This month we are highlighting a technique that a reader has been using for quite a while. Arthur J. Saarloos of Delta Junction, Alaska, sent it in hoping we would share it with the readership. Questions or comments regarding Saarloos’ technique can be forwarded to firstname.lastname@example.org.
The problem when doing trigonometric leveling with a total station is to find a way to rapidly get direct readout of the elevation of the point being observed (under certain conditions). Many total stations solve this problem if the elevation of the point occupied by the total station is known, and the height of the instrument (HI) and height of the reflector (HR) have been accurately measured, and of course a bench mark (BM) can be observed. There are situations surveyors face when the HR and HI can’t be measured and the elevation of the point being occupied is not known. As long as a bench mark can be observed, the method described here can be used.
In the early 1960s, I did my first surveying for the state of Alaska, Highway Department Design Section. We ran a “P” line through the rolling hills north of Fairbanks, then performed a cross-section survey of the “P” line using a rag tape, hand level and a Philly rod, making “turns” up and down the hill. One day the project engineer brought us a large box and a 6-foot stick. Inside the box was a large semi-circular plate that we discovered was called a Rhodes Arc. Our project engineer attached the disk to the stick and showed us how to take a reading by using the grid marks on the arc. The cross sectioning was faster and simpler. A similar story applies to the Lenker rod: no more grade computations. On a survey project in Southwest Alaska last year I found a way to read direct elevations with a total station. Although I am not in the same league as Messrs. Rhodes and Lenker, I am hereby pleased to outline the technique. Perhaps your readers will find a use for it, or at least find it interesting.
The procedure is as follows:
1. Set up your total station in a place where you can observe the bench mark.
2. Securely lock the position of the retroprism on your prism pole. (Saarloos suggests the lowest position, but surveyors may be comfortable with other positions as long as they have a reliable rod person who verifies that the position of the prism has not moved from the position it had when the bench mark is observed.)
3. Set the height of the instrument (HI) to zero (0.00).
4. Set the height of the reflector (HR) to zero (0.00).
5. Now have your assistant hold the prism pole on the bench mark and observe the difference in elevation (Elevï) that is computed by the total station. This computation uses a simple right triangle. Note the sign of Elevï; use positive (+) if the observation from the instrument to the prism rises upward, and negative(-) if the observation drops downward.
6. Use the Elevï value to calculate a new HR for subsequent observations using the following formula:
HR = – (ElevBM – Elevï)
Note that if Elevï is negative that the term inside the parentheses is calculated by subtracting a negative number from the elevation of the bench mark.Note also that there is a negative sign in front of the parentheses.
7. Set the HR on your instrument to this new value (keep the HI at zero) and reobserve the difference in elevation to the bench mark. If you have followed the procedure properly, your total station should now display for the difference in elevation, the elevation of the bench mark.
8. Now observe any subsequent point and get its elevation directly.
Saarloos suggests observing another known point if possible before beginning your survey, and to also check back on any of the known points to verify nothing has changed with your setup.
Scenario illustrated above:
HI set to 0.00; HR set to 0.00
In observation to BM, Elevï = -15
Using formula from above, HR = – (ElevBM – Elevïï )
Now set HR to -115.00 and observe unknown point. Elevï to this point is +20
Total station will display direct elevation of the point of 135.00
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