Survey Disk Placement/Usage Questions

[+frex3wv]

My wife went on her first benchmark hunt tonight and found it!

 

On our drive home I was telling her how much effort placing them was, especially so long ago (1930's, 1950's etc). She had some questions - maybe y'all can help.

 

1) Was it worth ALL that time and effort?

2) Were all these marks really used - especially the very rural ones?

3) How long were they used and would they really be used now with all the technology we have?

4) How accurate did they really end up being (computer ajdustments notwithstanding)?

 

Thanks in advance for anything you can share that I can pass along.

[Bill93]

Someone more expert may want to correct/elaborate these answers, but my understanding is that

 

1. Yes it was worth the effort because without it we would not have decent topo maps, construction of major projects would need to do more preparation work without those marks to start from, would not have had the data about the earth needed for accurate satellite orbit prediction, and many other reasons. They still set new marks today as a need arises.

 

2. All of the marks in the NGS data base were used for something, if only as stepping stones to get accurate measurements to other marks. The USGS additionally set thousands and thousands of marks that aren't in the NGS data, but were needed for the topo map effort. The only ones that may have gone to waste were some of the "C&GS and State Survey" disks which were done to employ more people in the depression and sometimes weren't measured to standards needed for the C&GS (now NGS), but some of those were useful for the topo maps.

 

3. Used from the beginning to present day. The horizontal (triangulation) stations are still sometimes used but are of much lesser importance now that GPS is common. When they are used it is often as the place for a local GPS reference to compare against a roving GPS. The vertical benchmarks are very much important today because GPS has a harder time getting that kind of accuracy in the vertical direction. Horizontal is easier for it.

 

4. The stations come in various orders of accuracy. The best horizontal ones from the old days are probably within a few centimeters (DaveD correct me if I'm wrong) although some have been disappointing when checked in recent times. The vertical benchmarks were supposed to be within a foot or so across the whole country and within tiny fractions of an inch in a local area.

 

From an old post, the horizontal order is specified as:

A-Order (1:10,000,000)

B-Order (1:1,000,000)

First-Order (1:100,000)

Second-Order Class I (1:50,000)

Second –Order Class II (1:20,000)

Third-Order Class I (1:10,000)

Third-Order Class II (1:5,000)

So two first order stations (best of the old ones) 5 miles apart should have a relative position error under 3 inches.

 

I can't find the table for vertical order right now, but I think first order is 0.017 ft * sqrt(miles) so that benchmarks 2 miles apart should give the correct elevation difference within 0.29 inch and two benchmarks 1000 miles apart should give the elevation difference within 6.5 inches.

Edited September 14, 2007 by Bill93

[holograph]

2) It is not unusual to find surveyor's paint or flagging near benchmarks, and that indicates the mark was used recently, since most flagging is designed to biodegrade, and paint fades rather quickly.

 

4) Also see this page.

[+Kewaneh & Shark]

1) Was it worth ALL that time and effort?

Yes. When you consider the survey work that developed from the benchmarks that were set, the infrastructure and development that was dependent on them, the ongoing progression of that infrastructure and development, together with all that come from that development, that marks become invaluable.

 

2) Were all these marks really used - especially the very rural ones?

Yes. They were set in areas where they were needed. Some get used more often than others, but they've all been used, and most (if not all) were intended to be used again.

 

3) How long were they used and would they really be used now with all the technology we have?

In theory, they could be used forever. Technology cannot replace what the marks and the associated data represent. In fact, in some cases, the old marks & data help to define that new technology. Technology can give us newer ways to access and/or relatively compare the data, using computers, new surveying instruments, and GPS.

 

4) How accurate did they really end up being (computer ajdustments notwithstanding)?

In most cases, the older marks were accurate and still are every bit as accurate, but it should be said: 'Accuracy' is a relative term, and there are varying degrees of measurable accuracy. Newer technology does not change the accuracy of a mark, nor does the accuracy 'end up' really being something else due to technology's advancement. Technology will continue to advance (so there really can't be an 'end', in that sense), but a benchmark's position and accuracy will always remain constant. That's what makes them benchmarks.

 

- Kewaneh

[+Klemmer]

2) Saw one I logged last year being used today. It is the vertical / scaled variety, if I recall correctly. From the way they were set up, and marking I saw there, I'm guessing curbs and street replacements are in store for that area. Needs it. Now more constrcution to go around between home & work. Uggghhh.

[+Renegade Knight]

...1) Was it worth ALL that time and effort?

2) Were all these marks really used - especially the very rural ones?

3) How long were they used and would they really be used now with all the technology we have?

4) How accurate did they really end up being (computer ajdustments notwithstanding)?...

 

Another angle. Even if it wasn't worth the time and effort it had to be done to solve certian land survey problems. Chiefly the US Government needed a way to describe land in such a way that they could give it away to homesteaders, and describe what they retained.

 

It worked since most of us are not suprised to find we built our homes in our neighbors yard.

 

Accurate for some (especially the ones denoting section lines) was "once placed this is the spot" meaning even if it wasn't accuratly located when it was placed it became the defacto control because property descriptiosn were derived from what was placed in the ground. In the section corners there have been some real doozies when it comes to error. Bechmarks in general have an accuracy they are reported as having. Some of the higher order ones are very accurat (and different than the section line markers set way back).

 

I can be confusing some NGS marks for other types of monumentation. We use survey markers of all sorts every day in my line of work. I'm not a surveyor and rely on them for control and to tell me what's what when we are working with marks.

[Bill93]

Knight is mixing cadastral survey (land boundaries) with the geodetic survey benchmarks and triangulation stations that were the original topic.

 

The land was surveyed either by "metes and bounds" using local landmarks or by the rectangular Public Land Survey System of townships and sections. At the time of settlement hardly any land boundaries were related to latitude and longitude other than perhaps a state boundary being an approximation to a parallel of latitude.

 

In the PLSS, error was allowed to accumulate for some distance and then a new start was made. There was no attempt to maintain the precision that would allow a unified coordinate system across the country at that time.

 

Only in recent decades have land boundaries been described in terms of latitude and longitude (or the related UTM or state plane coordinates), and not in all areas of the country at that, and such coordinates usually just to help find the old corners, not to control them.

[Z15]

From the NGS website, a simple explanation of what the marks accomplish.

 

Historical Coast & Geodetic Survey ~ Geodesy Albums

 

This network, the National Spatial Reference System (NSRS), provides the foundation for transportation and communication; mapping and charting; and a multitude of scientific and engineering applications.

 

 

Leveling on the "low line" at Hoover Dam. Note Hoover Dam in background. Level party of Emmett Sheridan

 

Image ID: theb1826, NOAA's Historic Coast & Geodetic Survey (C&GS) Collection

Location: Hoover Dam area, Nevada

Photo Date: 1941

Credit: C&GS Season's Report Sheridan 1941

Edited September 14, 2007 by Z15

[+Vines]

Great link Z!

[RedShoesGirl]

so what purpose does it serve now to report benchmarks. i enjoy the hunt but today was wondering why they encourage reporting.

 

and what do they do with discs that are missing? i discovered several today that are missing because it appears the railroad has bulldozes this area along the tracks where the markers are supposed to be.

 

curious i am.

 

rsg

[2oldfarts (the rockhounders)]

By logging the benchmarks that you find or do not find helps the next surveyor who intends to use that particular benchmark. They can read the log and see if the benchmark is still viable or if the disk is missing. They can then decide if they want to try and locate the disk and use it in their work or if it would be better to choose a different benchmark for the job at hand.

 

The ones where the bulldozers have been would probably fall under the choice of using a different benchmark from a nearby area rather than spend all the extra time trying to locate the benchmark where the ground has been heavily altered.

 

The disk may still be there, just not where the dozers ran or it may be under dirt that was pushed on top of it.

 

John

[Z15]

so what purpose does it serve now to report benchmarks. i enjoy the hunt but today was wondering why they encourage reporting.

 

and what do they do with discs that are missing? i discovered several today that are missing because it appears the railroad has bulldozes this area along the tracks where the markers are supposed to be.

 

curious i am.

 

rsg

 

Many of the marks have never been reported on since they were set many years ago. By searching out these marks and reporting on your findings helps update the data base as to what lefts out there. I k now when I worked at the DOT, Bench Marks were the basis of all our highway improvement projects surveys. All permits required for culverts, bridges, ditching and many other things had to be referenced to these marks per federal laws. In cases were none existed, it cost us time and money to go farther to find one or to establish new ones. Many in the surveying, engineering, environmental, aviation, navigation, pipeline, utilities among many others use them.

Edited September 15, 2007 by Z15

[AZcachemeister]

...All permits required for culverts, bridges, ditching and many other things had to be referenced to these marks per federal laws. In cases were none existed, it cost us time and money to go farther to find one or to establish new ones. Many in the surveying, engineering, environmental, aviation, navigation, pipeline, utilities among many others use them.

 

AH-HA! This explains the maddening series of marks along I-10 just south (east) of the I-8 interchange!

 

Typically:

CZ1587'DESCRIBED BY ARIZONA DEPARTMENT OF TRANSPORTATION 1975

CZ1587'4.65 MI NW FROM ELOY.

CZ1587'FROM MILEPOST 206, I-10, EAST ON THE INTERSTATE FOR 0.2 MILES AND TO

CZ1587'THE STATION ON THE RIGHT. ALONG THE SOUTH R/W FENCE LINE. MARKED BY

CZ1587'A P AND M SURVEY SIGN. A REBAR, UP 0.1 FT.

 

Might be recoverable if the signs were still there (which they are not), but how long will that rebar hold it's position?

Long enough to build/improve the freeway, I guess!

[+BuckBrooke]

I've recently (the past year) begun daily commuting between Albuquerque, NM and Socorro, NM, about an hour and 5 minutes drive (80 miles). I had long ago recovered (or not ) all of the PIDs along that stretch of I-25. The New Mexico Highway Commission placed a line of disks, a line of can/rods and a line of small steel I-beams (not PIDs, but sticking up about 4-6 inches) along the route in the 1970s/1980s, and given the info to the NGS. No one had logged them since then. I've been very pleased to see survey crews work their way along a stretch of the highway, using and tagging the marks there.

 

In particular, in one several mile stretch a landowner terraced a series of fields along one side of the highway, and had hired surveyors to make the field flat. I watched them use the thirty year old benchmarks I logged to do so. Kind of fun to know what's going on, when most driving by wouldn't.

Edited September 20, 2007 by BuckBrooke

[+cowpill]

I know here at Kirtland AFB they use the marks constantly; with new construction to the alignment of lasers and thier targets. There are some in the classified areas that rarely get used but are important for varing scientific venues.

[Wintertime]

I'm bumping up this topic because someone just asked me the other day why survey marks were placed and whether they're still used.

 

I found a USGS brochure that discusses how, before aerial photography, topographic maps were made by going to high points, determining those elevations, and sketching the intervening terrain. I think that answers my friend's question about how they were able to make maps that showed terrain in great detail when they had only determined the exact location of a finite number of points.

 

I'm having more trouble figuring out why they're still useful. Comments have been made here about GPS systems being better at horizontal than vertical measurements, but is that true even of the $30,000 professional ones? And certainly they're plenty accurate horizontally. Is it that not every surveying company assigned to work on a new strip mall has a $30,000 GPS system?

 

Would appreciate any clarification of the above issues.

 

Thanks!

Patty

[2oldfarts (the rockhounders)]

I'm having more trouble figuring out why they're still useful. Comments have been made here about GPS systems being better at horizontal than vertical measurements, but is that true even of the $30,000 professional ones? And certainly they're plenty accurate horizontally. Is it that not every surveying company assigned to work on a new strip mall has a $30,000 GPS system?

 

Would appreciate any clarification of the above issues.

 

Thanks!

Patty

 

It seems to me that one of the obvious reasons to use an existing benchmark is time. An adjusted horizontal control disk that is relatively easy to locate (as in being logged on GC,com) save an appreciable amount of time getting set up to use a needed point. With the $40,000 GPSr you still need to spend time acquiring the data from the satellites, then computer process it before you can use that position. With a "located" disk you set up your equipment and go since you already have the needed data for that position.

 

Don't know if this is an accurate observation, but it sounds good to me.

 

John

[CallawayMT]

Patty,

 

Yes, the existing survey monuments are still used and needed by survey and engineering companies. The horizontal marks are useful for as John mentioned, an instantly known value to base a control survey upon, but the horizontal marks need to be of a high order or one that has been recently adjusted so that it is on a datum and adjustment epoch which will agree with the latest adjustments, i.e. I can't sit on an old triangulation station which had coordinates collected by traditional methods; which means that it had angles turned to it to establish coordinate values. These older monuments generally are in the area of 3' off of a GPS established position and so there will be problems using older monuments with newer GPS established monument positions. Even a mark that was established with GPS 10 years ago and may have never been included in newer adjustments will have differences of up to 1' compared to newly readjusted or established positions. So we always look at the order of the horizontal position and the adjustment criteria located on the NGS data sheet before we use certain monuments for a project. Now that being said, most of the time I use CORS to establish my control projects and will use any existing horizontal or vertical marks that are in an area just to use an all ready established well set survey mark.

 

The next thing that we always try to do is shoot existing vertical control marks surrounding a project area, the reason being is that a CORS established position is very accurate horizontally, usually within 1-2 cm's within the NSRS system, but the vertical component is generally more of a question. So if we shoot a few known vertical bench marks then we can hold the elevation from the NGS data sheet from one mark and then see how that elevation translates via the geoid model to the other bench marks which were tied and hopefully run on the same level run, to see how accurate our vertical component is throughout our project. Without the existing bench marks we would have to run differential levels throughout, and depending on what we see in our adjustment, we may still run the levels.

 

Here is a recent bench mark tie on a project on Mount Hood and it is also a good photo to show the discussions we have in this arena as to GPSable. The mark may be difficult to occupy and there was a lot of trees surrounding it, but with good timing and enough time I was able to occupy and use this mark with good results.

 

 

CallawayMT

[Wintertime]

Thanks, John and Callaway. It's nice to know that existing marks are still useful--especially if they've been adjusted fairly recently. Good points about GPS receivers not being ideal for vertical control and forested areas, too.

 

Patty