TopoJSON (Topology-Aware JSON)
Open BetaTopoJSON er en topologibevidst udvidelse af GeoJSON. Den bruger delt buekodning (grænser mellem polygoner gemmes kun én gang) og heltalskvantisering af koordinater — den resulterende fil er 60-80 % mindre end den tilsvarende GeoJSON. Standardinput til D3.js, Mapshaper og Observable; ideel til interaktive koroplet-kort og webatlas.
Vigtigste funktioner
Om formatet TopoJSON
TopoJSON was invented by Mike Bostock, the author of D3.js, to solve a simple problem: in GeoJSON neighboring polygons store their shared border twice. TopoJSON moves all the lines into a common array of arcs that the objects then refer to. On top of that, coordinates are rounded to an integer grid and written as increments — the file gets sharply lighter.
The format is taken for web graphics and analytics: choropleths in D3, interactive atlases, boundary maps in Observable, lightweight layers for offline applications. The second motive is simplification: during generalization the shared borders move together, and no gaps appear between neighboring districts.
The output is a single document: every layer becomes a separate object inside it — roads, buildings, water and so on — and object attributes are preserved. Coordinates are in WGS84, up to 100,000 objects per layer. The topology is built by our own code, with no external dependencies.
The price of compactness is the rounding of coordinates. The grid step depends on the size of the area: for a district it is on the order of a meter, for a country hundreds of meters. For a web map that is invisible; for precise measurements take GeoJSON or GeoPackage.
Hvilket program åbner TopoJSON-filer
Følgende programmer og applikationer kan åbne, redigere og arbejde med TopoJSON (Topology-Aware JSON)-filer (.topojson), der er eksporteret fra OSM2CDR:
Format specifications
- Open standard
- 2D only
- Lossless (no quality loss)
- Stores feature attributes
- Output: 1 file
- TopoJSON (open spec). Topologically encoded GeoJSON: shared borders stored once, file size 30-80% smaller. Used by D3.js.
Hvem bruger TopoJSON-kort
Sådan eksporterer du TopoJSON
Sådan åbner du filen efter eksporten
- D3.js — include topojson-client and unpack the layer you need: topojson.feature(topology, topology.objects.buildings), then the usual d3.geoPath.
- Mapshaper — drag the file onto mapshaper.org: objects and attributes are visible, and simplification and export to SHP or GeoJSON are right there.
- QGIS — opens .topojson as an ordinary vector layer through GDAL; handy for checking the geometry with your own eyes.
- Converting to GeoJSON — topo2geo buildings=buildings.geojson < map.topojson from the topojson-client package, or ogr2ogr.
- Leaflet and MapLibre — they do not understand the format directly: unpack it into GeoJSON on the client with the topojson-client library and feed it on as usual.
Populære kort i TopoJSON
Få korteksport i TopoJSON (Topology-Aware JSON)
Download professionelle TopoJSON-kort fra OpenStreetMap. Øjeblikkelig generering, ethvert område.