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What Are Benchmarks?


FLMike

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I don't understand their purpose. How and who determines where benchmarks are placed? What criteria are used for placement? Is it a location grid?

 

I've been to the NGS website and didn't find it helpful.

 

Infosponge, how ya doing there in Florida? Still taking good care of my geocaches down there? I notice you archived the Cha Cha.

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Benchmarks are natural and man-made objects useful for identifying where you are.

 

There are several types of benchmarks. Some give you very accurate north-south/east-west location information. Some give you very accurate altitude above and blow sea-level. Some give you both, and others are useful either for finding another benchmark, or for ascertaining where true north is for a benchmark.

 

Given that benchmarks can provide this information, they are useful in a large variety of situations. Some examples are recording right of way information for highway and railroad development. Verifying that the dyke protecting your town has sufficent hight to accomplish it's task. Confirming the boundries of you and your neighbor's properties. Helping a farmer set up and manage water runoff features for his property. Determining if your property is in the floodplane of the local streams and rivers. Monitoring erosion of river banks. Undoubtably hundreds of other activities.

 

Before the advent of flight, benchmarks were the only way that mapping could be done. Road, geo-political and topographical maps were created with the assistance of benchmarks.

 

With the advent of arial photography, GPS, and subsequently DoplerGPS, it is far less common for benchmarks to be used in mapmaking and other directly related tasks. On the other hand when you need to know for certain what is where, on the ground, it is often less expensive to make a few measuremnts with a laser rangefinder, a coupld of laser targets, and a couple of known benchmarks, than it is to use a DoplerGPS, which is the only one of the tools reliable enough to be used in comparison.

 

This is becoming less true every day, and benchmarks are likely to be used less and less as the years go by, however I suspect that the ability to use the traditional surveyor's equipment, along with an understanding of the history behind benchmarks, is likely to be as much a mark of pride amoung surveyors as the ability to use a sextant and watch to determine your location at sea is to ships navigators.

 

Then again, I could be wrong.

 

-Rusty

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Rusty,

 

Thnaks for the reply. I understand how benchmarks can be useful but I'm still hazy about some of the details re: placement. What criteria are used to place these benchmarks? Are they placed where chosen minutes or seconds of latitude and longitude intersect? Is it random? Is there a pattern?

 

Today I tried to find the benchmark nearest my home, HZ1979. I couldn't find it but the description says it is in a culvert, which I did find, three feet below the adjacent road surface. Why would someone put a benchmark in a ditch? What purpose is served placing a benchmark that is so difficult to find? Who put it there? Another local benchmark is a cross on a church steeple. What's going on here? icon_cool.gif

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Survey Tech, gurubob, or some of the other people who use benchmarks daily may have better answers, however I will give it a stab as well.

 

There is no real "grid" organization as to why benchmarks appear at any specific location. In almost every situation, it is placed there at the decision of a surveyor who needs a benchmark for a specific task.

 

In the example of the culvert you found, though not the benchmark as part of that culvert, I would suspect that the mark was placed as part of the construction of the water management facilities for the resivour to the west of the mark. It is possible that the mark was placed long before the road was constructed. I see no notes from the monumenting of the mark, other than possibly the details of the description that the disk is embeded in a wing wall of the culvert. Also as part of the culvert it would be useful in the construction of the road, without being likely to be disturbed by that construction.

 

There are several factors that contribute to why a mark is established in a specific location. For example you do not want the mark disturbed for the duration of the project, or the useful lifetime of the mark. In almost all cases the mark is positioned some place where a tripod can be set over it to establish a secure "light" or laser target. That target is placed at a known or measured hight above the mark to be referenced back to the mark from other locations within the work area.

 

I also note that the this marker is a first class vertical order marker, which means that the altitude (or at least one of the given altitudes) is considered to be very accurate, note that they give it in hundredths of a foot, meaning it is believed to be off by less than half an inch. This is useful information when establishing water control facilities. You do not want one end of a dam to be significantly lower than the other end. Likewise if you set the dam too high up into a river bed you won't get enough water behind it to make it useful, and if you get too far down the river bed you are likely to loose the dam because of heavy runoff some year.

 

I also see that the horizontal accuracy is to within six seconds. If you have a handy straight stretch of east west, or north south roadway, find a point on that stretch that your gps says is an even multiple of 10 seconds, and walk in some direction along that road until your gps says you have moved six degrees from your starting point. You will probably be surprised at how far off from your starting point you have gone.

 

That is one of the reasons that most benchmark hunters rarely rely upon their gps when looking for a benchmark. In almost all cases reading the description will get you closer to the benchmark than your gps will.

 

I guess the short version is that a mark is put where the surveyor responsible for the project it is being used for believes it will do him the most good over the lifetime of the project. It has to be survivable or unlikely to be destroyed by the expected local construction activities. These decisions are entirely the responsibility of the surveyor for the project. They do have their own set of guidelines, both in relation to the federal as well as state, county and possibly city laws. Once a mark has been set, it may be used by any other surveyor who is aware of it. That is why it may end up in the NGS catalog.

 

-Rusty

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Rusty is right, except where he implies that GPS is making benchmarks obsolete. Surveys, even those done using GPS, are begun from existing benchmarks, checked against other existing benchmarks, and result in the creation of new benchmarks, although relatively few ever reach the NGS database, so those found in the NGS database are still as useful as ever, and will remain so. Unlike property boundary markers, which are set at specific locations, sometimes forming a grid pattern, benchmarks are set at random locations chosen by the project surveyor, primarily for convenience and, if possible, permanence. For example, a concrete culvert is a common setting for a benchmark because its a very solid object, in close proximity to the road, likely to remain in place for years, and therefore very useful for any projects along that road. Church buildings are also often chosen as benchmark sites, since they are usually centrally located, easily accessible, easily identifiable, and less likely than most other buildings to be destroyed during future development. The NGS website assumes the user has some knowledge of surveying and therefore has little fundamental information, aside from that of a historical nature. A conversation with a local surveyor or civil engineer would be the quickest and easiest way to get your specific questions about surveying practices in your area answered in detail.

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... that I am getting some of this right. Most of what I have written is based upon personal theory, conjecture and what seems to me to be common sense. Though I am fully in agreement with whomever it was that noted that good common sense is far from common. I am sure that I have made mistakes in the past, and will in the future. New question in a new thread comming up...

 

-Rusty

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quote:
Originally posted by FLMike:

I don't understand their purpose. How and who determines where benchmarks are placed? What criteria are used for placement? Is it a location grid?

 

I've been to the NGS website and didn't find it helpful.

 

Infosponge, how ya doing there in Florida? Still taking good care of my geocaches down there? I notice you archived the Cha Cha.


 

In simple terms a Bench Mark is a permanent point for which an elevation has been determined. Although a BM can have other data detrmined for it, e.g. latitude and longitude, its not a Bench Mark if no evelvation exists for it.

 

Bench Mark = Veritcal Control

Triangluation = Horizontal Contol

 

Many Horz. control marks have vertical control and vice-versa.

 

The org. that sets the marks have specifictions for establisment of the marks NGS has "Standards and Specifications for Vertical Control Surveys". That decribes the distance between marks, type of marks required for the various accuarcy of the Survey etc.

 

Geodetic Bench Marks (Need Acrobat Reader to view this)

 

Index of On-Line Publications

 

[This message was edited by elcamino on December 06, 2002 at 03:28 PM.]

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