Coordinate Systems Overview: How Locations Become Numbers
The four families of coordinate system — geographic, projected, grid-based, code-based — per ISO 19111, with EPSG codes and the WGS-84 vs NAD83 datum split explained.
By Steve K.. Published . Last updated .
A coordinate system is the set of rules — axes, units, origin — for naming a location with numbers. The four families are geographic, projected, grid-based, and code-based; ISO 19111 calls the combination of a coordinate system plus a datum a Coordinate Reference System (CRS).
The Empire State Building has different numerical coordinates in every coordinate system, but it occupies exactly one point on Earth. This article surveys the four families of coordinate system, distinguishes coordinate systems from datums (the most common source of sub-metre coordinate bugs), runs the Empire State Building through each family, and covers the EPSG-code conventions that name a full coordinate reference system unambiguously. The companion pillar /learn/what-is-latitude-and-longitude covers the geographic family in depth; /learn/what-is-a-geodetic-datum covers the datum side of the ISO 19111 split.
The Empire State Building in five coordinate systems
The same point in five common systems — all referenced to WGS-84 unless otherwise stated:
| Family | Empire State Building | Units |
|---|---|---|
| Geographic (EPSG:4326, WGS-84) | 40.7484° N, 73.9857° W | Degrees |
| Projected (EPSG:3857, Web Mercator) | 8,237,642 mE, 4,975,074 mN | Metres on sphere |
| UTM (EPSG:32618, WGS-84 zone 18N) | 18T 585628 mE 4511322 mN | Metres in zone |
| MGRS | 18T WL 85628 11322 | Zone + grid + metres |
| Plus Code | 87G8P2X7+9P | Open Location Code |
None of these is “the” coordinate of the building. The location is physical; the numbers are conventions. Picking the right system is the first design decision of every coordinate-handling workflow — web maps default to geographic+Web Mercator, surveyors default to local projected, NATO defaults to MGRS, address-poor regions default to Plus Codes.
The ISO 19111 split: CRS = CS + Datum
The international standard for talking about coordinate systems formally is ISO 19111:2019, "Referencing by coordinates." It splits the concept into three layers that are easy to confuse.
| Layer | What it specifies | Example |
|---|---|---|
| Coordinate System (CS) | Axes, units, axis order | "Latitude N−S, Longitude E−W, degrees, lat-first" |
| Datum | Reference frame: ellipsoid + orientation | WGS-84 (a = 6,378,137 m; 1/f = 298.257223563) |
| Coordinate Reference System (CRS) | CS + Datum together | EPSG:4326 (geographic WGS-84) |
A bare lat/lon pair without a datum is incomplete. 40.7484, −73.9857
is meaningful only when the reader knows the datum — almost always
WGS-84 in modern web work, but NAD83 in US federal data and ETRS89 in
European data. The EPSG registry names full CRSs
with numeric codes; software libraries (PROJ, GDAL, PostGIS, MapLibre)
all consume those codes directly. When two systems exchange coordinates
by EPSG code, the silent-CRS-mismatch bug class disappears.
Family 1: Geographic coordinate systems
Geographic coordinate systems locate points using latitude and longitude as angles on a reference ellipsoid. The dominant choice is WGS-84 (EPSG:4326), used by GPS, GeoJSON, every web mapping API and virtually every global database.
| EPSG | CRS name | Datum | Where used |
|---|---|---|---|
| 4326 | WGS-84 geographic | WGS-84 | Universal default; GPS, GeoJSON, web APIs |
| 4269 | NAD83 geographic | NAD83 | US federal data, USGS, USDA |
| 4258 | ETRS89 geographic | ETRS89 | Europe (INSPIRE directive) |
| 4612 | JGD2000 geographic | JGD2000 | Japan (post-2002) |
| 4283 | GDA94 geographic | GDA94 | Australia |
| 4267 | NAD27 geographic | NAD27 | Legacy US data (pre-1986) |
Geographic systems are the universal lingua franca of coordinate exchange — any modern web mapping library, GeoJSON file, GPS device or global database stores or transmits coordinates in geographic form, almost always WGS-84. They are not the right choice when you need to compute distances or areas directly from coordinates — angular coordinates require Vincenty's formula or equivalent geodesy. The /learn/what-is-latitude-and-longitude pillar covers the mechanics in depth.
Family 2: Projected coordinate systems
Projected systems map the curved Earth onto a plane through a map projection, then measure locations in linear units — metres, US Survey feet, or international feet — on that plane. Coordinates take the form (easting, northing) from a defined origin.
| EPSG | CRS name | Datum | Coverage / use |
|---|---|---|---|
| 3857 | Web Mercator | WGS-84 (sphere variant) | Slippy-map tiles (Mapbox, Google, OSM) |
| 32601-32660 | UTM Zone 1N-60N | WGS-84 | Northern UTM zones |
| 32701-32760 | UTM Zone 1S-60S | WGS-84 | Southern UTM zones |
| 27700 | British National Grid | OSGB36 | UK Ordnance Survey |
| 2154 | RGF93 / Lambert-93 | RGF93 | France national mapping |
| 3035 | ETRS89-extended / LAEA Europe | ETRS89 | European statistical mapping |
| 28992 | RD New / Amersfoort | Amersfoort | Netherlands cadastre |
| 2056 | CH1903+ / LV95 | CH1903+ | Switzerland cadastre |
Projected systems are the right choice for surveying within a region, for engineering and infrastructure work that needs direct distance and area measurement, and for printed maps where rectangular grids align with the map margins. They are not the right choice for global data exchange — a single projected system either covers a region with acceptable distortion or covers the whole world with substantial distortion, but not both at once.
Family 3: Grid-based coordinate systems
Grid-based systems overlay a projected system with regional labels — zone identifiers, grid-square letters, and digit counts — to produce shorter, voice-friendlier strings.
| Grid system | Zone structure | Cell size | Used by |
|---|---|---|---|
| UTM | 60 zones × 6° longitude | 6° × 8° (zone-band) | Surveying, topographic mapping |
| MGRS | UTM + 100 km lettered squares + digit-count precision | 100 km / 10 km / 1 km / 100 m / 10 m / 1 m | NATO military, US SAR, disaster response |
| US State Plane | One or more zones per US state | Subset of state | US civil surveying, deeds |
| British National Grid | 100 km squares (A-Z, with I excluded) | 100 km / 10 km / 1 km / 100 m | UK Ordnance Survey |
UTM is half projected and half grid: 18T 585628 mE 4511322 mN names
the zone (18T) followed by easting and northing in metres on the
zone's transverse Mercator projection. MGRS adds a 100-km
grid-square letter pair: 18T WL 85628 11322 names the same point with
a more compact string — the leading 18T WL identifies a 100-km
square within the zone, and the remaining digits give the position
within the square. The
/learn/utm-coordinate-system pillar
covers UTM and MGRS in depth.
Family 4: Code-based coordinate systems
Code-based systems encode a small region as a short alphanumeric string. The encoding is reversible: given the code, software can decode the rectangular cell it represents.
| Code system | Native cell at "typical" length | Alphabet | Designed for |
|---|---|---|---|
| Plus Codes (10-char + 1 + 1) | ~2.8 m × 3.5 m | 20-char (avoids 0,O,I,L,1) | Address-poor regions, Google Maps |
| what3words (3 words) | 3 m × 3 m | ~40,000-word dictionary | Delivery, emergency services |
| Geohash (8 chars) | ~38 m × 19 m at equator | 32-char (base-32) | Spatial indexing, Redis GEO |
| Maidenhead (6 chars) | ~9.3 km × 4.6 km | Letter-digit alternation | Amateur radio |
| Quadkey (zoom-level dependent) | One tile at the chosen zoom | 0/1/2/3 (base-4) | Bing Maps tile addressing |
Code-based systems trade precision for shareability: a Plus Code is readable in 10 characters; a what3words address is three English words. Geohash is unusual in the family because it is hierarchical — prefixes of geohashes represent containing cells, which makes geohash fast for proximity search in databases. None of these systems is suitable as a primary geodetic store; they all decode to and from geographic coordinates in WGS-84.
Conversion between systems
Conversion takes one of three forms, with different cost models:
| Conversion type | Same datum? | Same CRS? | Cost |
|---|---|---|---|
| Coordinate-format conversion (e.g. DD ↔ DMS) | yes | yes | Closed-form, lossless arithmetic |
| Coordinate-system change (e.g. WGS-84 ↔ Web Mercator) | yes | no | Forward map projection (closed-form per projection) |
| Datum transformation (e.g. WGS-84 ↔ NAD83(2011)) | no | no | Helmert 7-parameter transform + grid shifts |
The first kind is lossless. The second kind is lossless within the projection's usable region but introduces structural distortion (shape, area, or distance trade-offs — see /learn/what-is-a-map-projection). The third kind has measurable accuracy bounds: WGS-84-to-NAD83 in CONUS shifts coordinates by 1-2 m on the ground per NGS NCAT documentation; the exact shift depends on the NAD83 epoch (NSRS2007 vs 2011 vs the in-flight NATRF2022 successor).
The PROJ library — used inside virtually every modern GIS, including PostGIS, GDAL, QGIS, MapLibre, OpenLayers, and Mapbox — handles all three conversion types given just the source EPSG code, the target EPSG code, and the input coordinate. The library is the de-facto standard for coordinate transformation; the EPSG codes are the parameters.
How file formats handle the CRS question
File formats vary in how they record the CRS — or whether they record it at all. This is responsible for a non-trivial share of coordinate-integration bugs.
| Format | CRS handling | Implication |
|---|---|---|
| GeoJSON (RFC 7946) | Fixed to WGS-84 geographic by spec | No CRS metadata; any other CRS is non-conformant |
| Shapefile | .prj sidecar with WKT description | Missing .prj ⇒ CRS is ambiguous |
| GeoPackage | Embedded srs_id (typically EPSG) per feature class | Full round-tripping |
| KML | Fixed to WGS-84 by spec | Mirrors GeoJSON |
| GeoTIFF | Embedded EPSG / WKT in TIFF tags | Self-describing |
| CSV with lat/lon columns | None unless added | Most common silent-CRS-mismatch source |
| WKT 1 & 2 (OGC) | Self-describing CRS string | Lossless if produced and consumed by ISO 19111 implementations |
The robust pattern is to attach the EPSG code (or a full WKT) to every coordinate dataset at the file or record level — even when the format does not require it — so future readers can transform the data through PROJ and trust the result.
When to use each family
| Use case | Pick this family | Specific CRS |
|---|---|---|
| Global web mapping, GeoJSON, public APIs, GPS exchange | Geographic | EPSG:4326 (WGS-84) |
| Web slippy-map tile rendering | Projected | EPSG:3857 (Web Mercator) |
| Regional surveying / engineering (USA) | Projected (State Plane) | NAD83 SPCS per state |
| Regional surveying / engineering (UK) | Projected (National Grid) | EPSG:27700 |
| Regional surveying / engineering (France) | Projected | EPSG:2154 (Lambert-93) |
| Military / SAR / disaster response | Grid (MGRS) | UTM + MGRS |
| Address-style human sharing | Code (Plus Codes / w3w) | Open Location Code |
| Spatial indexing / proximity search | Code (geohash) | Hierarchical base-32 |
A typical web mapping workflow stays in EPSG:4326 for transport and EPSG:3857 for tile rendering, transforming into local projected systems only on demand. A typical surveying workflow operates in the local projected system (or grid) for field work and exports to EPSG:4326 only for archive and exchange. Military and emergency-services workflows operate in MGRS for voice and printed communication, with geographic coordinates as the underlying storage.
Common misconceptions
Related
- What Is Latitude and Longitude?— The foundation pillar — angular coordinates
- Coordinate Formats Explained— The six notations for writing coordinates
- What Is a Geodetic Datum?— The reference frame coordinate systems attach to (when shipped)
- What Is a Map Projection?— The mathematics behind projected coordinate systems (when shipped)
- Methodology— How content is sourced and verified
Frequently asked questions
What is a coordinate system?
A coordinate system is the rules that assign numbers to a location on Earth. ISO 19111 defines a coordinate reference system (CRS) as the combination of a coordinate system (the mathematical rules) and a datum (the reference frame the rules attach to). The same point has different numbers in different coordinate systems — the Empire State Building is 40.7484°N, 73.9857°W in WGS84 geographic coordinates, 18N 583960E 4507523N in UTM, and 18TWL 8396 0752 in MGRS.
What is the difference between a coordinate system and a datum?
A datum is the reference frame: an ellipsoid (mathematical model of Earth) plus its orientation in space. A coordinate system is the rules for measuring positions relative to that datum. WGS84 is technically a datum; "WGS84 coordinates" is shorthand for "geographic coordinates referenced to the WGS84 datum." ISO 19111 keeps the two concepts separate. EPSG codes typically combine them — EPSG:4326 names the WGS84 geographic CRS, which is WGS84 datum plus geographic coordinate system.
What are the four main categories of coordinate system?
Geographic (latitude / longitude in degrees, referenced to an ellipsoid). Projected (eastings / northings in metres or feet, on a planar map projection). Grid-based (zoned grid systems like UTM and MGRS that overlay projected coordinates with regional labels). Code-based (Plus Codes, what3words, Geohash, quadkey — each encodes a small region as a short alphanumeric string).
Which coordinate system should I use?
Geographic (lat / lon) is the default for global software, web mapping, GeoJSON, and any program-to-program path. Projected (UTM or a local grid) is the right choice for surveying, navigation within a region, and any work that requires distance and area to be computed directly from coordinates. Grid-based systems (MGRS, the British National Grid, State Plane) are the convention in military, search-and-rescue, and government surveying. Code-based systems are good for human-shared, short-lived references where conciseness matters.
What is EPSG and why do coordinate systems have EPSG codes?
EPSG (originally the European Petroleum Survey Group, now maintained by IOGP) publishes a registry of standardised coordinate reference systems, each with a numeric code. EPSG:4326 is WGS84 geographic; EPSG:3857 is Web Mercator (the projection most web maps use). EPSG codes are the universal shorthand for naming a CRS unambiguously: every major GIS package, database, and mapping library reads them. When integrating data from multiple sources, agreeing on EPSG codes prevents the silent CRS mismatch that produces metre-scale alignment errors.
What is the difference between EPSG:4326 and EPSG:3857?
EPSG:4326 is WGS-84 geographic — coordinates expressed as latitude and longitude in degrees, on the WGS-84 ellipsoid. EPSG:3857 is Web Mercator — coordinates expressed as easting and northing in metres on a sphere of radius 6,378,137 m, projected via spherical Mercator. EPSG:4326 is the universal exchange format (GeoJSON, GPS, APIs); EPSG:3857 is what every web slippy-map tile uses for rendering because it makes tile arithmetic clean.
How do I convert between coordinate systems?
Use the PROJ library (open-source) or any modern GIS (PostGIS, GDAL, QGIS, MapLibre) given the source and target EPSG codes. Conversion between geographic and projected systems uses a forward map projection (closed-form formula). Conversion between datums uses a Helmert 7-parameter transformation or a grid-shift file. For US-specific needs, NGS NCAT is the authoritative service. Always store and exchange the EPSG code with the coordinates.
What is a CRS in GIS?
CRS stands for Coordinate Reference System — the combination of a coordinate system (the rules for axes, units, axis order) and a datum (the reference frame those rules attach to). Every coordinate has a CRS, whether explicitly named or implicit. The EPSG registry names each CRS with a numeric code (e.g. EPSG:4326 for WGS-84 geographic, EPSG:32618 for WGS-84 UTM zone 18N). Modern GIS software treats the CRS as a first-class property of every dataset.
Sources
- ISO — ISO 19111:2019 — Referencing by coordinates (CS + Datum = CRS) · https://www.iso.org/standard/74039.html · Accessed .
- IOGP / EPSG — EPSG Geodetic Parameter Dataset — canonical CRS registry · https://epsg.org/ · Accessed .
- NGA STND 0036 — WGS 84 datum specification — ellipsoid a, 1/f, GM, ω · https://earth-info.nga.mil/index.php?dir=wgs84 · Accessed .
- NOAA NGS — NCAT — datum transformations + WGS-84 vs NAD83 1-2 m CONUS offset · https://www.ngs.noaa.gov/NCAT/ · Accessed .
- NGA TM 8358.1 — Universal Grids — UTM/MGRS zone, band, scale-factor 0.9996 · https://earth-info.nga.mil/ · Accessed .
- IETF — RFC 7946 — GeoJSON CRS fixed to WGS-84 geographic · https://datatracker.ietf.org/doc/html/rfc7946 · Accessed .
- OGC — GeoPackage Encoding Standard 1.3 — embedded srs_id (EPSG) · https://www.geopackage.org/spec130/ · Accessed .
- PROJ — PROJ 9.x — open-source coordinate transformation library (ISO 19111 reference impl.) · https://proj.org/ · Accessed .
- USGS — Snyder J.P. (1987) Map Projections — A Working Manual (USGS Prof. Paper 1395) · https://pubs.usgs.gov/pp/1395/report.pdf · Accessed .
Cite this article
APA format:
Steve K. (2026). Coordinate Systems Overview: How Locations Become Numbers. Coordinately. https://coordinately.org/learn/coordinate-systems-overview
BibTeX:
@misc{coordinately_coordinatesystemsoverview_2026,
author = {K., Steve},
title = {Coordinate Systems Overview: How Locations Become Numbers},
year = {2026},
publisher = {Coordinately},
url = {https://coordinately.org/learn/coordinate-systems-overview},
note = {Accessed: 2026-06-05}
}