Coordinate Systems & Datums

RTKdata delivers corrections in global geodetic reference frames. This page explains how to work with local coordinate systems like OSGB36, NAD83, GDA94, and others.

---

What RTKdata Delivers

RTKdata corrections are transmitted in these global reference frames:

Mountpoint	Reference Frame
AUTO	Automatic (ITRF2014-based)
AUTO_WGS84	WGS84
AUTO_ITRF2020	ITRF2020
AUTO_ITRF2014	ITRF2014

These are global geodetic datums. They do not match local coordinate systems directly.

---

Why Your Coordinates May Be Offset

If you see a consistent 50 cm – 1 m offset compared to local survey control, this is almost certainly a datum mismatch, not an accuracy problem.

The earth's tectonic plates move. Global frames like ITRF2014 track current plate positions, while local datums (OSGB36, NAD83, ETRS89, GDA94) were fixed at a specific point in time. The difference grows every year.

This is not a bug — it's physics.

---

Common Local Datums by Region

Region	Local Datum	Offset from ITRF2014
United Kingdom	OSGB36	~1 m
United States	NAD83 (2011)	~1–2 m
Europe (continental)	ETRS89	~0.5–1 m
Australia	GDA2020 / GDA94	~1.8 m (GDA94)
New Zealand	NZGD2000	~0.3 m

---

How to Use Local Datums with RTKdata

Option 1: Post-Processing Transformation (Recommended)

1. Collect data using mountpoint AUTO or AUTO_ITRF2014

2. Export your data (typically as WGS84 / ITRF2014 coordinates)

3. Transform to your local datum in GIS or surveying software:

   • QGIS: Layer → Export → Save As → Select target CRS

   • ArcGIS: Use the Project tool or define output coordinate system

   • Trimble Business Center / Leica Infinity: Set project datum in project settings

Option 2: On-Device Transformation

Some receivers and apps support real-time datum transformation:

• Emlid Flow: Set the coordinate system in Project settings (supports many local datums)

• Trimble controllers: Select local datum in the project coordinate system settings

• Survey apps (SW Maps, Field Genius, etc.): Configure output datum in app settings

Important: Set the NTRIP mountpoint to AUTO regardless. The datum transformation happens on your device, not on the correction stream.

Option 3: Transformed Mountpoints

For some regions, RTKdata offers pre-transformed mountpoints:

• AUTO_ETRS89 — for European ETRS89

• AUTO_NAD83 — for North American NAD83

Check availability on the Network & Mountpoints page.

---

DJI Drones and Datums

DJI Pilot 2 and DJI Terra work in WGS84 only. You cannot select OSGB36 or other local datums in the drone interface.

Workflow for local datum output:

1. Fly with RTKdata mountpoint AUTO — corrections are applied in ITRF2014/WGS84

2. Process in DJI Terra (outputs WGS84 coordinates)

3. Transform the final output to your local datum (OSGB36, NAD83, etc.) in your GIS software

---

Vertical Reference: Ellipsoidal vs. Orthometric Heights

RTKdata provides ellipsoidal heights (height above the WGS84 ellipsoid). These are NOT the same as:

• Mean Sea Level (MSL)

• Orthometric height

• Local vertical datum

To convert to orthometric heights, apply a geoid model in your software:

• EGM96 / EGM2008 — global geoid models

• OSGM15 — UK geoid model

• GEOID18 — US geoid model

• AUSGeoid2020 — Australian geoid model

Most surveying software and GIS applications can apply geoid corrections automatically when the correct vertical datum is selected.

---

US Survey Feet vs. International Feet

If you see a steady ~4 ft / 1.2 m offset in the United States, check your controller's unit settings. The difference between US Survey Feet and International Feet causes a noticeable shift at scale.

Fix: Set your controller/software project to use the correct foot definition for your state.