Understanding the Zone Boundary

The activation zone shown is the OS Terrain 50 contour line that sits immediately above the 25 m threshold — so the displayed boundary is always slightly inside the true SOTA zone, giving a small built-in safety margin.

Why the boundary may look unusual

On summits with a subsidiary top close by — such as Crib y Ddysgl near Snowdon — only the contour segment that directly encloses the summit is shown. You cannot count a position on an adjacent peak as part of the activation zone if the ridge dips below the 25 m threshold between the two tops, even if that second peak is itself high enough.

On very broad plateaus the zone may appear large, reflecting the gentle terrain. On knife-edge ridges it will be tight and elongated.

Data Sources

United Kingdom only
This tool uses Ordnance Survey mapping and data, which covers Great Britain and Northern Ireland only. It is designed for UK SOTA associations: GW (Wales), G (England), GM (Scotland), GI (Northern Ireland), GD (Isle of Man) and GJ (Jersey). It is not suitable for summits outside the United Kingdom.
About the data
Zone boundaries are derived from official Ordnance Survey data and are as accurate as the underlying dataset allows. Always use your own judgement on the hill — GPS, altimeter, and map reading skills remain essential.
Data Source Notes
Map tiles OS DataHub — Leisure 1:25,000 Full Ordnance Survey mapping including contours, paths and spot heights.
Activation zones (GB) OS Terrain 50 Contours © Crown copyright & database rights 2025 10 m interval contour polylines. Pre-computed for 1,046 active GB summits. Free Open Data product.
Activation zones (fallback) activation.zone — N6ARA Used automatically for any summit not in the local OS dataset. With thanks to N6ARA for providing this excellent free public API, which uses high-quality DEM data to calculate precise activation zone polygons for summits worldwide.
Summit data SOTA Database via sotl.as Summit coordinates, altitudes and points. Updated periodically from the official SOTA database.

Limitations

Daily search limit. To protect the OS map tile service, searches are limited to 50 per day. This resets at midnight. Zone data is cached indefinitely, so repeat searches for the same summit do not count against the limit.

This tool is for planning only. Always verify your operating position using your GPS device, altimeter watch or careful map reading. The zone boundary shown is a guide, not a substitute for your own assessment on the ground.