Establishing Onsite Treatment System Elevations Using a Benchmark

How to use instrument height and backsight to specify elevations for septic system plans

Establishing Onsite Treatment System Elevations Using a Benchmark

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As discussed earlier, a benchmark should be relatively permanent. So one is often set at a corner of a building, large boulder or some other location that will be there for a long time. If you were to work off a surveying post established by the U.S. Geological Survey, you would be working off a benchmark where the elevation is given in terms of elevation above sea level. In my part of the world, it means an elevation of approximately 1,100 feet above sea level. Obviously, we do not need to know how high above sea level we are to install a septic system.

When a benchmark is set on site, it is usually given an elevation of 100.00 feet. This is arbitrary and only for the convenience of not having to deal with negative numbers. The instrument (transit or level) should be set up to see all parts of the system if possible.

The first rod reading is taken by placing the rod on the benchmark. This reading is called the backsight, or B.S., as indicated earlier in the definition of terms. The height of the instrument is the rod reading plus 100.00 (the benchmark). If the rod reads 2.95, then the instrument height (H.I.) is 2.95 + 100.00 = 102.95. Let’s say the plans call for the invert (bottom) of the pipe into the septic tank to be at an elevation of 96.45; the rod reading should be 102.95 (H.I.) - 96.45 (desired elevation) = 6.50. During installation of the septic tank, the invert of the pipe needs to be at a rod reading of 6.50 when the tank is set.

Similarly, if the installer knows that the dimension of the septic tank below the inlet invert is 5.35 feet, then the bottom of the tank excavation or the top of the aggregate material beneath the tank needs to be 5.35 feet (tank dimension) + 6.50 feet (depth to the invert) = 11.85 feet for a rod reading. If aggregate is needed to properly set the tank according to specifications, the excavation will need to be deeper than the desired bottom of the tank elevation. The elevation specified on the plans for the bottom of the tank should be 102.95 - 11.85 = 91.10.

If the elevation change across the site is more than the length of rod available, the instrument will need to be moved. When this occurs, a turning point elevation and new H.I. needs to be established. The rod would be set at the turning point and a reading taken (this is called the foresight, or F.S.). If the rod reading is 10.50 at the turning point and the instrument is moved so the rod reading is 3.88 at the B.S., the elevations are as follows: Turning point (T.P.) = 102.95 (H.I.) – 10.50 = 92.45 and the new H.I. elevation is 92.45 + 3.88 = 96.33. Subsequent elevations will be determined using the new H.I. 

About the author: Jim Anderson is connected with the University of Minnesota onsite wastewater treatment program and is an emeritus professor in the university’s Department of Soil, Water and Climate. Send him questions about septic system maintenance and operation by sending an email to

This article is part of a series on site surveying.


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