How the numbers fit together
Use upright staffs and one unchanged level setup. HI is the elevation of the horizontal sight line relative to your datum, not the instrument’s height above the ground.
- Sight-line elevation
- HI = RL + BS
- New point elevation
- RL′ = HI − FS
- Difference from benchmark
- Δ = RL′ − RL = BS − FS
- RL · Known benchmark elevation
- 100.000 m
- BS · Backsight at the benchmark
- 1.250 m
- FS · Foresight at the new point
- 0.850 m
- HI · Sight-line elevation
- 101.250 m
- RL′ · New point elevation
- 100.400 m
- Δ · Difference from benchmark
- +0.400 m
Both modes require a consistent relationship to the same reference. An inverted levelling rod, a benchmark above the line or a changed instrument setup needs an explicitly checked sign convention; the page does not provide an automatic survey-network adjustment.
The browser uses one BS/FS method. The app guide below describes two reference forms with the same arithmetic; the app also supports saved calculations and export.
In the app · English
Tools → Benchmark elevation → Elevation from benchmark
Tools → Benchmark elevation → Elevation from known benchmark
For arithmetic after a levelling measurement. RL means reduced level: elevation relative to the chosen datum. The phone does not measure the levelling rod or replace a levelling instrument.
- Known reference level
- Elevation of the benchmark or reference point relative to the datum used for the work.
- Instrument / sight-line height
- The vertical offset from that reference elevation to the horizontal sight line. It is not an unrelated tripod height above the ground.
- Rod reading
- Distance downward from the sight line to the target point, using the positive reading on an upright levelling rod in this example.
The two forms use different field labels but the same arithmetic: known reference elevation + sight-line offset − rod reading. Keep the chosen datum and reading convention consistent.
- The target point’s elevation in the selected dimension format.
Two examples using the same line offset and reading but different reference elevations.
| Quantity | Value |
|---|---|
| Line offset; rod reading | 1.60 m; 1.20 m |
| Ground reference; raised benchmark | 0.00 m; 100.00 m |
| Ground-reference result | 0.40 m |
| Other-benchmark result | 100.40 m |
The sight line is 1.60 m above the reference. A reading of 1.20 m places the target 0.40 m above that reference. Add the known elevation to express it in the project datum.
Use metric or imperial lengths consistently and keep extra precision until the final display. Rounded diagram labels are not a measuring tolerance.
Both modes require a consistent relationship to the same reference. An inverted levelling rod, a benchmark above the line or a changed instrument setup needs an explicitly checked sign convention; the page does not provide an automatic survey-network adjustment.
This is an elevation calculation, not a bubble level or an automatic phone-sensor reading.
Why subtract the rod reading?
An upright levelling rod measures downward from the sight line to the point. A larger reading places the point lower, for the same line.
Can the reference be zero?
Yes, for relative work with an explicitly chosen local datum. Do not mix that local result with an absolute elevation without the correct offset.
- Why subtract the rod reading?
- An upright levelling rod measures downward from the sight line to the point. A larger reading places the point lower, for the same line.
- Can the reference be zero?
- Yes, for relative work with an explicitly chosen local datum. Do not mix that local result with an absolute elevation without the correct offset.
Measurements, units and material terms
Back to the category