proj4js-augment.md

proj4js Wrapper with Authority Code Resolution

A thin wrapper around proj4 that accepts multiple CRS input formats and automatically resolves authority codes via spatialreference.org.

Supported Input Formats

Format	Example	Handling
Proj4 string	+proj=stere +lat_0=-90 ...	Native proj4js
WKT1	PROJCS["Antarctic",...]	Native proj4js
Authority code	EPSG:3031, ESRI:2181	Fetch from spatialreference.org

Authority Code Resolution

When input matches pattern <AUTH>:<CODE> (case-insensitive):

1. Check if already registered via proj4.defs
2. If not, fetch: https://spatialreference.org/ref/{auth}/{code}/ogcwkt/
3. Register with proj4.defs('{AUTH}:{CODE}', fetchedWKT)
4. Return transformer

// Example resolution
"EPSG:3031" → fetch https://spatialreference.org/ref/epsg/3031/ogcwkt/
"ESRI:2181" → fetch https://spatialreference.org/ref/esri/2181/ogcwkt/
"SR-ORG:7483" → fetch https://spatialreference.org/ref/sr-org/7483/ogcwkt/

// Returns WKT1 like:
// PROJCS["WGS 84 / Antarctic Polar Stereographic",...]

API

// Async wrapper - handles all formats
const proj = await resolveProj('EPSG:3031');
proj.forward([lon, lat]);  // → [x, y]

// Or transform directly
const [x, y] = await transform('EPSG:4326', 'EPSG:3031', [0, -71]);

Implementation Notes

• Cache fetched definitions in proj4.defs for reuse
• Return meaningful errors on fetch failure (404, network)
• Timeout after ~5 seconds
• Authority names are lowercased for URL: EPSG → epsg