Surveying Basics for Septic System Installation

Surveying Basics for Septic System Installation

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Laser levels have become an important tool to set elevations during construction of septic systems. They are easy to use, and only one person is needed to operate them. 

A tripod is used to set a laser level. Its pointed legs need to be set firmly on the ground so that it does not move during use. Once the laser level is set on the tripod and leveled, it provides the user with a horizontal plane at the elevation (height) of the rotating laser head. The user can be anywhere on site. If the user has a line of sight to the laser level, then he or she can raise the laser detector to gain access to the horizontal plane created by the rotating laser head. 

Leveling rods are telescopic graduated rods that are used to measure from a benchmark or to set an elevation. When using laser levels, the laser detector is mounted on the leveling rod and can be slid up or down the rod to find the laser plane. A beeping or constant sound signals that the detector is close to the plane or right at it. 

The leveling rod typically has red numbers (1, 2, 3, etc.) that represent a whole foot. The leveling rod may be calibrated in inches or in tenths of a foot. When calibrated in tenths of a foot, every foot on the rod has 10 major tick marks instead of the regular 12 when calibrated for inches. These 10 major tick marks are called tenths of a foot. The internal tick marks within the major tick marks are also divided into ten. These are called hundredths of a foot.

For example, 6.75 feet (6 feet, 7 tenths and 5 hundredths) is the same as 6 feet, 9 inches. Depending on the region or designer preference, the design plans may come with elevations in inches or in tenths. The user will need to do a conversion or use the correct rod for the measurements. Leveling rods that come in inches are calibrated in feet, inches, and eights of an inch and read just like a regular measuring tape. 

Benchmark transfer

Once the laser level is mounted on a tripod and leveled, the user can take the rod and laser detector to the benchmark. The user places the leveling rod with the laser detector installed and moves the detector up and down with the rod to locate the plane of the rotating laser head. 

When the level plane is located, the user places the rod directly on top of the benchmark and slides the laser detector through the rod until the level plane of the rotating laser head is found. This needs to be done while the rod is vertical on top of the benchmark. Any angle on the rod will result in an inaccurate reading on the rod. After this measurement is done, the user records the measurement.

The height of the laser rotating head is calculated by adding the benchmark elevation to the measurement of the rod previously obtained. For example, the benchmark elevation is stated to be 100 feet on the plans (as a nail on a pole), and the measurement from the top of the nail to where the laser detector located the plane of the laser rotator head is 8.1 feet. Then the elevation of the laser rotator head is said to be at elevation 108.1 feet (100 feet + 8.1 feet). This elevation is called the height of instrument. 

With the height of instrument established, the user can use it to transfer the benchmark and to set elevations for the system. To establish a new benchmark (transfer), the user places the rod where he or she wants to transfer it, places the rod in a vertical position, and locates the plane of the rotating laser head. Once located, the user reads the number from the leveling rod at the locator. The elevation of the new benchmark is calculated by subtracting the number read from the height of instrument. For example, if the user reads 4.6 feet, then the new elevation of the benchmark is 103.5 feet (108.1 feet – 4.6 feet). Now the user can move the instrument to a more comfortable location in the lot to set elevations on the onsite wastewater treatment system.

Elevation transfers

Elevation transfers work almost in the same way as transferring benchmarks. The difference is that the elevations need to be set at a certain height. For example, when setting invert elevations on an onsite wastewater treatment system, the user has a set of elevations from the design plan and a benchmark.

After setting the instrument on a tripod and leveling it, the installer can obtain the height of instrument as stated in the previous section. Then he or she can set the elevation of an invert by subtracting the elevation of the invert from the height of instrument. This number indicates where the laser detector is set on the leveling rod. Move the rod up and down until the laser detector meets the plane of the laser rotator head and the bottom of the leveling rod is the elevation the invert needs to be set at.

For example, a plan calls for an invert into the septic tank of 98.9 feet and the benchmark is 103.5 feet. The user needs to set up the instrument and get the height of instrument. The user reads a benchmark of 5.4 feet and calculates the height of instrument (103.5 feet + 5.5 feet = 109.0 feet). Then the elevation of the invert is subtracted from the height of instrument (109.0 feet – 98.9 feet = 10.1 feet). The user slides the laser detector to 10.1 feet on the leveling rod and takes it where the tank is to be located. Then the user moves the rod up and down to meet the plane of the laser rotating head. The bottom of the rod at this position indicates where the invert should be.

The user can set up a table to minimize errors and record how the elevations were set up. The table can contain headers like benchmark (BM), height of instrument (HI), readings from, readings to, and elevation. Readings from an existing benchmark are listed under “readings from.” The user always adds those so the header can be simply a plus sign. “Readings to” are subtracted from the benchmark to determine elevation. A simple minus sign is sufficient.

A laser is an essential tool for designers, installers and regulators in the septic industry and understanding the basics of how they work is essential to using this tool effectively.

About the author
Sara Heger, Ph.D., is a researcher and educator in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota, where she also earned her degrees in agricultural and biosystems engineering and water resource science. She presents at many local and national training events regarding the design, installation and management of septic systems and related research. Heger is the President-Elect of the National Onsite Wastewater Recycling Association and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system design, installation, maintenance and operation by sending an email to


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