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A radar and a staff gauge can disagree without either instrument being incorrectly calibrated. They may use different elevation datums, observe different hydraulic locations or have been read at different times. A radar water-level survey should resolve those differences before anyone adds a permanent offset.
Identify exactly what each reading represents
Record whether the radar value is distance below the sensor, depth above a channel invert or elevation relative to a survey datum. Establish the zero mark of the staff gauge independently. A label such as "water level" does not identify any of these references.
Take paired observations under stable conditions where possible. Note the location of the staff gauge relative to weirs, gates, bends and pump discharges. A surface gradient or local drawdown can make readings at separate positions legitimately different. The reference method must observe the same required quantity.
Radar water-level survey record
| Survey item | Evidence to retain |
|---|---|
| Permanent mark | Identification and elevation of the stable reference used by the survey. |
| Radar origin | Surveyed elevation of the documented measurement reference plane. |
| Gauge zero | Elevation of the actual staff-gauge zero, including later repairs or movement. |
| Paired observation | Time, surface condition, reading locations and the two reported quantities. |
For a vertical measurement, water elevation equals the radar reference elevation minus measured distance. For example, an origin at 6.40 m above the site datum and a 2.10 m distance give a water elevation of 4.30 m. This is a geometric calculation, not a model accuracy claim.
Keep both input values in the commissioning record. If the sensor is subsequently remounted, its reference elevation may change even when the electronics and address remain unchanged.
Check the bracket before changing calibration
The METRAVON MV-RD12T measures from above the water. Review bracket stiffness, repeatable mounting and clearance from floodwater. A shifted bracket can create a real change of origin; correcting it with an unexplained software offset hides the physical cause.
Compare several suitable water levels when available. A nearly constant discrepancy suggests a different investigation from an error that grows with level. Neither pattern alone proves the cause, but both are more informative than a single reading taken during rapid flow changes.
Q1. Can the staff gauge be assumed correct because it is mechanical?
No. Its mounting, scale, zero and local water surface still need verification. A damaged or moved gauge is not an independent reference merely because it has no electronics.
Q2. When is an offset justified?
Only after the measurement quantities and physical references are reconciled and the correction is supported by the approved procedure. Record its reason, sign and value so later maintenance can reconstruct the calculation.
Summary
Reconcile the survey and observation locations before correcting the display. Provide the datum sheet, bracket drawing and paired readings for an MV-RD12T site review.
