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Bill93

Transit Testing

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This isn't directly about benchmarks, but is about surveying equipment that I may find useful in locating benchmarks. In particular there are a couple I can't find that are within a 1/4 mile of others that have been found. So leveling might provide clues, such as whether enough dirt has been moved they can't be there or to deeply bury them.

 

I recently pulled out a damaged Dietzgen 6100 transit from I guess the 1950's. I bought it pretty cheap several years ago because it had been knocked over and bent the vertical standards that hold the telescope. The vertical circle is scraped because of the misalignment. I hope this winter to figure out how to disassemble and at least crudely straighten those pieces.

 

I tested it by sighting about 10 feet on something, then rotating a half-turn and inverting the scope. The difference in readings was 18 minutes, which I work out to mean the telescope is 0.29 inch off center. Pretty bad, huh.

 

Now I finally get to the real question. What is the best way to test whether the horizontal motion or circles have been knocked off axis and eccentric? The surveying text book that I have describes a number of field tests and adjustments but does not cover this problem.

 

I tried repeating an angle of about 5 degrees for 75 times (whew!) and plotting the difference between each reading and N times the average. I found out that my old eyes aren't so good. I obviously didn't always read the closest mark on the vernier. Averaging each point's discrepancy with the 3 on each side of it produces a reasonably smooth curve that shows about 1 minute max to min variation. Would you take this as a reasonable estimate of the amount it is bent?

 

I'm thinking now that if I bumped the instrument or took sights off a little bit the result could be what I'm seeing because the error affects all the remaining angles the same way. I think it would be a better check if I turned 80 or 100 degree angles, so that a blunder would not just appear in the middle of the curve but would be scattered around the circle and would average out.

 

Am I on the right track?

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Whew,

I use to calibrate mercury and dry flow meters,Once something is bent it always has an error,as with the inherint error in all things.

 

I would try to find some newer used parts for the instrument.

Or get a gooder instrument.

 

For leveling any distance you need precision tools.

In my opinion,you can get just as close,as with what you are trying to do using your GPS in the same way....as a transit.

That is Use the mark you have (BM) as a go to,you can use the functions,distance heading,bearing of your GPS to get distance,and bearing and /or heading

(directions).

I have used triangulations and reference marks,other BM's along Railroads this way,the data sheets give the directions distance,azimuths most of the time in the original Data Sheets.

 

You can do the same thing with your computer mapping programs by using the distances and bearings from one point to another,there are several options within the programs which will allow you to do it to many decimal points,and in the correct datums,or even the various datums for references.

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Bill,

 

You sound like you are on the right track, but I have a few thoughts...

 

You are an engineer so I assume you are familiar with the basis for differential leveling. As you know, leveling isn't about turning angles, so in effect, a "level line" of Bench Marks can follow a meandering line even go around corners and skip over a block or two here and there. They basically just put them in places that seemed stable and would provide longevity. They were never really triangulated so it is a best guess and a good hard reading of the description to find them.

 

For leveling, the vertical circle has to be right. At the stadia, you have to be able to turn 360 degrees bubble level. If this varies at all you, cannot perform leveling. Hopefully you will be able to repair this so as to be sure that at zero, you are bubble level at the stadia. After being bent, it may not be level at zero, nor can you count on it being the same twice. With precision instruments, damage like that almost never comes back and replacement parts do not exist. You could rent a rotating construction laser, used for leveling, and set yourself up a permanent calibrating range in your yard for the transit, but renting a laser even for one day is not always cheap. The other trick to this process is getting a custom stand set up so you have the transit stadia match the exact height of the laser eye. The H.I has to be exactly the same. All this will do is calibrate you for level however. You may never know where you are on a bent vernier if you get off. If all you ever plan to do with this transit is leveling which is all it may every be able to do based on the damages, you could lock it level, and calibrate it so that it optically will shoot your leveled range, and going forth the vertical and horizontal circles will not be all that important.

 

As you are aware, this leveling will take two people to perform, and you will likely need to have a leveling rod which reads in tenths of a foot or meters, if you would prefer not to convert in the field. I would recommend a plastic telescopic leveling rod over the metal or wooden ones. The safety factor from electrocution by overhead power lines is a factor. Leveling is a leap frogging process, one that will have you moving the transit after nearly every shot and you will have to mark the location of the last shot you took and keep it's elevation so you can re shoot the Back Sight from that for each move. For those who may be interested in some of the details I am skipping over here, the gist of it is Here.

 

As for the field tests to check for eccentric wear, you would almost have to set up a tripod and establish a series of hubs around it. Shoot them with a calibrated instrument and someone steady with a prism pole. Record your findings after double checking, and be sure to vary, and record the height of the prism pole at each shot. Then take the known good instrument off the tripod without disturbing the tripod and mount up your transit. Turn your angles again, both vertical and horizontal and compare the data. It will be important as both kinds of angles will need measured at once.

 

For triangulation, you will need to make use of the vertical and horizontal circles. Of course, Trigonometry will require you to shoot an azimuth to establish the first angles. You will need to establish a base line to triangulate from, so as to have two known points to establish the third. There will be a lot of trigonometry involved. You are going to be line of sight dependent. and this is a limiting factor, Literally. Geodetic Triangulation was based long lines of sight, and often required tall towers, then to add, the optical power of the theodolite was a big factor in how far you could see. Further complicating this is that the transit also does not have the accuracy on the verniers as would a theodolite let alone the optical power. The best you may be able to achieve is an open, or closed traverse between stations. Lines of sight are the hardest thing, then doing all the trig...

 

Today's professional instruments do a lot for us but it still is a stretch sometimes. We have to use a lot of tricks. At the consumer grade level, GPSr will get you within 5-10 feet of any triangulation. With leveling being scaled you are still having to live with the narrative description and a tape measure for location. For quick and easy short range leveling, you may get by with a Hand level and a lufkin folding ruler. They make those in engineering scale tenths and hundredths, as well as metric too.

 

Beyond this, a call to a local surveyor may steer you towards a shop who may be able to repair it enough for leveling, especially if the repairer is an old timer... Some of them were miracle workers.

 

I fear the worst may have befallen this instrument Bill, but I will hope for the best.

 

Good luck!

 

Rob

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Yes, the primary job is getting it so I can turn 360 degrees and stay level. That's what I need for the two benchmarks referred to.

 

It does reasonably well at holding the striding level bubble as I turn it. I need to get the vertical action working smoother so I can get it set exactly by iterating between th plate adjust knobs and the vertical adjust. If the bubble stays centered I'm in good shape.

 

Then there is the problem of making the optical axis and the bubble agree and that's where the calibration range comes in. It is possible to set up the cal marks using the uncalibrated instrument so long as it meets the 360 degree no-change test. I would set points A and B on a line of sight from a position past A and then set A' and B' on the same posts while sighting from a position past B'. Level is halfway between the marks at each post.

 

The horizontal angles aren't significant there, but I may think of things to do with them also and I hoped to better understand what all was bent by testing both horizontal and vertical. After thinking it over I think I tested a combination of the lower and upper plate motions. What I should have done was record both A and B vernier readings. The difference in these tells me more about how angle readings are affected.

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Find two lightposts (or similar) about 150 feet apart. Set up halfway between the two and level up. Now sight post #1 and instruct someone to make a level line where your crosshair hits it. Turn ~180° and sight post #2 and do the same thing.

 

Next, setup close to post #1 (about 10 feet away, or as close as you can reasonably focus). Level up and sight the same line. Have someone hold a pocket tape up so you can record the distance between what you are sighting and your line. Turn ~180° and sight post #2 and record the difference again. It should be the same but it probably isn't.

 

Whatever the difference between the two pocket tape readings is the amount you are out of level in 140 feet (in this example).

 

If you need more information on this, a better place to post would be: RPLS.COM Message Board, which is a message board for registered professional land surveyors.

 

Enjoy,

Milan

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Your method is equivalent to the one I described using AB and A'B', but as you point out it is not necessary to set up the instrument outside the space between the marks. That may be more convenient, since you can see your mark on the same side of a big post, but does require you to determine the midpoint between the posts to whatever accuracy you are interested in.

 

In your example, the amount you read for "out of level" is for a distance of 130 feet, not 140. The instrument will be off the same amount at 130 (10 feet on one side of the setup) as it is at 150 feet (10 feet the other side), so the vertical difference is the error per 130 feet.

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I'm not sure that an EDM baseline would offer any benefit, since those are only for calibrated distances, and this transit doesn't have an EDM...

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