How to use
- areas.geojson
Open a GeoJSON file
- Map (no map tiles)Flags swapped lat/lng
Show on a map and validate
- areas.kmlpoints.csv (points only)
Convert to another format
Paste GeoJSON or other text into “Input”, or drop a file (or use “Choose file”). “Try the sample” shows a result straight away.
In “Preview”, the coordinates are drawn in Web Mercator. Drag or use the arrow keys to move, and + / − to zoom. Select a feature on the map or in the list to see its properties, area and length.
In “Check”, the problems against RFC 7946 are listed as errors and warnings. Swapped longitude and latitude and open rings are found there.
In “Convert”, choose GeoJSON, KML, GPX, CSV or WKT. You can reduce the decimal places and drop properties, then save the file.
In “Measure”, the area, length and centre of the whole file and of the selected feature are calculated on a sphere.
What it does
Checks against RFC 7946
Finds swapped [longitude, latitude] pairs, out-of-range values, open rings, ring winding, self-intersection, bbox and the old crs member, and points to the feature number.
Plain preview without map tiles
The background is plain and the coordinates are drawn as SVG in Web Mercator. No map image is fetched, so the preview causes no network traffic. A link opens GSI Maps or OpenStreetMap only when you click it.
Six input formats and five output formats
Reads GeoJSON, TopoJSON (basic), KML, GPX, CSV (latitude and longitude columns) and WKT. Writes GeoJSON, KML, GPX (points and lines), CSV (points) and WKT.
Smaller files and closed rings
Round coordinates to 3–7 decimal places and drop properties before saving. Open rings are closed automatically for KML and WKT, and the count is shown.
Area, length and centre on a sphere
Uses a sphere of radius 6,371 km: polygon area (holes subtracted), line length, perimeter and the mean of vertices. Compared with ellipsoid results, the difference is at most about 1% for 1-degree cells (0.3% or less around Japan).
Use cases
- Check that a GeoJSON file draws correctly and that longitude and latitude were not swapped
- Turn latitude and longitude columns from a spreadsheet into KML or GPX for a map app or GPS device
- Export a list of points as WKT or CSV for a database or GIS software
- Get a quick rough area or route length (not a survey-grade value)
Supported formats
Input
Output
Capability matrix
| Feature | Support | Notes |
|---|---|---|
| GeoJSON view, check and convert | Supported | — |
| RFC 7946 checks (order, range, rings, winding, self-intersection, bbox, crs) | Supported | Self-intersection is a planar approximation; very long rings are not checked |
| Reading KML, GPX, CSV and WKT | Partial | KML styles and images are ignored. GPX: waypoints, routes and tracks. CSV needs latitude and longitude columns |
| Writing KML, GPX, CSV and WKT | Partial | GPX takes points and lines; CSV takes points only. Other shapes are left out and counted |
| Reading TopoJSON | Partial | Basic Topology only (arcs, transform and objects) |
| Area, length and centre (spherical approximation) | Partial | Treats the Earth as a sphere. Not for survey or land-registry areas |
| Rounding coordinates and dropping properties | Supported | — |
| Showing map tiles | Not supported | No map images are fetched or shown. The background is plain |
| Reading shapefiles (.shp), KMZ or GeoPackage | Not supported | Zip and binary files cannot be read. Convert to GeoJSON or KML first |
| Altitude (third value) on output | Partial | Kept in KML and GeoJSON. GPX, CSV and WKT write latitude and longitude only |
| Coordinate system conversion (plane rectangular, UTM and so on to latitude and longitude) | Not supported | Input is treated as latitude and longitude (WGS 84). Use the coordinate tool for conversion |
| Resolving XML DTDs and external entities | Not supported | XML that contains a DTD or ENTITY declaration is refused for safety |
Limits & notes
- Up to 20 MB, 100,000 features and 2,000,000 vertices. Larger input is refused.
- The preview draws up to 300,000 vertices and 20,000 shapes. The list, checks, measurements and conversion cover every feature.
- Area and length assume a sphere of radius 6,371 km. Do not use them for surveying, land registration or boundary decisions.
- Self-intersection is checked in the plane (on the raw longitude and latitude values). Rings with more than 2,000 vertices are not checked.
- Input latitude and longitude are treated as WGS 84. Other coordinate systems are not converted.
Privacy & safety
Files you open and text you paste are processed only in your browser. Nothing is sent to or stored on a server. The GSI Maps and OpenStreetMap links open a URL with the centre coordinate only when you click them.
FAQ
What is the coordinate order in GeoJSON?
Can I view GeoJSON without map tiles?
Should I convert to KML or GPX?
How accurate is the area?
Are my files uploaded?
Can it read shapefiles (.shp)?
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