---
title: Heights & Orthometric Elevation
order: 11
---

# 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.

![RoboDot ARP and APC](/assets/img/b353b4e9cd88.png)

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.

:::info
Every orthometric number RoboDot shows or exports is **labeled with the geoid model that produced it** — for example `412.301 (EGM96)`. If a height has no model label, it is ellipsoidal. You can always tell which is which.
:::

:::info
Regional geoid model files can be added to the SD card as additional `geoid*.bin` files — they appear in the settings dropdown automatically.
:::