Heights & Orthometric Elevation

Getting a height right means knowing exactly which height you are looking at. This page walks the whole chain, from the antenna to sea-level elevations.

The height chain

The antenna phase center (APC) is located 9.4cm (0.094m) from the bottom of the RoboDot.

This distance is available on the RoboDot Touch Screen settings page.

When you mark a point, RoboDot computes the ground elevation for you:

ground = measured height at the APC − antenna offset (0.094 m) − pole height

That ground elevation — like the raw GNSS height it comes from — is ellipsoidal: measured against the WGS84 ellipsoid, the smooth mathematical model of the Earth GPS works on.

Ellipsoidal vs orthometric

Most plans, benchmarks, and stakeholders use orthometric heights — elevations above sea level. The difference between the two (the geoid undulation) is not a constant: it varies from roughly −100 m to +85 m around the world. Handing someone an ellipsoidal height where an orthometric one is expected is one of the classic ways a survey goes wrong by tens of meters.

RoboDot converts on the device using a geoid model file — no internet required.

Turning on orthometric heights

1. Open the Web UI Settings page.
2. Under Ortho Height, pick a geoid model from the dropdown. The list shows every geoid*.bin model file present on the SD card; the file's header names the model (e.g. EGM96).
3. The change takes effect immediately — no restart.

Where orthometric heights appear

• Status page — the position card shows Ortho H alongside the ellipsoidal height.
• Point exports — CSV and GeoJSON exports add an orthometric ground column for every point when a geoid model is active.