Tech Talk: Calibrating Equipment
January 25, 2011
I fully subscribe to A.C. Mulford, who wrote in his 1912 Boundaries and Landmarks: A Practical Manual: “For after all, when it comes to a question of the stability of property and the peace of the community, it is far more important to have a somewhat faulty measurement of the spot where the line truly exists than it is to have an extremely accurate measurement of the place where the line does not exist at all.” But we must be careful how we use this quote.
I have heard many point to Mulford's quote and say that the accuracy of measurements done by a surveyor is not all that important. Surveyors, however, are supposed to be experts at measurement, and it is not possible to do credible work if the dimensions that a surveyor reports are consistently no better in quality than a layperson could accomplish, regardless of whether it is a property boundary survey or some other type.
To accomplish accurate measurements (measurements that are close to the true value, not measurements that compare well with each other), surveyors must do more than rely on third parties (manufacturers and instrumentation repair and service businesses) to keep the instruments performing correctly. What surprises me is how infrequently the question is asked: “How do I know if my measurements are accurate?” Instead, the assumption is often made that the instruments are delivering accurate measurements.
There are several components to having accurate measurements. The first is that the people making the measurements must know how to make correct measurements, and that requires more than teaching them which buttons to press or how to set up or hold a part of the measuring system. The second is that the measurements must be made in such a way that inaccurate measurements can be detected. The third is that the technologies used by the surveyor must be inspected to make sure they are delivering accurate measurements, and if they are not, they must be adjusted so that they are.
Calibration is often used as a shorthand term to reflect this last process. I prefer the term adjustment since calibration always implies comparison to a standard, and not many adjustment processes get to that point, although there are some check processes that do. However, I use calibration in the title of this column since the process is often called calibration by surveyors in the U.S.
While many think of total stations or the old theodolites and transits as the equipment that needs checking and adjustment, the truth of the matter is that every piece of technology used by the surveyor should be checked and adjusted: total stations, GPS, laser scanners and levels, to name a few, plus all peripheral equipment such as prisms, antennas, prism/antenna poles, optical plummets, tripods and level rods. Keep in mind that sometimes the adjustments are not physical but rather are in the software that applies the correction in real time, or later during some part of the post-fieldwork processing phases.
To begin the process of checking and adjusting, first learn all there is to learn about the technology, whether it is a peripheral device such as tripod or prism pole, or a primary piece like a laser scanner. This means not only reading the instruction manual but also understanding how this technology is put together. You don’t have to be a scientist, but you do have to be scientific in amassing and assimilating the information. It is helpful to write out what you learn so that you can organize the information. This process can have the added benefit of helping you educate your colleagues on how to check and adjust with full knowledge of what they are doing.
If you do this first step thoroughly, you might discover parts of the system or parts of components where nothing is published. This is the time to start digging. Start with the business that sold you the product, but track it down to the manufacturer if you have to. The manufacturers owe this information to you. Any oversights in not publishing this information in user manuals and other documentation should not be a barrier to you pursuing every issue you identify.
Next, follow whatever instructions you have to begin the checking procedure. Also, develop a list of checks that can be done easily taking practically no time from a typical day in the field. Good examples include checking the level vials (whether spirit bubbles or electronic sensors) and optical plummets that are mounted in the alidade (rotatable part) of the instrument.
Now assess how the other processes for checking and adjusting should be done. For example, for optical plummet tribrachs, the best method is to use a tripod and tribrach along with a metal cylinder that looks like hockey puck in the tribrach. Then place the tribrach to be adjusted upside down on the cylinder and rotate it while observing a point on the ceiling. For prism/antenna poles, use some kind of wall-mounted system that allows you to verify in seconds that the circular level vial is in adjustment.
Finally, for the more tedious processes, work out a set of instructions that you and your team understand that relate to the particular points, baselines or other local features they might use. Even with itinerant teams, describing the needed baselines and checkpoints that are real--perhaps near your office--will help the team visualize what to do on another site.
While many of the checks and adjustments for your equipment will truly make it accurate, some will only bring you closer to high precision (i.e., repeatability). Remember that you can have a highly inaccurate instrument that is of very high precision. Thus, you must figure out how to get to the required accuracy. For EDMs, the oft-ignored EDM baselines are still the surveyor’s best friends. Many of these are going out of service because they have been ignored by surveyors as a valuable resource. Team up with other surveyors in your area and petition the pertinent authorities to reestablish or recalibrate them. For GPS, whether RTK or kinematic, accuracy is much harder, as points and lines must be measured whose true positions are known. Observing points that are part of a high accuracy network is one way to test them. Again, team up with surveyors in your vicinity to conduct high-accuracy surveys from these points to densify an area within the network so that various regimens for RTK can be tested under a variety of epoch, antenna support (handheld pole, pole with bipod, tripod and tribrach) and satellite constellation configurations. Tree cover, multipath reflectors and other accuracy-reducing features can also be introduced into a standard testing scheme.
In the end, know how your equipment works; know how to determine whether it is working properly; and know what to do if it isn’t.