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Generally, most surveying books provide instructions for the setting up of tribrachs and optical plummets over known physically existing points. This article provides instructions for setting a tripod/tribrach combination over an unknown point whose position has been previously calculated and is to be physically established in the field with maximum precision.
This is an important field procedure that I have yet to find in any surveying book. Besides me, a few surveyors that I have worked with know this technique, but not enough as should know it, and that is why I am writing this article to share expertise with other surveyors. I have heard some surveyors say that you can measure a point more precisely than you can set it. While there may be some truth in that statement, utilizing the following procedure makes it a debatable point.
I have seen this procedure used to set control monuments at predetermined coordinates within chemical plants, monuments from which additional precise construction layout work is to be done. I myself have used the technique myself for double centering, laying out brick points on concrete footings, and for precisely setting points in obstructed positions. I have enough confidence in this method that I would certify to the precision of the position of the points set utilizing it. I have never had the opportunity do so, but have written a certification for this article, and in the event that I have opportunity to do so:
The required equipment for this procedure are: sliding leg tripod, tribrach, reflector, and a hand tape. This article assumes that the reflector and EDM or Total Station are calibrated and that the tribrach is in adjustment. The calibrating of reflectors and the adjustment of tribrachs is outside the scope of this essay. Essentially this technique utilizes the tribrach and tripod to set the point, and like most things it is easier said than done.
The first step is to determine approximately where the point is to be set. If you are on a construction site you may be able to position yourself close enough by the position of surrounding structures. This can be accomplished using the reflector on a range or stakeout pole as you would normally do. At times I have used the entire tripod, tribrach, and reflector combination to obtain an approximate distance by placing the tripod with legs together on line so that the instrument man can measure the distance to the reflector.
The second step comes when you have determined the approximate position of the point to be set, within about a 0.5 foot radius of the points final position. You set the tripod and tribrach up as you would normally do, with the exception that you want to place the eyepiece of the optical plumb on the line of sight of the instrument (see Figure 1). The closer this first approximation is to the desired point the quicker and easier the point will be finally set.
Once the tripod is set up and the tribrach is leveled, the instrument man tells the head chainman (tribrach operator) which way to move it and estimates in hundredths of a foot how far to move it on line. (In our example (Figure 2a) the tribrach needs to be moved 0.12′ to the instrument man’s right.) At this point you have the option to directly move the tribrach on the head of the tripod if it is close enough to do so by loosening the screw that secures the tribrach to the tripod and moving it directly. If not, you place the hand tape on the ground where you can see it through the optical plumb at right angles to the instruments line of sight (see Figure 2a) The tribrach operator (head chainman) notes the position of the cross hairs of the optical plum on the tape. Then using the leveling screws on the tribrach at A and B (see Figure 1) the cross hair of the optical plumb is moved along the hand tape the number of hundredths as previously determined by the instrument man. The tribrach is then re-leveled using tripod leg screws at D and E (see Figure 1). By doing this the reflector is positioned on the instrument’s line of sight. Essentially, you are measuring the amount the tribrach will move with the crosshair’s line of sight on the hand tape.
In the third step, the instrument man measures the distance to the reflector and it is determined how far either to or from the instrument the tribrach has to move to be at the correct point. The tribrach operator (head chainman) then turns the hand tape parallel to the line of sight of the instrument and looking through the optical plumb notes the position of the crosshairs on the hand tape. He then moves the crosshair along the tape the required distance, using the leveling screw at C in Figure 1. He then re-levels the tribrach using the tripod leg at F. Again if the tribrach is close enough to the point then the tripod screw can be loosened and the tribrach moved on the head of the tripod directly to the necessary point. The advantage to moving the cross hairs instead of the entire tribrach is that the line of sight of the cross hairs can be moved a greater distance then the entire tribrach can.
The instrument man then checks the line and distance again. Both are adjusted again as above if necessary. When the tribrach is in the desired position the point is then set on the ground by looking through the optical plumb and setting a nail, an iron rod or a scribe mark whatever the situation requires. One advantage to this method is that the reflector does not move off of the point as it is being set, it remains fixed in the correct location.
Another important aspect of using the above procedure is that it enables the user to set points behind obstructions walls, vehicles, dirt piles and what not. By placing the reflector on top of a 4-foot section of range pole and placing it on top of the tribrach you raise the reflector above obstructions and will be able to set points where it would not be otherwise possible. Placing the reflector on two 4-foot sections of range poles in addition to the height of the tripod enables the reflector to be about twelve or thirteen feet off of the ground. This works well only on a completely windless day. I don’t recommend using more then two 4-foot sections of range pole.
Some individuals in the surveying profession may find the above procedures cumbersome and time consuming. The goal is precision not speed. This is an advanced technique and prior experience with tribrachs is required. With practice, speed in setting points by the above procedure is acquired. I have not yet had the opportunity to use this technique with a robotic total station or a GPS antenna, but I am confident that it would work well with them when extreme precision is required.
Sidebar:
Suggested certification:
I, Terrance Mish, Registered Professional Land Surveyor #4981 hereby certify that the horizontal linear dimensions depicted on this plan are exact as shown hereon to within +/- 0.005 feet (1.5 millimeters) and meets the requirements as specified in …
Terrance Mish received a Bachelor of Science Degree in Surveying from Ferris State College in 1981. He is a licensed land surveyor in Virginia and Texas.
A 2.009Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE