Compasses were familiar to the Chinese a thousand years ago, and they have been continuously used ever since. The classic Chinese geomancer carried an elaborate compass that not only located the poles, but also helped to determine the directions of the Tiger and the Dragon. These early surveyors traced energetic forces and invisible lines much as we do today and they prepared site plans and reports that served as the basis for tremendous private and public works projects. Compasses have remained the surveyor’s friend for centuries and they are still an essential component of our toolbox.
The compass is easily transportable, easy to use, and when employed by a skilled operator is capable of delivering results in the order of 100′ per mile of traverse through all types of terrain.
Some considerations regarding the magnetic compass;
Legal descriptions and survey maps are oriented to astronomic north. This is close to the position occupied by Polaris in the night sky. It is not the same direction as magnetic north and the difference between these two locations is referred to as the magnetic declination or deviation. Astronomic and Magnetic north are coincident in a line running through the middle part of the United States, but the further west one might be from that line, the greater effect the magnetic deviation will be. In our area it is approximately 17 1/2 degrees east, but because the magnetic pole is generated by the shifting magmatic core of the Earth, declination does not remain stable. Subracting 17 degrees from every bearing becomes tedious, so most surveyors compass are equipped with an adjusting screw that allows us to set declination to the local area and forget about it thereafter.
Compasses may also be affected by local magnetic variation. In heavily mineralized areas compasses sometimes are skewed.
For most accurate line determination with a compass the best method is to simply prolong the straight line by careful backsighting and foresighting and making proportional correction when the known error is determined at the end of the traverse. This technique was very commonly used by pioneer surveyors using a box compass and a Jacobs staff.
Perhaps there is no tool that has been associated with the profession as consistently as the transit. Just as an anvil represents a blacksmith, a tripod with a transit attached is the symbol of the surveyor. Transits and theodolites have been evolving since the seventeenth century, and although the terms have apparently referred to various different instruments over the years, both labels describe devices for measuring vertical as well as horizontal angles around a single center point. modern theodolites generally have some sort of optical or electronic magnification of the circles that allows for more precise divisions and most transits in use have a vernier for sub minute divisions, but fundamentally these instruments function as they have for centuries, measuring angles.
Both instruments evolved from 16th. century compasses as mariners and land-navigators became more sophisticated at prolonging lines and describing angles. Compasses first developed limbs and vanes and then sighting devices (telescopes and crosshairs) were added. Older transits all were constructed around a central compass and as late as the 1990’s I routinely included compass bearings with all field measurements.
During the 1960′ and 1970s, various electronic distance measuring devices were developed. Most of these utilized infra-red or lasers, although there were some models that worked with other wave-lengths of the electro-magnetic spectrum. The first electronic distance measurement devices were large and heavy and frequently required several men to carry the meter and the battery and other associated equipment. By the 1970’s distance meters had become small enough to attach directly to the top of a theodolite, and a survey instrument man could record angles and distances from a single set-up.
The total station merges the angle and distance measuring function into a single unit. Total stations have become standard equipment for conventional surveying crews. Most of these now also feature electronic data collection which makes data transfer to the office computer a simple affair.
The global positioning system has become another useful tool for land surveyors. Where conditions allow, it has made accurate measurements over large areas much easier than other methods. Where there is heavy tree canopy or deep canyons however, satellite reception is too inconsistent for reliable measurements.
An important thing to understand about property line delineation is that boundaries depend on existing monumentation, so that even with the most accurate measurement devices, land surveys require assessment and evaluation of existing evidence on the ground. Corner markers and lines of occupation (fences, walls, roads, etc.) may all have an effect on the boundary resolution, and need to be located and evaluated. Typically property information is not dependent on Latitude and Longitude or wide-area coordinate systems, but is controlled by the iron pipe near your fence post or the street right-of-way. GPS allows the surveyor to accurately locate the relative position of the controlling evidence. It is particularly effective for establishing survey control over a large area or locating isolated points such as aerial photo control targets or radar or cell phone towers.
Machetes are perhaps the single most essential tool for forest surveying. This all-purpose instrument is ideal for clearing lines, uncovering buried monuments, discouraging angry dogs and pot-growers, and even slicing melons or salamis. Razor-sharp machetes require a good leather sheath which will end up costing you more than the machete itself, but it is worth the investment in convenience and cut fingers.
“You see the machete, over the last three centuries, has proven to be the ultimate outdoor and survival tool. It will cut, chop, slash, hack, split, scrape, scoop, hammer, dig, crush, carve, whittle, crack or smash just about anything you can put in front of it.”
The ubiquitous, brightly-colored plastic tape that decorates so many property corners and tree limbs is sometimes referred to as “surveyor’s spoor.” This vinyl ribbon is available in every color imaginable as well as stripes, prints and polka-dots. It is widely used by many resource professionals as temporary location markers, and there are places in the woods that have been visited and flagged for so many different purposes that all the trees look like Christmas trees. The flagging breaks down fairly quickly when exposed to UV radiation so that direct sunlight can deteriorate the stuff in less than a year, while if it is in the shade, or buried, it may last for many years. This is quick and temporary stuff. “Flagged” lines are approximate.
The plumb bob is a living link with gravity. This simple device allows us to experience a mystical union with the center of the Earth, the ancient Maya and all our ancestral engineers. I don’t know how many hours I have spent willing myself to stand still, quieting my body and mind while the plumb bob tip hovered over a point on the Earth. Understanding the plumb line can really help foster a profound understanding of gravity and of the stately dance of the cosmos.
When I was child in the Boston area I often visited the Museum of Science and spent many hours in the foyer mesmerized by the Aztec calendar stone and the giant plumb bob suspended above it. I don’t know if this was an Aztec or a Boston invention, but the pendulum and the calandar stone are always united in my mind. This was a Foucalt’s Pendum. It wasn’t util many years later, when I found myself staring down the string of a burnished brass bob, the iron tip describing lemniscate patterns over a tiny punchmark, that I discovered my own empiric experience of being a part of natural matter.
Levels are another ancient tool that remains in common use. The oldest level of all is, of course, water. A tranquil lake surface provides an adequate method of conveying an elevation over a large distance. Water levels were employed in the construction of the pyramids, and are still employed by builders. The ancient Egyptian water-level was a groove chiseled in the top course of masonry and flooded. Modern water-levels are clear glass or plastic tubes filled with water. These simple devices may be as long as required and accurately transfer elevations.
At a recent Permaculture design course I was introduced to the A-frame level. This is a simple and elegant device that allows an elevation to be transfered by means of a two-legged plumb bob. An A-frame level may be assebled on site from locally available materials. It does not require watyer, plastic, glass or precision measurements. The A-frame consists of two rigid legs fastened at the apex and secured by a cross bar. A plumb line depends from the apex and hangs freely between the legs. A scale is inscribed along the cross bar and calibrated by setting the instrument up off-contour and noting the loation of the string along the scale. The A-frame is then inverted at the same location, and the point midway between the two scale marks denotes level. By maintaining the plumb line at this location it is then possible to walk a level line across the landscape.
The surveyor’s level is a spirit level. The level vial allows the instrument person to sight a plane perpendicular to the direction of gravity. By leveling the scope and sighting to a point of known elevation, the HI (Height of Instrument) can be determined. Sighting to a leveling rod allows the surveyor to determine the heights of subsequent points by subtraction. Old-style levels used 3 or 4 leveling screws to adjust the vial. The auto-level pictured needs only to be set roughly level with a bullseye, and internal compensators will set the sighting device.
Because level sighting devices do not adhere to an arc concentric with gravitational forces, but prolong a plane tangent to a plumb line, shots taken with a with a spirit level should not be more than 300′ and backsight and foresite distances should be about equal.
Most grading outfits today use rotating laser levels. these utilise a pinpoint lazer that can be set level and which rotates rapidly to project an arc at a continuous elevation. If this is set at some point on a construction site, and a rodman with a graduated pole or a measuring tape can easily transfer elevations from known control to the working surface.
On all of our jobs we keep a set of field notes. This may be a bit of an anachronism in the days of electronic data-collection, but I have found an old-fashioned set of notes, with date, crew members and a good sketch of monuments found or set, and the location of stations along with their relationship to the rest of the world to be an invaluable aid when returning to the site after a passage of months or years. We no longer write down the measured angle and distances at each set-up, the data collector is faster and more accurate for that, but we include each set-up and backsite, and describe what is actually shot-in. The field notes provide a check on what is recorded electronically, and when a bust is discovered in the data reduction, I am often able to resolve the error with the aid of the notes.