Feature Guide: Local Coordinates

Overview

OxTS navigation units (INS) can output measurements in a local coordinate system defined by the user. This system allows you to express position, velocity, and acceleration data relative to a nearby reference point instead of global latitude, longitude, and altitude.

⚠️ Important: The local coordinate system configured on an INS is independent of any local coordinate system configured in an RT-Range system. Changing one does not affect the other.

What Are Local Coordinates?

A local coordinate system defines a flat, user-specified frame of reference. It’s particularly useful when global coordinates are unnecessary or inconvenient for analysis.

Two parameters in the local reference frame (LRF) define the system:

1. Origin (zero point) – The latitude, longitude, and altitude of the origin.

   • Should be within a few miles of the test area for accuracy.

2. Orientation (axis directions) – The angle of the X-axis relative to true North.

   • The Y-axis is automatically perpendicular to X, pointing left.

   • The Z-axis points upward.

Setting Local Coordinates on the INS

Local coordinates can be configured using either NAVconfig or NAVdisplay.

Setting Local Coordinates in NAVconfig

1. Power the INS and connect to it.

2. Open NAVconfig and select the unit.

3. Navigate to Environment → Local Coordinates.

4. In the Local Coordinates section:

   • Enable Local Coordinates.

   • Enter the latitude, longitude, and altitude for the origin.

   • (Optional) Select Use current position to use the unit’s current location.

   • Enter the X-axis angle relative to true North:

     • 0° = X points North, Y points West.

     • 90° = X points East, Y points North.

5. Commit Configuration to the device.

   • The setting is stored permanently and will remain after power-off.

Setting Local Coordinates in NAVdisplay

1. Power and connect the device.

2. Open NAVdisplay and select the unit.

3. From the Utilities menu, select Quick Config → Local Coordinates.

4. If the INS is initialised:

   • Click Set Origin to use the current position as the local origin.

   • Drive approximately 100 m to define the X-axis, then click Set Axis.

5. The local coordinate system becomes active immediately.

   • To save it permanently, tick Save settings before defining the origin and axis.

   • Otherwise, the configuration will be cleared on restart.

Local Coordinates in Post-Processing

Even if local coordinates weren’t configured during testing, they can be added later in NAVsolve.

1. Open the .rd file in NAVsolve.

2. Go to the Process page → Local Coordinates.

3. Scroll to the desired point in the preview data and click Set origin here. The LRF file should populate.

   • When the file is processed, local coordinates will be present in the NCOM file

How Local Coordinates Are Calculated

The local coordinate frame is treated as a flat plane, which provides sufficient accuracy when used over a small area. Over larger distances, however, XY position errors will gradually increase due to the curvature of the Earth.

Enabling local coordinates adds an additional layer of processing on top of the core navigation solution that produces the standard NCOM outputs. This process recalculates all position and velocity measurements relative to the defined origin point to generate the corresponding local coordinate values.

Any configuration or setting that influences the standard output data will also be applied to the local coordinate outputs.