

I can tell you that if I were building a huge telescope for high accuracy astronomical observation, I would guess that I could be expected to hold some of the work to 1/1000 foot precision, and it would be necessary. Imagine rotating 360 degrees on a pedestal that was not absolutely level, and expecting a computer to train the scope on a particular part of the sky, exactly, and with repeated accuracy? The accuracy has to be built in on every level, starting with the soil.

Bench Marks are important things! Horizontal control is too!

Hope that Helps,

Rob

Edited by evenfall

Rob,

While I can't add much to your grading/surveying/construction information, as to astronomy...typically the needs are precise, but not over a large scale. The back structure for a typical large telescope sits on a precision concrete pedestal which extends downwards to bedrock, and often is thermally isolated from the surrounding dirt so that it doesn't expand/contract.

An odd example is the pedestal for the Green Bank 100m radio telescope. It is a 210 foot diameter circular track that surface is accurate to 1,000th of an inch. That it has an 8,000 ton telescope slowly rolling over it is impressive.

More typically, telescopes rest on a cylindrical concrete pier up to 6 meters across that extends to bedrock, with similar accuracies.

## Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

×   Pasted as rich text.   Paste as plain text instead

Only 75 emoji are allowed.