On a sloping site, which ground point should define wind-sensor height?

The short answer

On sloping ground, measurement height needs an explicit local reference rather than an unlabelled number. The fixed sensor position, mast base, terrain beneath the sensor and wider upwind slope describe different geometry. Document the arrangement so the assessor can interpret exposure and compare it with the proposed turbine position.

Draw the measurement arrangement

Record the mast location, sensor mounting position, stated vertical height and the ground point used as the reference. Where a boom projects over a slope, include a simple section drawing or survey information showing that offset.

Do not substitute the distance measured along the mast, slope or access route for a vertical height without explaining the geometry. The aim is a reproducible definition, not a universal rule inferred from a convenient tape measurement.

Keep height and terrain transfer separate

A correctly surveyed sensor height does not make sloping terrain behave like a horizontally uniform open site. Upwind topography and direction can influence the measured flow. Ask the assessor whether a simple height extrapolation is appropriate to the actual arrangement.

Likewise, matching the proposed turbine's nominal height does not resolve a horizontal displacement across the slope. Document both position and height changes so any transfer has a clear physical basis and stated limitations.

Resolve ambiguity before modelling

Review the height field in the resource file and the hub-height field in the energy model against the same drawing. If historical metadata are ambiguous, preserve that uncertainty rather than assigning a precise datum retrospectively without evidence.

For a new campaign, agree the height convention and survey record before installation. The resulting handover should let another engineer reconstruct the vertical references and assess the relevant flow assumptions. This reduces avoidable modelling confusion while keeping resource interpretation separate from foundation, tower and structural design requirements.