- SPECIAL REPORTS
- THE MAGAZINE
The solutions are not rocket science, and there are plenty of them: regular inspections and calibration, backup units, having a good maintenance source, and so on. One ingredient in the remedy recipe is paramount: knowledge.
“Composite materials—especially for prism rods—are definitely in the surveyor’s future,” says Warner Allen of Allen Precision, a nationwide supplier based in Duluth, Ga. “The light weight and the material’s resistance to environmental effects such as heat and moisture expansion are definite benefits.”
Ash Puri, president of Chicago Steel Tape, Wateseka, Ill., agrees, but doesn’t expect composite materials to dominate accessories such as tripods, at least not in the near future. “In addition to stability and light weight, durability must be built into any tool that will be subjected to the rigors of surveying and construction,” Puri says. “And when you put enough composite material into a tripod to make it sufficiently durable, the light weight advantage disappears.”
TripodsAs far as tripods go, the surveyor will be most interested in three characteristics: material, weight and leg-locking mechanism. Actually, an even more important issue is the stability of the device, but we’ll take for granted that any exceptions to the stability characteristics (at least for a new tripod) will be rejected as unacceptable. Let’s also take for granted that the correct tripod is selected for a given use. You wouldn’t mount a heavy robotic total station on a flimsy tripod intended for holding no more than a lightweight auto level. Stability, of course, has a relationship to weight; a tripod can be so light as to inherently be unstable. Once a tripod is heavy enough for stability, the lighter the better (just ask the person who carries one all day).
As to material, the three common materials are aluminum, fiberglass and wood. Aluminum has the advantage of being lightweight and cheap. The mechanisms and hinges, however, are more subject to wear and more easily damaged. Recently, heavier-duty aluminum tripods have been produced, which many surveyors use without problems. The one thing a manufacturer cannot improve with aluminum tripods is the metal’s high conductivity and high coefficient of heat expansion. An aluminum tripod sitting under the summer sun has just too many molecules moving around for most surveyors’ comfort. By and large, it is fair to say that serious surveying instruments usually sit atop wood or fiberglass tripods. Usually.
For expertise on leg-locking mechanisms, we turned to Rodney Wagner, a survey instrument technician in Austin, Texas, with over 10 years experience. He confirmed that the two primary methods for locking the upper and lower sections together are the thumb-screw lock and the quick-release lever. The former is a threaded fitting at the bottom of the top leg section that squeezes the top section’s two dowels against the solid bottom section, which holds the tripod’s feet. The quick-release lever method, by contrast, is housed on the lower, solid section. The lever activates a cam, which in turn activates two plates that “grab” the outer dowels. The advantage of the quick release over the screw lock is that it is, well, quicker. But the cams, which for some reason are often made of plastic, can round off as they wear, blurring the distinct “locked” feeling the user expects. It can also blur the locking itself so the upper section slips down. “But,” Wagner says, “there’s no reason the quality and durability of the cam can’t be upgraded to eliminate that problem.” The mechanism also has a bearing on the material used in the tripod’s construction. Even a wood tripod will have fiberglass dowels when a quick-release clamp is used because wood dowels develop compressed places where the plates contact them for locking. It’s probably fair to say that where ultra high-reliability is called for, the screw lock would probably get most surveyors’ nods.
There’s more to say about tripod components, such as the construction of the feet, smoothness and durability of leg hinges, and characteristics of the mounting surface. But let’s move on to our next indispensable accessory…
PrismsThese mysterious mirrors, which reflect your dominant eye at whatever angle they’re positioned, are the targets for every shot we take. Mary Sansom, an instrument and accessory specialist who has been with Miller Blue Print in Austin, Texas for over 10 years, says, “Manufacturers have come out with some bells and whistles for prisms. Nowadays we sell more tilting prisms, and prisms with battery-powered strobe lights in the housing are really catching on. Also, there are the housings that can present the prism at either zero offset or 30 mm.” The strobes, Sansom explains, are for visually locating the target in difficult settings.
Speaking of 30 mm prism offsets, that value has emerged as the de facto standard. It means, of course, that the center of the female fitting into which the rod goes is 30 mm (about a tenth for the metrically-challenged) in front of the optical reflection center of the mirrors. There was a time when some manufacturers used a different value and the surveyor had to be really careful to not mix prisms. Even now, some surveyors swear by an ironclad bond between instrument and prism set. Once an instrument and prism are used together for a calibration (goes this theory), they should be marked accordingly and used together (until legally divorced, I guess).
Occasionally, one might need a temporary substitute for a prism, especially in areas of difficult access when high precision is not a requirement. Bicycle reflectors work for these, as does plain reflective tape. Just climb up to Jefferson’s nose on Mt. Rushmore, slap a piece of tape on a nostril, climb down and shoot your distance.
Prism PolesWell, we’ve got our instrument on the right tripod, shooting infrared beams to our nitrogen-purged, highly polished 30 mm-offset prism. Now all we need is a good standard to hold the prism directly over our object point. Enter the prism pole. The usual pole is in telescoping segments of around 4' each, expandable to 8', 12' or even more. The inner segments are usually graduated in feet and tenths, or meters. There’s a level bubble to indicate plumb. This bubble must of course be checked regularly, preferably daily. Ease of bubble adjustment should be a major selection factor when choosing prism poles. These poles are beginning to utilize composite materials, but most are still metal or fiberglass.
“Almost all of our sales of prism poles are the graduated, precise models,” Sansom says. The word “precise,” she says, refers to a specific characteristic indicating availability of an adjustment the user makes to ensure the rod’s graduations are correct.
There seems to be a consensus that the small additional cost of the precise adjustment is money well spent. The worst of all prism pole worlds is a model with graduated measurement markings but without the precise adjustment. That arrangement is a recipe for error, with the graduations appearing so useful and believable, but with no way to ensure their accuracy.
Leveling RodsThese “vertical rulers” are not used so much with the total station/prism configuration (although they can be) as they are with a level. Although there are special purpose rods available, such as the direct-reading type popular with some construction outfits, a huge percentage of rods sold today are telescoping rods made of fiberglass. Both surveyors and construction crews use these rods, with surveyors normally opting for a 25-ft version and construction crews often using shorter ones. The wooden Philadelphia and Frisco rod are getting rare in most parts of the country. Many surveyors feel that, even though the wooden ones are somewhat more precise, the small difference in precision is a small price to pay for the benefits of the fiberglass models.
What are those benefits? Well, for one thing, the telescoping rods are longer. A 25-ft rod can drastically reduce the number of setups required in hilly terrain. Another goody: most of the fiberglass rods can be fitted with an adapter on their tips to hold prisms, creating in effect a 25-ft prism pole. Needless to say, the application for this configuration would lean toward the topographic rather than the higher precision survey. The fiberglass models are generally easier and faster to operate; a spring-loaded button pops into place to fix an extended section, pretty much automatically. Caution: if a section is extended without the “click” of the button lock, error can result. This is of course true for other kinds of rods as well.
Of major concern to us all is safety. Who hasn’t heard of a surveyor who has been seriously injured or killed by a rod contacting electrical conductors? Another hazard is lightning. A metal prism pole, for instance, is basically a lightning rod when carried vertically. “The fiberglass rod is a big improvement in this area,” says Steve Crain of Crain Enterprises, Mound City, Ill. Crain concurs that the fiberglass telescoping models are by far the most widely used.
There also exists issues about caring for leveling rods, just as with all survey gear. Fiberglass rods are particularly susceptible to wear and distortion on the graduated face, particularly when casually tossed into a truck unprotected. These rods come with a case, and personnel should be taught to use them any time a rod is put away.
The experts agree that a little time spent in selecting the appropriate accessories is well spent. They make another reasonable point: “Why scrimp and agonize over a few dollars spent on accessories that can make a very expensive instrument more productive?” Why indeed?