HOW TO USE
1. Export your alignment from QGIS as KML (WGS84) — LineString geometry.
2. Load the .kml or .kmz file here.
3. Enter the beginning station (e.g. 125+00).
4. Tap START — iPhone GPS updates station & offset in real time.

OFFSET SIGN: LT = left of stationing direction  |  RT = right.