TopoJson Class

Each object of the topology is a collection of features: a GeometryCollection object gives a feature for each of its geometries, with their id and properties, and another object gives one feature. A GeometryCollection inside a feature becomes a GeoGeometryCollection, and a geometry whose type is null becomes a feature without geometry (or is skipped inside a GeoGeometryCollection).

The reader stitches the arcs, reversing those referenced by a negative index (~i), applies the transform of a quantized topology to the arcs (delta-encoded) and to the positions of the points (not delta-encoded), and keeps a third value of a position, which is never transformed, as the altitude. It ignores the bbox, the foreign members, and the id and properties of a GeometryCollection object. The properties are a String, a Double, a Boolean, null, or a JsonElement for an object or an array. A numeric id becomes its JSON text.

The writer writes each collection as a GeometryCollection object, and finds the arcs as topojson-server does: the lines and rings are cut at the junctions, the positions where they meet or part, and the identical arcs, in either direction, are written once. The rings are also cut at their first position, so they are read back starting at the same position. It closes the rings that are not, writes the altitudes as a third value (which GDAL does not read), the identifiers as strings, and the properties as GeoJson does. Without TopoJsonWriterOptions.Quantization, the features are read back as they were written; with it, the positions are rounded to a grid and the arcs delta-encoded.