Land surveying is the act of measuring portions of the earth's surface to determine their boundaries and area, usually for the legal purpose of conveying property. It's and ancient occupation that demands honesty and accuracy.
Surveying has changed over the centuries. Once, a surveyor's chain really was a chain. Then it became a steel tape. Now it has been replaced by a laser beam. We'll talk here about both the modern and traditional forms that surveying takes, since both are in use in various parts of the world today.
To survey, two things must be measured as accurately as possible: distance and angle. The tool for measuring distance has undergone the most changes over the years. In the past, a surveying chain was really a chain. It needed to be metal, since it must not stretch when it was pulled tight. In more modern times (eg, past 1930), the surveyor's chain became a steel tape, with feet and tenths etched into the surface. The tool for measuring angle has changed as well. In earlier days, it was a compass, possibly with some extra sighting guides. Subsequently, the transit came along: a telescope with crosshairs and vernier scales for measuring angles. Then came the modern theodolite, capable of measuring angles to within one half of one second of arc.
The act of surveying a piece of property involves doing a traverse of its bounding points. If you own a piece of property and look at your deed, you will often find a metes and bounds description of the property that reads something like:
Beginning at the iron pin at the southeast corner, continue 125.23 feet North 3 degrees 11 minutes and 7 seconds West to a 1/2 inch iron pipe;
53.12 feet North, 87 degrees 4 minutes East to ...
And so on, you get the idea. The description will traverse
of the property
and wind up back at its starting point. Since surveyors know all about analytical geometry, they can calculate all of those angles and distances and prove that, starting from a given point, they return to the same point, "proving" that the survey was accurate.
Often, the actual traverse points (the bounding points of the property) are inaccessible (they may be tangled up with a fence post or underneath a wall or, in very old surveys, too vague - a tree or fence post rather than an iron pipe or piece of rebar). When this happens, the surveyor will use their own traverse points - set near the property corners - and "shoot" an angle and distance from them to the real corners. Thus the traverse can still be "closed" (by calculating the effects of all the angles and distances to all of the new traverse points) and the accuracy of the survey be guaranteed.
A survey party is traditionally (and technology has changed this quite a bit) composed of four persons (sorry for the male dominated terms, but surveying has been a male dominated field): the survey party chief, instrument man, head chainman (who often serves as rodman) and the rear chainman (who also gets pressed into duty as rodman, as circumstances require).
Put simply, the party chief runs the show (and is responsible for documenting everything in very stylized "notes" that describe the survey in mathematical and pictoral detail); the instrument man uses the theodolite to turn all of the angles (and often, in modern times, measure all of the distances); the chainmen measure distances (unless electronic instruments are used for this purpose as they would be in most modern circumstances today); and the rodman holds the rod (or, at closer distances, a chain and chaining or plumb bob) over the traverse point so that the instrument man can sight on it.
In modern times, a survey party can consist of only one person, who fills all the roles. This works because the modern theodolite (the successor to the transit used up until the 1970's or so) can also measure distances very, very accurately by bouncing a laser beam off of a reflector set up on a tripod over the distant point. Modern theodolites even remember the points and distances and can computationally "close" the traverse at the end, saving no end of work on the part of the party chief who traditionally had to do it by hand.
Modern equipment allows surveys to be incredibly accurate. Distances can be measured to within one part in a million and angles can be measured to within one second (and some instruments boast a half a second) of arc (a second of arc is roughly the width of a quarter seen from three miles away). By "turning" angles multiple times to distribute error and "shooting" distances many times, surveys accurate to less than one part in a million are routine (so a traverse of a couple of miles around a good-sized property might "check in" to its starting point to within a couple of hundreths of a foot - about a quarter of an inch).
Surveying is a demanding occupation. Like the mail, a survey must "get through". Obstacles, such as mud, snow, ice, steeply inclined hills, brush, tree limbs and so on must be dealt with daily. Often markers left decades previously are difficult to find and may require a good deal of digging to unearth. Sighting a line through wooded property may require hours of brush cutting.
Be sure you thank the crew that surveys your lot before you sell.