Land surveying

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 Now it has been replaced by a laser beam.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;
THENCE
53.12 feet North, 87 degrees 4 minutes East to ...

And so on, you get the idea. The description will traverse the bounds 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.

Elevation Survey

Besides marking off precise measurements of property boundaries, surveying equipment is also used to take elevations. This is critical information when constructing bridges, laying railroad track, installing sewer lines, and building level house foundations. Taking elevations is a two-person job: one to hold the surveying rod and one to operate the theodolite.

To take relative elevation measurements, all that is strictly needed is a theodolite, a surveying rod, and a nice stable, permanent point, such as a cement porch step, to use for reference. This will allow the surveyor to measure elevations relative to the reference point, which is often all that is necessary. To take absolute elevation measurements, the surveyor will need an absolute reference point. In the US, these are typically bronze-capped concrete markers set and maintained by the United States Geological Survey to permanently mark the elevation above sea level of known points around the country. By taking measurements off of one of these elevation reference marks, a surveyor can determine the absolute elevation above sea level of any point.

Equipment

Setting up the theodolite properly is critical. This can take a few minutes, a steady hand, and patience. The theodolite is basically a small telescope on a tripod with leveling screws. Once the tripod is set securely on the ground, the leveling screws are adjusted to make the theodolite perfectly level, referencing a bullseye spirit level set in the instrument. It must be level for its entire 360 degrees of travel. The theodolite will also have a dial that shows the angle it is pointed at, but it is not needed for taking elevations.

The surveying rod is little more than a yardstick. Actually it's quite a bit more than a yardstick, as they're typically 3 yards or more long. To ensure they don't wear down with use, wooden surveying rods will have metal endcaps to protect them. The surveying rod itself will be marked off in feet and hundredths of a foot (rather than the less-precise inches). Typically this is done with large numbers at the foot marks and ten smaller numbers at each tenth of a foot. Five black and white lines then divide each tenth of a foot into ten sections, the dividing lines between black and white stripes marking the hundredths of a foot.

Procedure

The first elevation to take should be the reference point, or the elevation reference mark if taking absolute elevations. The rodman will put the surveying rod on the point to be measured and hold it plumb (this can be verified from the theodolite with the vertical crosshair in the scope). The surveyor will then look through the scope to see where the crosshairs line up on the rod. Since the rod measures in hundredths of a foot, the scope magnifies the view for precision at a distance. A guide built in to the top of the theodolite can help the surveyor roughly line up the scope with the rod to make it easier to find when looking through the scope.

The surveyor will then note the measurement the crosshairs line up with on the surveying rod. If the theodolite happens to be on ground that is level with the reference point, it will typically measure about 4-5 feet, depending on how tall the tripod is. This is the most critical measurement in the survey, as all the other measurements will be referenced relative to it.

Once the reference mark has been identified, the rodman will move the surveying rod to the various points around the site that need elevation checks (these points will have been identified and marked ahead of time). If the survey is being done to level a spot for a house foundation, these points will be the corners of the foundation, and probably also a point on the street to ensure that the elevation of the house meets the minimum building code requirements for height above street level. This ensures the house will not flood easily and that the sewage lines will drain properly.

Sometimes the elevation reference mark is too far away from the surveying site to shoot accurately. In this case, one or more new reference points must be identified between the reference mark and the site. Once a point closer to the site has been identified and measured, the surveying site can be measured relative to that reference mark. An experienced and careful surveyor can take multiple new reference points without losing accuracy.

Example

To provide an example, let's assume that the foundation must be 1 foot above street level, and that the reference mark used shoots at 5.00 feet on the surveying rod. The rodman then goes to the street and the surveyor shoots street level at 7.00 feet. This means that the street is 2 feet lower than the reference point, because the surveying rod is down lower, so the theodolite will shoot a higher number. Since the foundation must be 1 foot higher than street level, we need it to be at 6.00 feet. The rodman then goes around to the four corners of the house and the surveyor shoots them at 6.10, 5.80, 5.75, and 5.25 feet.

Obviously if a foundation were to be poured on the ground as it is, it would not be level and all your pens would roll off of your desk. So markers need to be set near each corner and adjusted so that they sit at our desired 6.00 foot level. Unfortunately the 6.10 corner is 10 hundredths of a foot too high, so we need to dig out the ground here before setting the marker. Once the markers are set at the appropriate elevations, string can be stretched tightly between the markers. The string level designates the top of the foundation, providing a reference to grade and level the dirt the foundation will sit on. If the measurements were taken carefully and accurately, the finished foundation will provide a stable, level surface to build the house.