Shared map borders stay intact when parcels are stored as a planar partition

A developer building a web map editor found that moving a corner of one land parcel often opened a gap along the border it shared with a neighbor. The cause is that most editors store each polygon as an independent ring, so the shared edge exists twice and nothing keeps the copies in sync. The author instead proposes storing each border once as an edge, with faces on either side, which keeps adjacent shapes consistent. The model means that dragging a shared corner moves both faces and drawing an outline across parcels splits them. The approach is less suitable where independent, overlapping shapes are needed, and the post includes TypeScript example code using the vectorstream library the author maintains.