Coordinate Reference Lab
Transform WGS 84 geographic positions and Web Mercator display coordinates with their coordinate systems made explicit. EPSG:4326 formally uses latitude then longitude axes, while RFC 7946 GeoJSON fixes longitude then latitude; choose the order of plain coordinate pairs. GeoJSON always enters as WGS 84 longitude/latitude. The tool reports the projected extent and numerical inverse check for every point. It does not perform a datum shift or certify positional accuracy.
Key features
- Two-way WGS 84 geographic and EPSG:3857 spherical Pseudo-Mercator transformation
- Explicit latitude/longitude or longitude/latitude input for plain geographic pairs
- Bounded RFC 7946 Point, MultiPoint, LineString, MultiLineString, Polygon and MultiPolygon input with properties retained
- Output CRS, axis, units, extent and per-position numerical round-trip check
- CSV coordinate rows and JSON report with projected geometry clearly marked as non-GeoJSON
- Out-of-range latitude, oversized input, malformed rings and unsupported geometry rejected
How to use
- Choose coordinate pairs or GeoJSON. GeoJSON always uses WGS 84 longitude then latitude.
- Set the source and target CRS; choose the input axis order for geographic coordinate pairs.
- Paste coordinates or load the worked Seoul example, then run the transformation.
- Inspect transformed rows, target extent, units and the numerical reverse-check value.
- Download CSV or the JSON report; do not treat projected geometry as RFC 7946 GeoJSON.
Use cases
- Prepare display positions for a Web Mercator map from geographic coordinates
- Check whether a copied pair was entered latitude-first or longitude-first
- Inspect the extent of a small GeoJSON feature collection without a remote map service
Frequently asked questions
Why can EPSG:4326 and GeoJSON have different axis order?
The formal EPSG:4326 axis order is latitude, longitude. RFC 7946 GeoJSON coordinates use WGS 84 longitude, latitude (OGC CRS84). The pair input offers either order explicitly; GeoJSON is always longitude, latitude.
Why is latitude limited near 85.0511°?
The map-compatible square Web Mercator world ends near ±85.05112878°. The projection is singular at the poles. Values outside the map-compatible range are rejected rather than silently clipped.
Is the EPSG:3857 output valid GeoJSON?
No. RFC 7946 GeoJSON uses WGS 84 geographic longitude/latitude. A projected geometry appears only inside a JSON report with explicit EPSG:3857 metadata; do not rename that report to GeoJSON.
Does a small round-trip error prove positional accuracy?
No. It measures only numerical inversion of these formulas. Source measurement accuracy, datum realization and epoch, grid shifts, ellipsoidal distance distortion and survey requirements are outside this tool.
Which GeoJSON shapes are accepted?
Two-dimensional Point, MultiPoint, LineString, MultiLineString, Polygon and MultiPolygon geometries, optionally in a Feature or FeatureCollection, are supported. Other geometry types, third coordinates, extra fields and malformed polygon rings are rejected to avoid silent data loss.
Does this convert to UTM or local national grids?
No. Only EPSG:4326 WGS 84 geographic coordinates and EPSG:3857 Pseudo-Mercator are implemented. A UTM zone or a local datum cannot be inferred from these two codes.
Privacy
All coordinates and GeoJSON remain in this browser tab. The calculation and optional CSV/JSON download happen locally; the tool does not upload input to its server API.
Comments & questions