Q: I know that the bubbles on my prism/antenna poles can go out of adjustment. How much of a problem is this? And how do I check them to see if they're in adjustment?
A: The critical factor is the sensitivity of the bubble on the antenna or prism pole. Bubble sensitivity, whether for tubular bubbles or circular bubbles, is given in terms of angle change per 2 mm of bubble shift. On prism and antenna poles, this is in the range of 40 to 60 arc-minutes. If the pole has a 60-minute bubble, and the top of the pole is 6 ft above the tip, it is simple to calculate the shift in position from the true vertical when the bubble is not centered by 2 mm. Take the tangent of 60 minutes and multiply by 6 ft. This works out to 0.09 ft, or more than 1 inch! If you are able to keep the bubble within 2 mm of the center during a GPS or total station observation but there's constant movement within that 2 mm radius circle, the radius (not diameter) of the antenna phase center or target/prism is 0.09 ft. If you are not sure how much you are off-center because you have trouble visualizing 2 mm, you can track this by sticking a piece of transparent tape on the bubble glass and marking off a 2 mm circle (a 2 mm radius circle has a diameter of just under 3/16 inch). The easiest way to adjust the bubble is to buy an adjusting jig that attaches to a wall. You can also use wood and/or metal to fabricate your own adjusting jig. A schematic of an adjusting jig is given to the right. You need to be able to center the bubble when it is in the jig, and then without changing the direction of the vertical axis of the pole, rotate it until the bubble is 180Â° from the starting location. Any movement of the bubble indicates twice the actual error in it. Also check with the pole manufacturer to see if it has recommended maintenance and adjustment procedures.
A: To check and adjust a tribrach (which has a circular bubble with relatively coarse sensitivity), set it up on a firmly positioned tripod, and then place and lock an instrument that has a much more sensitive tubular bubble, such as a total station or theodolite. First level up the instrument carefully. Keep in mind that the instrument's level vial may not be in adjustment either. You will know it is not level if, after centering the bubble in one position, you rotate the instrument 180Â° and find that the bubble has moved. You don't have to adjust the tubular vial. Simply use the footscrews of the tribrach to bring the bubble halfway back to the center. Now check the position of the bubble in the four principal directions. If you've done your job correctly, the bubble should remain in the same position as you rotate the instrument. This is referred to as the bubble's reversing point. Once the instrument is accurately leveled, observe the position of the tribrach's circular bubble. If it is not centered, you are observing the total error in its adjustment. Now, using capstan screws or a screwdriver (depending on how the tribrach is constructed), turn all of the bubble-adjusting screws to bring the bubble to the exact center. As with all adjustments, check that the adjustment has been done correctly by repeating the entire process, beginning with setting up the tripod firmly.
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