Tomstrails GPS Duo is a free iPhone app developed by John Spinks, designed to provide users with a clear and concise display of their position in map coordinates. It offers various grid formats, including British Grid, Irish Grid, MGRS (Military Grid Reference System), UTM, and Latitude & Longitude in decimal and sexagesimal notation, making it versatile and widely applicable.

One of the standout features of Tomstrails GPS Duo is its ability to optimize battery life. The app intelligently adjusts the frequency of position updates based on battery charge and speed of travel. This ensures that users can rely on their phone's battery throughout multi-day trips, without sacrificing the accuracy of their positioning. It's worth noting that the GPS is never enabled when the app is running in the background, further conserving battery life.

The app's display is crisp and clear, with a high contrast design that allows for easy reading of the position in any light condition. This is particularly useful in emergency situations, as the simultaneous display of multiple grid formats enables ground and aerial support coordination by emergency services.

Tomstrails GPS Duo is a reliable and user-friendly app that provides accurate positioning and extended battery life. Whether you're a traveler, adventurer, or emergency responder, this app can be a valuable tool in navigating and coordinating your activities.