esriwkt

Parse ESRI-flavored WKT (as produced by ArcGIS .prj exports) and convert it to an EPSG code, standard OGC WKT, or proj4 string.

Only new-style pseudo-OGC WKT (PROJCS[...] / GEOGCS[...]) is supported; the legacy Arc/Info 7.x key-value .prj format is not.

This library bundles a subset of EPSG entries for matching and 316 datum transforms from PROJ data. It works entirely offline. WKT and proj4 rebuilding works for any parseable ESRI WKT, regardless of whether an EPSG match exists.

Table of contents

• Installation
• Usage
• API
• How EPSG matching works
• How WKT/proj4 rebuilding works
• Limitations

Installation

npm install esriwkt

Usage

import { parseEsriWkt } from "esriwkt";

const wkt =
  'PROJCS["ETRS_1989_UTM_Zone_32N",GEOGCS["GCS_ETRS_1989",DATUM["D_ETRS_1989",' +
  'SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],' +
  'UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],' +
  'PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",0.0],' +
  'PARAMETER["Central_Meridian",9.0],PARAMETER["Scale_Factor",0.9996],' +
  'PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]';

const crs = parseEsriWkt(wkt);

crs.toEpsg(); // 25832
crs.toWkt(); // OGC WKT: PROJCS["ETRS89 / UTM zone 32N",GEOGCS["ETRS89",...AUTHORITY["EPSG","25832"]]
crs.toProj4(); // +proj=utm +zone=32 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs +type=crs

parseEsriWkt() returns an EsriWkt object. Each conversion method (toEpsg(), toWkt(), toProj4()) is lazy and caches its result. The params property exposes the normalized parameters directly.

API

parseEsriWkt(wkt: string): EsriWkt

Parses an ESRI WKT string. Throws if the WKT cannot be parsed.

EsriWkt

Member	Return type	Description
params	NormalizedCrsParams	The normalized parameters extracted from the WKT (angles in radians, distances in meters).
toEpsg()	number | null	Returns the matched EPSG code, or null if no match was found.
toWkt()	string | null	Returns the rebuilt OGC WKT1 string, or null if the projection is unrecognized.
toProj4()	string | null	Returns the rebuilt proj4 string, or null if the projection is unrecognized.

All three conversion methods are lazy and cache their result, so repeated calls return the same value without re-computation.

How EPSG matching works

1. If the WKT has an embedded AUTHORITY["EPSG", code], that code is returned directly.
2. If it has an embedded AUTHORITY["Esri", wkid], a crosswalk table maps known legacy ESRI WKIDs (e.g. 102100 -> 3857) to EPSG codes.
3. Otherwise, the WKT is parsed into normalized parameters (projection method, ellipsoid, origin, false easting/northing, scale factor, etc.) and compared against the bundled dataset within numeric tolerances. Geographic CRSs that share identical ellipsoid parameters (e.g. NAD83, ETRS89, GDA94 all use GRS80) are disambiguated by datum-name similarity.

How WKT/proj4 rebuilding works

The parsed parameters are re-serialized using small lookup tables that map ESRI naming conventions to their OGC WKT and proj4 equivalents:

• Projection names: ESRI names like Lambert_Conformal_Conic -> OGC Lambert_Conformal_Conic_2SP / proj4 lcc.
• Parameter names: ESRI names like False_Easting -> OGC false_easting / proj4 x_0.
• Ellipsoid names: proj4 codes like GRS80 -> OGC GRS 1980.
• Datum transforms: 316 +towgs84= entries auto-generated from PROJ data, plus manual entries for grid-based datums like NAD27.
• Prime meridians: offsets like 2.337° -> PRIMEM["Paris",...] / +pm=paris.

Angles are converted from radians (internal) to degrees (output). Linear false easting/northing values are kept in meters for proj4 (per proj4 convention) and converted to the CRS linear unit for WKT. When a datum transform is unknown, the datum-shift clause is omitted rather than guessed.

Non-Greenwich prime meridians are serialized as PRIMEM["Paris",...] in WKT and +pm=paris in proj4. EXTENSION["PROJ4_GRIDS",...] clauses from the source WKT are preserved and preferred over the built-in nadgrids value. Oblique Mercator (RSO) projections emit +gamma= for the rectified grid angle and +no_uoff in proj4. Mercator 1SP vs 2SP is distinguished by the presence of a non-zero standard parallel.

When no EPSG match is found, projected CRS names are canonicalized from ESRI naming patterns (e.g. WGS_1984_UTM_Zone_33S -> WGS 84 / UTM zone 33S).

Limitations

EPSG matching coverage depends on the bundled dataset

toEpsg() can only return codes for CRSs in the bundled dataset (or those carrying an embedded AUTHORITY clause). The dataset is a small curated subset, so some CRSs may not resolve to an EPSG code even though toWkt() and toProj4() will still produce output.

Grid-based datum shifts are limited

The library includes 316 +towgs84= datum transforms auto-generated from PROJ data, plus a handful of grid-based (+nadgrids=) entries for common datums like NAD27. Datums that require a grid file for accurate transformation (e.g. OSGB36 with OSTN15, NAD83 CSRS) will produce valid proj4 output but without the grid reference, so datum transformation may be less accurate.

No WKT2 output

Only WKT1 (PROJCS[...] / GEOGCS[...]) is produced. The newer WKT2 format (PROJCRS[...] / GEODCRS[...]) is not supported.

Datum transforms are not region-specific

Some datums have multiple regional transforms to WGS84 (e.g. ED50 has 38 variants for different countries). The library picks a single general-purpose transform for each datum. This is accurate to a few metres but not optimal for a specific region.

NAD27 grid reference may differ from epsg.io

The library uses a chain of optional NTv2 grid files (@ntv2_0.gsb,@conus,@alaska), while epsg.io uses a single grid file. Both are valid, but the output may not match epsg.io exactly for NAD27-based CRSs.

Unmatched CRS names are best-effort

When toEpsg() cannot find a match, toWkt() attempts to derive a canonical name from the ESRI naming pattern (e.g. WGS_1984_UTM_Zone_33S becomes WGS 84 / UTM zone 33S). Complex or non-standard names will pass through unchanged.

Prime meridian coverage is limited

Ten prime meridians are recognized (Greenwich, Paris, Madrid, Lisbon, Rome, Jakarta, Bern, Stockholm, Athens, Oslo). Unrecognized meridians are emitted with a numeric offset from Greenwich rather than a named reference.