Questions and answers about cross hairs and EDM measurements.

Typical prism cross section where the parent prism apex and the plumbing line do not coincide.
Q: Measuring an angle, I aligned my cross hairs with the center of the prism (the “star shape”). My rod person then pointed the prism more directly at the gun. My cross hairs were no longer on the prism center, changing the angle five seconds. How could this be?

A: Precise angles should be measured with a target provided with the prism by the manufacturer. Such targets are designed to coincide with the prism’s plumbing point, which may or may not coincide with the apparent center of the prism. For precise work, be aware that there may be an offset on the order of tens of millimeters (40 mm is not unusual) between the location of the plumbing point and the apparent intersection of the three plane surfaces at the back of the prism glass that come together in the “star shape.” Note that the physical location of the point of the back of the prism versus the plumbing point is not the alignment discrepancy you are dealing with; rather it is the apparent center versus the plumbing point.

If using a zero offset prism that is mis-aligned horizontally or vertically by 10?, assuming a 40 mm offset in the apparent center of the prism with respect to the plumbing point, the lateral displacement will be 40 mm x tan 10?=7.1 mm. If observing this prism from 200 m away, the resulting mis-pointing will be tan-1 (0.0071/200) = 7.3 arc seconds. Many manufacturers offer prisms, usually a premium line, which eliminate or greatly reduce the errors in horizontal and vertical angles (and distance) due to prism aiming error.

One simple remedy to the confusion a prism’s apparent center causes is to simply aim instead at the pole itself, preferably sighting as low on the pole as possible. Many surveyors feel that this practice simplifies the aiming issue and eliminates prism confusion.

Q: I have told my crews when making EDM measurements with a prism pole, that it is OK to take a shot as long as the bubble in the rod vial is not more than half outside the circle that forms the inside of the bullseye. How much error is potentially caused by not being perfectly level?

A: It depends on the equipment you are using. But here is how to determine the error. Level vials, whether tubular or circular are specified for accuracy by stating the amount of angle change from horizontal that is required to move the center of the bubble 2 mm. The spec for a rod level vial may vary from 20 or 30 minutes per 2 mm down to three minutes per 2 mm (find out from the manufacturer). Next you need to know the diameter of the inner circle of the bull’s-eye (often 4 mm, but measure to be sure), to figure the actual range of the bubble’s travel. Finally you need to know the height of the top of the vial above the point of the pole. If the rod you are using is rated at 10 minutes per 2 mm, the circle is 4 mm and the vial is located 3.5 feet above the point, the error is 3.5 x tan 10'=0.01 ft=3.1 mm. Note if your standard for “level” is tighter or looser than halfway outside the inner circle, then you’d use less or more than the 10'. In this case, 10' was used because when the bubble has moved from dead center in the circle to halfway outside, the center will have moved from the center to the circumference of the circle. Now that you know the horizontal offset, remember that random motions can be in any direction. So the impact can be on your horizontal angle or distance, or both.

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