FIELD CALCULATOR / FEET
Benchmark, backsight, foresight.
Shoot a known point, shoot the point you care about, and find out how far off grade you are. Enter a target and this returns cut or fill against the tolerance you are working to.
Enter the readings above.
The result shows the point elevation and, if you supply a target, the difference from it.
How it works
Differential levelling, which has not changed in a very long time:
- Height of instrument = benchmark elevation + backsight. This is the elevation of the line of sight, not the height of the tripod.
- Point elevation = height of instrument − foresight.
- Difference = point elevation − target elevation.
A positive difference means the ground is high and you cut. A negative difference means it is low and you fill. When the difference is within your tolerance the reading is marked PASS.
You set up on a benchmark at elevation 100.00 ft and read 5.25 ft on the rod. Your line of sight is at 100.00 + 5.25 = 105.25 ft.
Move the rod to the point you are checking and read 4.75 ft. That point sits at 105.25 − 4.75 = 100.50 ft.
The plans want 100.25 ft there. 100.50 − 100.25 = +0.25 ft, so it is a quarter of a foot high: cut 0.25 ft, or about three inches.
Reading the rod the right way round
The backsight is always taken on a point of known elevation and always adds. The foresight is taken on the point you are establishing and always subtracts. Swapping the two is the single most common way to get a clean-looking number that is wrong by twice the difference between the readings.
A useful sanity check: a larger foresight than backsight means the new point is lower than the benchmark. If your arithmetic disagrees with what you can see, believe your eyes and check the arithmetic.
Choosing a tolerance
The tolerance is yours to set, and it should come from the specification for the work rather than habit. Rough grading is usually held to a looser figure than finished subgrade, and a concrete pad is tighter still. This tool marks PASS against whatever number you enter — it has no opinion about whether that number is appropriate for what you are building, and PASS is not a statement that any contract or standard has been met.
Where this goes wrong on site
A benchmark that moved. Every elevation you set from it inherits the error and nothing in the arithmetic will reveal it. Close your level run back onto a known point when the work justifies it.
Mixed units. Rods marked in feet and tenths and rods marked in feet and inches look similar at a glance and are not interchangeable. Everything here is decimal feet.
An instrument out of adjustment. Balancing your backsight and foresight distances cancels most collimation error. A two-peg test now and then is cheap.
Sights that are too long, into heat. Shimmer near the ground on a hot day will cost you more than the arithmetic ever will.
The calculator warns on a negative foresight, on rod readings over 25 ft, and on differences over 20 ft, because those usually mean a typo or a unit mix-up rather than genuinely unusual ground.
Turning points and longer runs
To carry elevation past a single setup, take a foresight onto a solid turning point, move the instrument, and take a new backsight onto that same point. The turning point's elevation becomes the new known value. Run each leg through this calculator and carry the elevation forward.
Keep the readings, not just the answer
An elevation on its own is hard to defend weeks later. The Driftless Field workspace keeps each observation with its benchmark, backsight, foresight, target and tolerance, numbers your points as you go, and prints the whole set as a job report with the arithmetic shown.
Related: slope and grade calculator for turning two elevations into a percent or a ratio.
Useful on your job?
Share this free calculator with someone who could use it, or tell Jesse what needs improving.
Shares the tool link only, without your measurements. https://driftlesssolutions.us/tools/grade-check/