What is Differential Leveling?

Differential leveling is one of the fundamental techniques used by surveyors to measure height differences accurately between points on the Earth's surface. But what exactly is differential leveling, and how does it work?

Imagine you're standing at the base of a hill, and you want to determine the height difference between your current location and the hill's summit. This is where differential leveling comes into play. Essentially, it's a method used to measure vertical distances (elevations) accurately. By using a level you can accurately measure the differences between “objects” using proper survey methods, even if the objects are further apart than you can see looking though the scope.

There are a many kinds of survey levels, but ultimately, they all do the same thing. By setting up the level between two “objects” and using a level rod, they allow the user to measure up off of one object (back station) to determine the height of the level, and then measure down to determine the elevation of the next object or location (ahead station).

Starting at a Permanent Benchmark or reference point

Differential leveling typically starts at a known fixed point, possibly a published benchmark or simply at an arbitrary point that won’t be disturbed that, can be used again in the future. This benchmark serves as the starting point for our elevation measurements.

In the below “Differential Leveling” graphic, you can see on the left side, the benchmark elevation is published as 9.26’ NAVD88. (For those not sure what NAVD88 is, please see our blog about datums located here: Vertical Datums)

Traversing Ahead

The survey level is placed close enough to accurately read the level rod, and usually in the general direction toward where the object is that you want to measure and then instrument is leveled. In the sample above, the level is set between two monuments and the level line is 5.26’ above the benchmark, which makes the level line (plane) an elevation of 14.52’ (aka height of instrument or “HI”). Since levels are made so that when you spin the level head horizontally it maintains level, you can turn to any position to the ahead rod and measure down to the next station. In the sample above, the monument is 3.43’ below the level line, which makes the ahead monument an elevation of 11.09’. This portion of the process is often referred to as one “leg” and is only part of the overall process.

What’s next?

If only elevating the ahead monument this is the extent of the task then it may be able to be completed by a single setup, BUT it would be best to move the tripod (sometimes referred to as “breaking the legs”) and then repeat the process in reverse. Measure up the from the monument, and then measure back down to the benchmark and demonstrated in the below graphic after moving the tripod.

By doing this it allows us to verify that our starting benchmark elevation, matches our end benchmark elevation. This allows us to have a high level of confidence with all of our elevations that we may have measured through along the way. This process is called a “level loop” and checking the “error of closure”.

Often these level loops, requires repeating the process to measure multiple objects and may take many “legs” to complete and close the loop. It is best practice to “break the legs” between each measurement, to make sure the level was actually level and in proper working order. It’s also quickest way to do the math afterwards, by not having multiple measurements from one setup, usually referred to as “side shots”.

What does the math look like and the notes?

It is very simple math, and is often done on the field notes before the Surveyor leaves the site, so that if there any issues, they can verify measurements or start over if need be. Depending on the situation and the necessary precision, we would typically rerun a traverse that has a misclosure of greater than 0.01’, especially in an urban environment. There are situations where that is not feasible, especially on long distance leveling efforts, and adjustments may need to be performed.

The following note sample is in reference to the information shown in the above ahead and reverse graphics:

Differential leveling plays an important role in high precision Land Surveys where elevations are important. It is imperative to understand this principle and how to best utilize leveling in conjunction with GPS and conventional traversing methods.