Temporary benchmarks, anyway. If 5 years is considered temporary. Comments were made in another thread about the wisdom of using fire hydrants as bench marks. I wish to make some counter arguments in support.
Certainly the stability of a fire hydrant is less than absolute.?ÿ But so is the stability of a great many things that brass has been cast or set into.?ÿ For example, the City of Portland - like a lot of cities and towns across this great country, and others - has been maintaining a network of benchmarks since 1895. There are thousands of beautiful brass monuments across this city, most of them in the top of curbs. Curbs that have sunk into the mud. Curbs that have been thrust up by frost heave and tree roots. Curbs that have been rubbed and mashed by innumerable automobile and truck wheels over the last 123 years.?ÿ I've had the pleasure of working in a number of cities, 3 states and 2 countries, and IMO Portland's benchmarks are no worse than those in any other city of my experience and a lot better than some.
NGS monuments are frequently no better. Some few are cast into granite batholiths of undisputable stablity. But most are in culvert headwalls and such that are less than perfect. Datasheets acknowledge that.
Meanwhile fire hydrants all over the city sit next to the very same curbs and sidewalks. Sure, they may move from hydraulic pressures and so on. But I'll assert that they are - at the least - no less stable that those curbs next to them.?ÿ Hydrants are directly connected to pipes that are well bedded below the frost line. The temperature of that pipe is moderated by the depth of bury.?ÿ?ÿ
When I drop an elevation on a fire hydrant near a construction site I am assured that the hydrant will be the last thing that gets demolished. Commonly the contractor goes to a great deal of trouble to save the hydrant, because he would have to provide alternative service for the duration of his project.?ÿ ?ÿ?ÿ
When using a hydrant as a bench mark one must take great care that the hydrant has not been replaced altogether. It is prudent to check into something else. But that is?ÿ true of anything one might use.?ÿ So I'll finish by asserting that in our imperfect world a fire hydrant has a lot to recommend it as a benchmark.
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Temporary benchmarks, anyway. If 5 years is considered temporary. Comments were made in another thread about the wisdom of using fire hydrants as bench marks. I wish to make some counter arguments in support.
Certainly the stability of a fire hydrant is less than absolute.?ÿ But so is the stability of a great many things that brass has been cast or set into.?ÿ For example, the City of Portland - like a lot of cities and towns across this great country, and others - has been maintaining a network of benchmarks since 1895. There are thousands of beautiful brass monuments across this city, most of them in the top of curbs. Curbs that have sunk into the mud. Curbs that have been thrust up by frost heave and tree roots. Curbs that have been rubbed and mashed by innumerable automobile and truck wheels over the last 123 years.?ÿ I've had the pleasure of working in a number of cities, 3 states and 2 countries, and IMO Portland's benchmarks are no worse than those in any other city of my experience and a lot better than some.
NGS monuments are frequently no better. Some few are cast into granite batholiths of undisputable stablity. But most are in culvert headwalls and such that are less than perfect. Datasheets acknowledge that.
Meanwhile fire hydrants all over the city sit next to the very same curbs and sidewalks. Sure, they may move from hydraulic pressures and so on. But I'll assert that they are - at the least - no less stable that those curbs next to them.?ÿ Hydrants are directly connected to pipes that are well bedded below the frost line. The temperature of that pipe is moderated by the depth of bury.?ÿ?ÿ
When I drop an elevation on a fire hydrant near a construction site I am assured that the hydrant will be the last thing that gets demolished. Commonly the contractor goes to a great deal of trouble to save the hydrant, because he would have to provide alternative service for the duration of his project.?ÿ ?ÿ?ÿ
When using a hydrant as a bench mark one must take great care that the hydrant has not been replaced altogether. It is prudent to check into something else. But that is?ÿ true of anything one might use.?ÿ So I'll finish by asserting that in our imperfect world a fire hydrant has a lot to recommend it as a benchmark.
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Way back in 1954 when I was first introduced to the surveying profession when I went to work for the City of Sheridan, Wyoming City Engineers Office, 99% of the City's bench marks were on the nut that that was used to open the hydrant for water flow. That was or had been the standard of practice for City bench marks from probably the time when the water mains and hydrants were put in place. At that time I never questioned using a fire hydrant for bench marks, because what did I know. I was told to go hold the rod on the nut for a backsight?ÿ or a foresight. Apparently the City never had a problem with setting bench marks on those nuts on fire hydrants. When a new water line was installed, we would go out after a certain length of time and run a level circuit to put the new fire hydrants into the bench mark system.
I think hydrants work well as long as they aren??t replaced. The other opinion I like in densely populated areas are stoops.?ÿ
I think hydrants work well as long as they aren??t replaced.
When I was surveying in south Florida, I used to see bench mark descriptions like "Set "x"-cut in north bolt of top flange of fire hydrant located at ... ." Sometimes the "x"-cut was hard to see because the fire hydrant had been repainted. I never set bench marks on the top flange bolts of fire hydrants because I was told that sometimes the top flanges get replaced.
When a car hits a hydrant the crews repair them very quickly. Usually within an hour or two.
You would never know it. The do a great job of cleanup. The Hydrant looks just the same!
If they use a riser that in not the same length as the "original" the elevation will be off, but usually only half a foot or so.
(the risers are cast iron, they break real easy. The new one just bolts on. The work crews usually keep a couple spares in their rigs, of varying length)
Hydrant could be good for?ÿ TBM, for a few hours, not longer.
We did a project in Chicago, where the city had been using a fire plug for elev. for a long time. Trouble was, it was 1/2 foot off. That whole intersection and 1/2 way down some of the streets, was off by that amount.
I saw the results. It made a low spot...
Agree with Peter. Using hyd. nuts is nuts for any longer than the survey takes. We've seen the city move the hyd. and use it over. Did the following contractor do enough checking? No. Is it an accident waiting to happen? Yes.?ÿ
redundancy, check to another source of data. Even if it is just a few spot elevations on the existing plan/design/topo... we need to check.
I remember a few brass disks NGS Bench Marks set in the Face of abutments. Solid, no doubts. But maybe, just maybe, somebody made a typo or transposed a number (maybe it was Me).
GPS doesn't lie. But remember, GPS is an average of thousands of measurements... and the software throws out the outliers (probably).
Book it Danno!
I enjoy the challenge of setting a TBM that is unique, easy to identify and not susceptible to subsidence or disturbance. I do not feel that a hydrant nor a spike in a wood telephone pole meet the mark and have seen both cause problems, admittedly only once with any consequence when a plat was built off a spike in pole where the pole had been removed and put back a couple feet lower.?ÿ
The major objection seems to be that a hydrant can be moved or replaced without it being obvious to future users. Agreed, that's an issue. Buts it's an issue with all the alternatives I can think of as well.?ÿ A piece of uniquely stamped brass is always desirable.?ÿ But sometimes curbs that have sunken get levered back up and re-levelled.?ÿ Sometime sidewalk sections get cut out and reused. Not often, but it happens. Sometimes the published information is just flat wrong. And sometimes the user transposes the data. Bottom line, even brass needs to be confirmed.?ÿ?ÿ
When it comes to project site benchmarks lets examine the alternatives. It would be nice to be setting uniquely stamped brass tablets around every project site. It is a special site indeed where that happens. Common site benchmarks might be any of the following:
- Mag (or PK) Nail
- Iron rod with (or without) plastic cap
- manhole rim
- spike in pole
It's really common to find?ÿ more than one nail on every street corner in town. And if yours is the only one when you set it, there my be others by the time you come back to it. Super easy to get mixed up. Their permanance is questionable.
Iron rods are somewhat more permanent but just as easy to get crossed up or disturbed. If that is what you use for traverse points you have set several around the site.?ÿ
Manhole rims are routinely raised during repavings. And around here they frequently aren't flat. What part of the rim are you going to use??ÿ The part with the chisel mark you say??ÿ Did it get rotated during the repaving?
Spikes in poles have already been covered by others.?ÿ
In short, the objections to hydrants as benchmarks can be applied to most everything else I've seen used. If someone has what they believe is a foolproof solution, I'd like to hear of it.?ÿ But I think it likely that you have underestimated the ingenuity of fools. Anything you might use, including brass, should be checked.?ÿ?ÿ
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I have used fire hydrants as bench marks. Yes at times I have found they have been replaced. Anyway, I always check into two bench marks before beginning work. Who uses only one bench mark?
Historic boundaries and conservation efforts.
One of the great things about this site is that you learn so much about how people around the country and even the world might have different viewpoints regarding what seems like a trivial matter.?ÿ Not that TBMs are trivial, of course, but that's something that I'm sure most of us don't spend very much time considering.?ÿ I'll share my experiences.?ÿ This is from my perspective, as someone who is not a licensed surveyor, who has been doing design work for the last three years, and who hasn't spent more than a few days surveying in the last dozen years.
I started surveying in Houston during the late '70s. Common TBMs that I found and set include spikes in poles, manhole rims, chiseled boxes in inlets or on sidewalks, bolts on the bases for traffic signals, and yes, fire hydrants.?ÿ On fire hydrants, we didn't use bolts, we would use the "LL" in Mueller (standard fire hydrant for City of Houston), or on the occasional oddball hydrant that was stamped Tennessee we would use the "EE" (or maybe one of the other double letters, I really don't remember).?ÿ I think the spikes in poles fell out of favor when Houston Lighting & Power let it be known that they didn't want people putting spikes in their poles, the manhole rims fell out of favor for the reasons that Mark stated above, and the use of traffic signals became more prevalent as signals became more and more common.?ÿ Through it all, the use of fire hydrants remained common.
I surveyed one project in southern California that was a boundary survey that needed no benchmarks.?ÿ I spent a few years in the mid 2000's doing topo surveys in Fort Wayne.?ÿ I had no licensed surveyor on staff to instruct me as to local customs, so in setting TBMs for each projects, I stuck to spikes in poles, which seemed very common there, and fire hydrants, which also seemed common but not as popular.
So here are the questions I have after reading many of the responses.?ÿ Do people really set brass tablets for TBMs??ÿ I guess I look at it as these are truly temporary.?ÿ I've only set them for construction purposes.?ÿ Construction schedules vary.?ÿ Again, when I started in the '70s, we were mostly staking subdivisions, which might take a couple of years to finish.?ÿ As such, since it wasn't uncommon to spend months going to the same construction site every day we would routinely check the TBMs to make sure they were still accurate.?ÿ Later, when construction became site work instead of subdivisions, it wasn't uncommon to set a TBM on a site and within six months have a Taco Bell or a Kroger or a Target open and ready for business. After that, their existence became meaningless.?ÿ Now, doing mostly INDOT related work here in Indianapolis, it's not uncommon to finish a design and still not having construction started two years later.?ÿ Does this play a part in how people view the term "temporary"? Is it a rural vs. city thing??ÿ Is what you all do your own personal preference, or is it generally accepted practice in your area??ÿ Do you go back five years later and use a TBM that you set previously, or that someone else has set?
No matter what you end up doing, as many people have already pointed out, verifying the elevations is critical.?ÿ This is my two cents.
I find hydrants are one of the more reliable benchmarks especially when the point is marked up somehow (or ID'd).?ÿ It's not perfect but among all the others it works very well.?ÿ The top spindle is not very reliable point and should be avoided.?ÿ Cap bolts with "X" marks are good.?ÿ Having something else to check is always good especially if the check is not another hydrant...if a line is replaced then chances are all the hydrants are different.
Have compared one FH to the RM in a small town for 30 years. ?ÿThe differential has been constant. ?ÿHowever, there is a spike in power pole that has changed twice due to resetting of the pole. ?ÿAlways use more than one record BM.
Performed a topo on a planned hotel site a few years back and tied into one MH and four nearby DOT BM's. ?ÿFellow called one day who was to do work on he same site and was told the MH elevation was off by nearly three feet. ?ÿNothing had changed to that MH. ?ÿNot by one inch let alone three feet. ?ÿHope he finally figured out why he was wrong and everyone else was right. ?ÿ(Not 29 vs 88, BTW)
First LS boss said Never Ever use a fire hydrant for a BM no matter how temporary. There was an accompanying story ab0ut the fire hydrant BM that went up and down 0.3' in the course of a day depending on the water pressure and the finger-pointing festival that went on until they figured it out.
First LS boss said Never Ever use a fire hydrant for a BM no matter how temporary. There was an accompanying story ab0ut the fire hydrant BM that went up and down 0.3' in the course of a day depending on the water pressure and the finger-pointing festival that went on until they figured it out.
If you properly identify/mark the BM then replacing the hydrant should be a issue.?ÿ However, if the hydrant is improperly used, then a water hammer in the system can move the hydrant vertically without any outward signs of disturbance.?ÿ ?ÿ
Just arrange a time with your local water dept or fire dept to watch them open a hydrant to 100%. You will see the water hammer in full effect, and it may change how you set BM's. Also over the past 10 years a lot of newbies like to use the highest point of a hydrant, which is the grease fitting on the valve nut. I have even seen them use a valve nut, and a bonnet bolt for two different hydrants on the same sight, without proper description.
I know of a local older NGS brass disk that was reset when the city added an ADA ramp, looked good until you relized it had to have been moved as ADA ramps weren't typically done 50+ years ago. Since that time I believe that sidwalk was rebuilt again and there is no disk at all now (which I guess is better). The first replacement that had taken the time to chissle the old brass out, polish it and reset it, was real pretty!
The NGS SS rods are pretty good and hard to move plus being stable!
SHG
Just arrange a time with your local water dept or fire dept to watch them open a hydrant to 100%. You will see the water hammer in full effect, and it may change how you set BM's. Also over the past 10 years a lot of newbies like to use the highest point of a hydrant, which is the grease fitting on the valve nut. I have even seen them use a valve nut, and a bonnet bolt for two different hydrants on the same sight, without proper description.
I don't doubt that it happens, but it doesn't happen to all of them.?ÿ Or this one.
Well the City that I work for put all their vertical control in hydro poles, tell me what you think about that one? I can't tell you how may times I've had issues with this. A pole is removed, hit and broken or replaced BM is toast. I've had the issue where a pole was replaced with a pole that had a?ÿspike in it. Extremely frustrating when you are doing a loop and you are closing on another and it's way off. After some constructive criticism we have moved away from this method. We would do yearly checks on the poles and speak to our Utilities commission to see what work has been done to existing poles but its a lot of extra work that doesn't need to be done. I'd welcome using fire hydrants at?ÿone point but yes if you aren't careful you could get?ÿinto some trouble. Just be careful and check against?ÿa known good elevation and you should be fine.?ÿWhere I'm from, freeze?ÿand thaw are?ÿa problem and can cause issues with brass tablets in walks and curbs, as of late we have been using our GPS to establish vertical control. When it is done correctly it has been working just fine.?ÿA few of us still check into a BM in the area just to be sure. ?ÿ?ÿ?ÿ