A side selective line simplification algorithm for nautical chart generalization

Depth contours are essential chart features, portraying seabed morphology and delineating the depth areas used by navigation systems to assess route safety and trigger alarms. Their portrayal must simultaneously satisfy operational safety and visual clarity. For navigational safety, contours must be displaced only toward the adjacent deeper-depth area, ensuring that charted depths never appear deeper than the source bathymetry. Contours must also remain easily interpretable and adhere to Electronic Navigational Chart data constraints, such as the minimum length of line segments. Despite numerous efforts, existing methods suffer from limitations, including parameters difficult for cartographers to interpret, unnatural shapes, uncontrolled contour displacement, and increased vertex density. This paper introduces a side-selective line-simplification method that enforces the safety constraint and controls line displacement, segment length, and bend geometry using parameters expressed in millimeters at the target scale, to produce visually coherent and application-compliant contours. Tests conducted on six testbeds demonstrate a reduction in geometric complexity while preserving the structural form of contours. Comparisons with representative alternatives highlight the method’s advantages in reducing data volume and producing scale-appropriate, consistent outputs. The side-selective method forms an essential building block toward automated workflows for the cartographic generalization of depth contours and other relevant chart features.