Many SITECH clients use their leveling rods for their GPS or Total Station, but do you use the bubble on the rod to get it plumb and lined up accurate? But, more importantly, is your bubble calibrated correctly?
What’s the problem?
Coming from my background in laser machine control and grade checking with a laser, I am always concerned with elevation. I also know that if I lean my rod over too far, then my elevation will be off. But I did the math and found that I would have to lean it over pretty far to get a significant error. That is why most laser detectors have 40 min vials on them.
However, one day I was talking to a surveyor, and he was concerned about his bubble being out of calibration. That’s when I asked him, “what’s the big deal?” since the elevation is not going to be off by much, I didn’t see what the big deal was. To which he replied “true, but my horizontal will be way off’’.
His answer surprised me and I had to do the math calculations to get a better understanding of what he meant. I’m sure you remember your high school Trigonometry, right?
Time for a pop quiz!
Take a look at this geometry math:
The vertical error is equal to the Pole Height (6.56 ft) divided by the Cosine of the angle minus the Pole Height
Example for 40 min bubble, which is Plus/Minus 20 min. 20 min= 20min/60 deg = .333 deg
6.56 / (Cosine.333 – 6.56) = +.0001 ft not much at all.
The horizontal error is pole ht (6.56) * Sine of angle
6.56 * Sine .333 = .038 ft, that’s significant when using GPS or Total stations
But on survey rods we use 20 min bubbles which are Plus/Minus 10 min so the same error on one of these rods would be like 10 min/60 = .16666 deg
In this instance, the Horizon Error is then 6.56 * Sin .16666 = .019 ft, which is much better.
So, is your rod bubble calibrated correctly?
It does make a difference in making your site calibration as tight and accurate as possible. If you are noticing that your horizontal distance is off in its calculation, it might be time to have it checked. I would check it before every site calibration because it will save you time and aggravation down the road.
If you are unsure about how to check your rod, come into our office and use our self-service range pole calibration station. If you do not know how to use it, someone in our office will be glad to show you.
You can also reach our support team to help troubleshoot any calibration errors.