Searching For A Benchmark

[+stellarscapes]

I'm relatively new at hunting benchmarks. I've only found 2 benchmarks so far, and both were easy finds that had been found many times. I've come across one now that is really on my "must find list" now. I'm currently looking for a benchmark that apparently has never been found since it was placed there in 1958. The driving directions on the information sheet are no longer completely valid, as the fire road mentioned was bulldozed over when an interstate was constructed in the early 1960's. I was able to access the area from another direction, and there is a trail that goes right near the coordinates listed for the benchmark, so I'm assuming that trail was once the fire road. I searched the location for about 30 minutes (by trying to follow the instructions on the data page) and couldn't find anything, (the coords listed on the page were adjusted - and I've read that "adjusted" coordinates are fairly accurate). Anyway, there is a main surface disk, which is on a concrete post 4 inches above ground, a disk for ref mark 1 and a disk for ref mark 2, and a standard metal witness post. It also states that below the marker is an underground disk set in a mass of concrete 44 inches below the ground. My questions are these: WHAT IS THE PURPOSE OF THE UNDERGROUND DISK 44 INCHES BELOW GROUND? Also, what is a "STANDARD METAL WITNESS POST". WHY ARE THERE 3 DISKS (main station disk, and 2 reference disks)? From what I understand from reading the data sheet, no one has ever found (logged) this since it was "monumented" in 1958. It's in a remote area of a Pennsylvania State Gamelands, and I'm sure the only people who have been in the area have just been hunters. Right now the area is full of thick bushes and lots of trees that were much smaller 48 years ago. Are there any hints anyone can suggest that might help in find the benchmark? Thanks.

[+m&h]

Could be tough, could be very worthwhile.

 

The buried mark is the permanent mark, and should not be disturbed by any person not employed or assigned to the task by NGS. The surface mark is directly above the buried mark, and except in extraordinary circumstances, the surface mark is the one everybody uses. A witness post is a visible marker, made these days of fiberglass but in those days of steel fence-post stock, slender and three or four feet high, bearing a small sign announcing the presence of a survey marker. In the relentless disintegration process, the witness post is usually the first thing to go.

 

Triangulation stations are among those marks that are equipped with reference marks; that's probably what you have here. The purpose of the reference marks is to provide a means of replacing the primary mark, if it is lost and the reference marks remain.

 

We haven't got quite enough information to be more precise here, but we would suggest narrowing down the GPS reading as much as you can, perhaps by setting it on a tripod and turning on the AVG function for twenty or thirty minutes, and then going to work with a sharp eye or a metal detector or both.

 

Good hunting!

[+Black Dog Trackers]

WHAT IS THE PURPOSE OF THE UNDERGROUND DISK 44 INCHES BELOW GROUND?

The underground disk is there for security purposes. In case the surface disk is gone, the point can be resurrected from the underground disk if necessary. DO NOT dig for any "underground disk". That is generally considered beyond our scope as amateur benchmark finders. If a surface disk is buried under a few inches of dirt, that is OK to dig up (unless it's in someone's garden!) but I'm talking about the ones specifically referred to in the description as "the UNDERGROUND DISK", like the one you're mentioning.

 

what is a "STANDARD METAL WITNESS POST"

There is no real standard witness post - there are a few kinds - the main ones being a steel stake about 4 feet tall with a metal sign on it, and the other is a orangish flat plastic stake about 4 feet tall. You can examine the Benchmark Gallery to find some examples and also you can look at this site which pictures different kinds of witness posts, benchmarks, etc.

 

WHY ARE THERE 3 DISKS (main station disk, and 2 reference disks)?

Triangulation stations often have reference marks. Like the underground disk, these are for resurrecting the use of the main station if it is gone. Reference marks are also used to help a surveyor to find the main station. By the way, the main station will likely have a triangle in the center, while the reference marks will have an arrow that points toward the main station. You can count a Find only if you find the main station. Finding just the reference marks doesn't count as a find.

 

A very important thing for you to do is to get the 'box score' for this mark. On the benchmarks's web page, click on "view original datasheet". You will then be looking at a circa 2000 copy of the datasheet in NGS format. Look for the ascii box that names the reference marks and shows their distance and azimuth (dddmmss.s) from the main station. Bring this information with you when you hunt for the mark! You will also need a tape measure and a compass. Hopefully you are not in an area of large declination.

 

Note that a current copy of the NGS datasheet can be obtained here.

 

Welcome to benchmark hunting!

[Bill93]

>>apparently has never been found since it was placed

 

Possibly, but many users of benchmarks are too busy to report them. It isn't in the project schedule or budget.

 

>>I searched the location for about 30 minutes (by trying to follow the instructions on the data page) and couldn't find anything,

 

That happens to everybody sometimes

 

>>I've read that "adjusted" coordinates are fairly accurate)

 

Unless there was a clerical error, they are more accurate than your handheld unit. Don't confuse adjusted vertical with adjusted horizontal, howerver.

 

>>WHAT IS THE PURPOSE OF THE UNDERGROUND DISK 44 INCHES BELOW GROUND?

 

A backup so that if the "surface" disk gets disturbed they can dig down and use the underground mark to reset a new surface mark. It is unlikely to be done nowdays because the work often exceeds that needed to establish a GPS station of similar accuracy, but it has happened.

 

>> a "STANDARD METAL WITNESS POST"

 

usually a steel post looking much like what a farmer would use to hold fence wire, but in this case having a sign saying "Please do not disturb nearby survey mark" or words to that effect. The ones used by the C&GS or NGS are usually white with black lettering. The newer posts are often orange "carsonite" plastic.

 

>> WHY ARE THERE 3 DISKS

 

The reference marks are to help find the main disk, especially in the days before GPS. The RMs are often more exposed to be visible, and thus more vulnerable.

 

>> any hints anyone can suggest that might help in find the benchmark?

 

Since it has adjusted horizontal coordinates, and you have done the best you can with the description, the only thing left is to probe the ground with a rod (camp fork, golf club handle, etc) to see if you hit something, or bring a metal detector. Disks tend to get covered up over the years, particularly when there is a lot of vegetation to make new soil. In a remote area digging probably won't cause any problems (fill your divots) but in developed areas you have to be very careful about buried utilities. For that reason most of my digging is done with a garden trowel instead of a shovel.

 

Boy, you have to get here early to get ahead of some people. Two other good answers while I was trying to untangle my sentences.

Edited July 13, 2006 by Bill93

[Z15]

STANDARD METAL WITNESS POST

 

Meaning what they were using in that time period.

 

Standard witness posts, in chronological order of use

 

1st type - 4inch square white wooden posts

2nd type - Metal fence post with 4x6 in metal sign (oftern removed by souviner hunters)

3rd type - Orange fiberglass post with NOAA logo decal

 

Its rare to still find the wood posts but the other 2 are still out there and still being set.

Edited July 13, 2006 by Z15

[mloser]

Here is how I would search for that mark, which I am thinking may be LY2691. First, I would download the NGS datasheet directly from their database, which is the link above. Then, after finding my way to the mark, and armed with a probe, metal detector, compass and 100' tape measure, I would:

 

Use my GPSr to get close to the mark. You are right that the coordinates are adjusted and therefore accurate. As Bill93 mentioned they are more accurate than your GPSr can get, so you will find the reading getting squirrelly when you approach the mark location. The description of the station is a bit vague and relies on the witness post remaining, but they quite often disappear. Being 10 feet from the center of the trail is not very helpful either because the trail is linear and the mark is a point, but it is something to start with. I wouldn't measure that distance but would pace it off or guess it, then walk the trail where my GPSr kept reading in the 1-20 foot range, probing or metal detecting. A benchmark that was exposed 4 inches in 1958 could well be under dirt, logs and leaves by now, and in fact will most likely be covered by something. A simple metal stick might make all the difference--when you tap concrete you will know it. That works well only in areas that are relatively free of rocks. The mark is also 46 feet northeast of the crest of the hill, which may or not be helpful. It may be hard to tell exactly where the crest of the hill is.

 

One way I use my GPSr on an adjusted station is to use the squirrelliness of the reading when close to the mark to my advantage. I know that because of the innaccuracy of the unit when it says 0 feet it is probably not right at the mark. It is up to 9 feet off at the best reception. However, by walking away from the area where I tbink the mark is at the GPSr will point TO the station with what seems to be fair accuracy. By walking around the area I can mentally triangulate to the mark by averaging where the arrow points when I am standing 10 feet or so away from it at different locations.

 

After failing to find the main station I would look for either of the two reference marks (which I would try to find anyway) and use the reference mark to track back to the station, this time taping the distance and measuring the angle as carefully as possible.

Both RM1 and RM2 are as vaguely described as the station, but I would start by using my GPSr to get me where it thought the station might be, then pace the distance to one of the reference mark--whichever has less obstructions between me and it. How do I know the distance?! The Box Score, that BDT mentioned. It is not on the Geocaching site but you will see it on the NGS datasheet and in this case it looks like:

 

LY2691|---------------------------------------------------------------------|

LY2691| PID Reference Object Distance Geod. Az |

LY2691| dddmmss.s |

LY2691| CRESCENT AZ MK 0250623.0 |

LY2691| CRESCENT RM 2 6.993 METERS 26902 |

LY2691| CRESCENT RM 1 8.264 METERS 34230 |

LY2691|---------------------------------------------------------------------|

It lines up better in a monospaced font so that the headings are over the correct information.

 

Reference mark 1 is 6.993 meters (I am sure there is an easier way but when I convert meters to feet I multiply the meters by 3, then add that same number of inches to the distance. Thus 7 meters becomes 21 feet plus 21 inches, or 22' 9") at a compass direction of 269 degrees 02 minutes. That is degrees from TRUE NORTH, and the declination at that place is 12.6 degrees (I got this by selecting Topozone from the Geocaching page. It is at the bottom of the page). This means I add 12.6 degrees to the reading and come up with 281 degrees. I would walk that approximate distance from where my GPSr seemed to zero out the most and see what I could find. If I failed, I would check on RM2. Once you find either reference mark you can measure exactly BACK to the station and concentrate your search. If you find BOTH RMs you can be even more accurate by triangulating for the station, measuring the distances from each and where they cross is the station, I would hope.

 

And don't forget to look for the azimuth mark--it is described at the bottom of the sheet and to me finding a triangulation station isn't complete until I have looked for the station, all reference marks and the azi.

 

Good luck.

 

Edited for content clarity.

Edited July 13, 2006 by mloser

[+stellarscapes]

I'd like to thank everyone for the answers to my questions and ideas on how to search

for the benchmark. The benchmark is LY2691 (Crescent). I plan on going back there

early one morning (before it gets too hot) and searching for it again. I just got a metal

detector, so I'll take that along with the tape measure, metal rod, compass, etc...

Thanks again for the hints and ideas.

[Team Fawlty]

You are right that the coordinates are adjusted and therefore accurate. As Bill93 mentioned they are more accurate than your GPSr can get, so you will find the reading getting squirrelly when you approach the mark location.

 

Where do they get the adjusted coords? Did they find and measure them but not log them?

 

Brendan

[Bill93]

Most of the adjusted horizontal coordinates come from the days before GPS, much of them in the 1930's.

 

They were obtained by making optical sightings with the best available theodolites. In many cases they used 100 foot towers over the disk to get high enough to see other towers as well as church spires and water tanks. Then they spent an enormous amount of effort (with mechanical calculators that could barely multiply and divide) calculating the positions of all the points relative to each other across the country.

 

The accuracy that they attained was essentially the limit of what human ingenuity, technology, and massive effort could achieve at that time. Today the accuracy is improved only by professional grade GPS with long observation times. It was an accomplishment of grand proportions like that of the transcontinental railroad, the Empire State building, Hoover Dam, or the Panama canal, but rarely gets mentioned in the history books. The NGS site http://www.ngs.noaa.gov under "Our History" has good text and some fascinating pictures.

Edited July 14, 2006 by Bill93

[+m&h]

We would add to Bill93's excellent note that the adjustment itself is usually a much later process than the one he describes so well.

 

Having obtained coordinates by those classical methods, and having a certain number of calculations for the same point, all of them different by tiny distances (usually!), a mathematical procedure called a least squares adjustment can be run on the set, and the result will be the most likely set of coordinates, carried out to five decimal places. Least squares procedures come out of statistics and probability. They work best when the set of quantities to be adjusted is fairly sizeable.

 

Datasheets bearing adjusted coordinates usually have a line saying when the adjustment was made--early 1990s in many cases. But usually the sets of coordinates for the point were gathered long before--often decades back.

[+Klemmer]

AND - again, in case you missed it - the coords from the 1930's or even earlier are WAY BETTER than our handheld consumer grade GPS receivers. Amazing!

[kc2ixe]

...snip...

Then they spent an enormous amount of effort (with mechanical calculators that could barely multiply and divide) calculating the positions of all the points relative to each other across the country.

...snip...

 

They also used "computers" - not these new fangled electonic or mechanical things. There was a job called "computer" - as in a person who computes...

 

My Father In Law spend a BUNCH of time as a young teen during WWII working as a computer for ballistic tables for the Govt

[68-eldo]

Then they spent an enormous amount of effort (with mechanical calculators that could barely multiply and divide) calculating the positions of all the points relative to each other across the country.

 

.

 

Here's a site thats shows some of those early mechanical calculators. My father had a slightly newer version of these. Neat old machines.