To determine the suitability and location of a sewage treatment system during site evaluation work, I use an Abney level. While it is not quite as accurate and is only useful for relatively short distances, it is good enough to locate the system and determine if there is sufficient elevation change for installation. With this level, I can also directly read slope.

When using the Abney level or using a simple hand level, the bubble in the tube should be centered on the horizontal crosshair. For an Abney level, make sure the pointer for measuring slope is at zero on the scale. To make sure the level is accurate on a level site take a rod reading at your eye height next to you; then take a reading on the rod at a distance. The numbers should be the same; if not, the level needs to be adjusted to give an accurate reading. You should have your eye height memorized for use in determining elevations and contour intervals.

It happens that my eye height measurement on a surveying rod is 5.5 feet — an easy number to remember and an easy number to use in calculations. Using the Abney level to determine percent of land slope is done by siting on a rod, other person or point on an object that is at the same height as the eye level (in my case, 5.5 feet). The pointer on the slope scale is moved up or down depending on whether it is pointed upslope or downslope until the bubble is in line with the crosshair in the eyepiece. Slope is given directly by where the pointer rests on the scale.

Slope can be determined by a hand level by standing at the bottom of the slope, sighting through the eyepiece and determining a location upslope in the horizontal line of site, then measuring the distance between the hand level and the identified point (called the point of intercept). Slope is determined by the height of the hand level divided by the distance to the point multiplied by 100. Using me as an example: If the distance to the point I sighted is 50 feet, the slope percent would be 5.5 ÷ 50 x 100 = percent slope, or 0.11 x 100 = 11 percent slope.

To determine the elevation difference between two points with either an Abney or hand level, have another person hold the rod or have a rod holder so the rod can be placed at the point of interest. Similar to determining land slope with a hand level, here a point is determined on the line of site; for me, this point would be 5.5 feet higher than the starting point. If I moved to the new point and sighted again, identifying a second point, the elevation difference would be 11 (5.5 feet + 5.5 feet = 11 feet). Then if from point two I read 2.5 feet on the surveyor rod, the elevation difference from the initial starting point would be 5.5 feet + 5.5 feet + (5.5 feet – 2.5 feet) = 14 feet of total elevation difference. The rod reading is subtracted from my eye height to provide the 3-foot difference between the last two points. 

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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 kim.peterson@colepublishing.com.

This article is part of a series on site surveying.

• Site Surveying: Terms You Should Know
• Equipment Options for Site Surveying
• Using a Hand or Abney Level
• Surveying: Checking Relative Elevations
• Establishing Onsite Treatment System Elevations Using a Benchmark