Set the vertical datum
Record what your heights are measured from, such as GEOID18 (NAVD88), EGM96 or a local benchmark, so exports and the QA report state it.
On this page
A GNSS receiver measures height above the ellipsoid. An elevation on a plan is height above the geoid, and the receiver converts one to the other using whichever geoid model it's set to. Different models give elevations that can differ by half a metre to a metre and a half across the continental US. Nothing in the data stream says which model was used.
The Vertical datum setting is where you state which one your project is on. It declares; it doesn't convert. It records what your receiver is set to. That statement travels into every export, and the QA report can tell you when the data doesn't look consistent.
Before you start
- An open project. The Vertical datum row is greyed out without one.
- Check your receiver's own geoid setting, in the receiver's app or configuration tool. That's the value you're declaring here.
Set the vertical datum
- Go to Tools → Vertical datum. The row shows the current setting. New projects start at Not stated.
- Under Heights in this project are, tap the option that matches your receiver.
- Tap Save. Tap Cancel to leave it unchanged.
| Option | What it means |
|---|---|
| GEOID18 | NAVD88, ±0.02 m in the continental US. The current US model. |
| GEOID12B | NAVD88, ±0.02 m in the continental US, superseded by GEOID18. |
| EGM2008 | EGM2008 global mean sea level, ±0.15 m in well-surveyed regions. |
| EGM96 | EGM96 global mean sea level, ±0.5–1 m in the continental US. Often a receiver's factory default. |
| Local benchmark | Heights reduced to a benchmark on site. Internally consistent, but not tied to a published datum. |
| Ellipsoidal (WGS84) | Heights above the WGS 84 ellipsoid. These aren't elevations: they're tens of metres from any sea level. |
| Not stated | Heights are stored as the receiver reported them, with no record of which geoid it used. This is the default. |
Why the receiver has to apply the geoid
No geoid grid ships with the app. The Geoid grids section of the sheet says so: No geoid grid is installed, so nothing here is checked against a model. Every elevation you collect is the height the receiver reports after applying its own geoid model. With phone GPS alone, it's the height iOS reports.
So the vertical datum is only as right as the receiver's setting. If you work in NAVD88, confirm the receiver is loaded with GEOID18 before you trust an elevation.
What the QA report checks
Even without a grid, the QA report can catch the failure that happens most in the field: the geoid model changing partway through a job, for example when a second receiver joins or a setting is reset.
- Mixed geoids. The geoid is smooth. Across a site it changes by centimetres, not metres. If the geoid separations recorded with your points disagree by more than about 0.25 m plus 0.2 m per kilometre of distance, the report flags "N heights use a different geoid from the rest". Elevations from the two groups aren't on the same datum.
- No stated datum. While the setting is Not stated, the report lists "N heights have no stated vertical datum" and points you to this setting.
The report's vertical datum line also repeats what you declared. For a geoid option it notes that the datum was declared by the project and not verified against a geoid grid.
Where it appears in exports
The same statement is written into every format that has a place for it:
| Export | Where the vertical datum goes |
|---|---|
| CSV | A vertical_reference column on every row, next to horizontal_datum. |
| GeoJSON | A vertical_datum entry in the file's metadata, plus geoid_model when a geoid is declared. |
| LandXML | The verticalDatum attribute of the coordinate-system element, with the geoid model named. |
| DXF | A header note, for example Heights: NAVD88 (GEOID18) — orthometric where measured. Entities without a measured height sit at 0. |
| QA report | The vertical datum line. |
The wording depends on the option. For GEOID18 it reads NAVD88 (GEOID18) — orthometric where measured. For Not stated it reads orthometric (MSL) where measured — geoid not stated rather than claiming a datum.