Definition
Topmarks resolve ambiguity when buoy shape is unclear or colour is degraded by daylight, glare or fading. Standard forms are the can and cone for lateral marks, two cones for cardinals, two spheres for isolated danger, a single sphere for safe water, an X for special marks and an upright cross for emergency wreck marks. On pillar and spar buoys, where the body gives no shape information, the topmark is the primary daytime identification and should always be sought before committing.
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