Smoothing

-smooth generalizes polygon and polyline geometry by replacing the original vertices with smooth curves. Unlike -simplify, which thins existing vertices, -smooth recomputes the line's shape. It also filters out small intricate details and preserves sharp corners, such as surveyed borders.

mapshaper coastline.shp -smooth 5km -o smoothed.shp

The <distance> parameter sets the smoothing resolution: detail finer than this scale is removed.

Usage tip

For best geographical fidelity, apply -smooth to unsimplified linework. Smoothing can also be applied to simplified paths, but low-resolution inputs give the smoother less information to work with.

Smoothed low-resolution contour lines:

Prefiltering intricate detail

When line smoothing is applied to small, intricate features — jetties, narrow inlets, docks, spikes — the output can include kinks or self-intersections. To prevent this, -smooth first removes detail below the smoothing scale.

With the prefilter, the small basin, dock outline, and other fine detail are dropped before smoothing.

Without the prefilter, the same details pull the smoothed line into unwanted bends and loops.

Use no-prefilter to skip this step and smooth the input as-is.

Corner preservation

By default, -smooth protects long straight sections from being reshaped by smoothing.

Here, the straight lines are preserved and the riverine section is smoothed:

Use corner-bias= to adjust the sensitivity of corner detection, or no-corners to smooth the entire line uniformly.

# Round off corners instead of preserving them.
mapshaper borders.shp -smooth 5km no-corners -o smoothed.shp

Adaptive vertex density

Output vertices are placed adaptively: more on tight bends, fewer on straight or gently curving runs.

The dots below mark output vertices along a smoothed line.

The max-bend-angle= option (default 8 degrees) controls output density. A larger value uses fewer vertices; a smaller value uses more.

Technical notes

• The -smooth command uses a Gaussian smoothing kernel.
• Curve compensation reduces the shrinkage that smoothing causes around bends. The gain= option controls this correction.
• Lat-long data is smoothed on a sphere: coordinates are converted to 3D geocentric coordinates, distances are measured along great circles, and the result is converted back to longitude and latitude.

See the -smooth reference for the full set of options.