seadistance

Seadistance is a general term used in maritime contexts to describe the distance over the sea between two geographic points or between a vessel and a reference feature such as a coastline, waypoint, or another ship. The concept is central to navigation, collision avoidance, voyage planning, and search and rescue operations. In practice, seadistance is expressed in units such as nautical miles or kilometers, with one nautical mile equal to 1.852 kilometers. The choice of unit depends on the standards of the operating domain and the tools used.

Distances can be determined from coordinates derived from GNSS, radar range measurements, or automatic identification system (AIS) data. For routing and safety calculations, great-circle or rhumb-line distances between coordinates are commonly computed, though local chart projections may affect small differences near the coastline.

Seadistance is used in route planning to estimate travel time, fuel, and exposure to hazards; in collision avoidance to maintain safe separation, in anchorage planning, and in the distribution of search patterns in search-and-rescue operations.

Limitations: Accuracy depends on geolocation precision, update rate of AIS and radar, and environmental conditions that affect radar performance. In crowded waters, propagating errors could be significant, and chart projections or tide and current effects may introduce additional discrepancies.

See also: nautical mile, great-circle distance, haversine formula, AIS, radar.