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.


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.


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.


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.