Topographic isolation (also called radius of dominance or simply isolation) measures how far a mountain peak stands from higher ground. It is defined as the shortest horizontal distance from the summit to any point of equal or greater elevation. In other words, the isolation of a peak is the radius of the circle within which the peak remains the highest point. This metric complements topographic prominence, which measures how much a peak rises above its surroundings. Prominence is a vertical measurement (height above the lowest connecting saddle), while isolation is horizontal.
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How isolation is calculated
The OpenTopoMap project computes isolation values using digital elevation models (DEMs). The isolation algorithm first searches for nearby higher peaks from a list of summits and uses these to obtain an initial estimate of the isolation distance. It then refines this estimate by scanning the DEM directly to find the nearest cell whose elevation matches or exceeds the summit’s height. The resulting distance is stored as otm_isolation in the database
Why isolation matters in cartography
On maps that show many mountains, it is important to avoid clutter. Using only elevation to decide which peaks to label can produce confusing maps: a moderately high hill in a flat area may be more visually important than a higher peak surrounded by taller mountains. Isolation provides a natural measure of a summit’s dominance. Peaks with large isolation values rise alone over a broad area and should stand out on the map; peaks with small isolation values are surrounded by higher mountains and can be left unlabeled at small scales.
OpenTopoMap’s rendering rules use the otm_isolation attribute to decide which peaks to draw or label at different zoom levels. For example, the style file for peak labels (text-peaks.xml) shows names only for peaks whose isolation exceeds 50 km at low zoom levels; as the user zooms in, the isolation threshold drops (30 km, 10 km, 5 km, etc.), eventually labelling all peaks at large scales. Similarly, the symbol file (symbols-peaks.xml) draws a peak icon for very isolated summits at small scales (isolation ≥ 75 km) and gradually includes less isolated peaks at higher zoom levels. This approach ensures that the most dominant summits appear first while keeping the map readable.
Illustration
The diagram below contrasts topographic prominence (vertical) with isolation (horizontal). Point B is a summit; its isolation is the horizontal distance to the nearest higher point A. The prominence is the vertical drop from B to the lowest contour connecting it to a higher peak (point C). Isolation distances govern when peaks appear on the map, while prominence helps classify their height relative to surroundings.
By Original: Andrew pmk Vector: Giorgio136 – This file was derived from: Topographic isolation and prominence.jpg: , CC BY-SA 3.0, Link
Conclusion
Topographic isolation is a simple yet powerful concept for understanding the geographical dominance of mountains. By combining isolation with prominence, cartographers can produce maps that highlight significant summits at appropriate scales. OpenTopoMap uses isolation to filter and rank peaks, ensuring that the map shows the most important mountains without overwhelming the reader with too many labels.