Datums and coordinate systems in surveying: WGS84, UTM, state plane and transformations
Key takeaways
- A datum defines the size, shape, origin and orientation of the reference ellipsoid; a coordinate system defines how positions are expressed on it.
- GNSS receivers compute positions in a global frame such as WGS84 or ITRF; project data is often in a national datum like NAD83 or ETRS89.
- In the conterminous US, WGS84 and NAD83(2011) positions differ by roughly one to two metres.
- UTM divides the world into 60 zones 6° wide, with a central meridian scale factor of 0.9996.
- Ellipsoidal height from GNSS must be converted with a geoid model to give orthometric height above the vertical datum.
What is a coordinate system and what is the datum?
A coordinate system is a set of rules for describing a position with numbers, such as latitude and longitude in degrees or easting and northing in metres. A datum is the reference those numbers are measured from: an ellipsoid of defined size and shape, fixed to the Earth by an origin and orientation. Together they form a coordinate reference system (CRS).
A simple datum definition: the datum tells you where zero is and what shape the Earth is assumed to be. The same physical point has different coordinates on different datums, sometimes by more than 100 m with older local datums. That is why "what is the datum?" is the first question to ask about any coordinate list, drawing or GIS layer.
In GIS, the CRS is often identified by an EPSG code, for example EPSG:4326 for WGS84 latitude and longitude. A coordinate system in GIS without a stated datum and EPSG code is a common source of shifted layers. For the wider GIS context, see our guide to geospatial data and GIS mapping.
What is a datum in surveying and GPS?
A datum in surveying is the reference framework, horizontal and vertical, to which all measured coordinates and heights on a project are tied. A datum in GPS is the global reference frame in which satellite orbits and therefore receiver positions are computed.
Horizontal (geodetic) datums
A geodetic system or geodetic datum uses a reference ellipsoid, such as GRS80 or the WGS84 ellipsoid, and a realisation through coordinates of control stations. Modern datums are Earth-centred and realised with GNSS and other space techniques. Older local datums were fitted to one region and can differ from global datums by hundreds of metres.
Vertical datums
A vertical datum, the reference datum for heights, approximates mean sea level, for example NAVD 88 in the United States. GNSS measures ellipsoidal height (h). To get orthometric height (H) you subtract the geoid height (N) from a geoid model: H = h − N. The geoid can be tens of metres above or below the ellipsoid, so using the wrong model or none is one of the costliest mistakes in GNSS levelling.
What is a datum point?
A datum point, in the survey sense, is a physical mark with published coordinates or height, such as a benchmark or control station, used to tie a project to the datum. Datum point meaning varies slightly in engineering drawings, where it can also be the reference point all dimensions are taken from.
Which datums and frames do surveyors use?
Surveyors mostly work in either a global frame (WGS84 or ITRF) or a national datum fixed to the local tectonic plate, and need to know the relationship between them.
| Datum / frame | Region | Ellipsoid | Notes |
|---|---|---|---|
| WGS84 | Global | WGS84 | GPS broadcast reference; recent realisations agree with ITRF at the centimetre level |
| ITRF (e.g. ITRF2020) | Global | GRS80 | Scientific frame; coordinates change with plate motion over time |
| NAD83 (2011) | United States | GRS80 | Fixed to North American plate; about 1–2 m from WGS84 in the conterminous US |
| ETRS89 | Europe | GRS80 | Fixed to Eurasian plate; drifts from ITRF by about 2.5 cm per year |
| GDA2020 | Australia | GRS80 | Replaced GDA94; continent moves about 7 cm per year |
| CGCS2000 | China | CGCS2000 | National geocentric system |
The WGS84 datum, or geodetic datum WGS84, uses an ellipsoid with semi-major axis 6,378,137 m and flattening 1/298.257223563, and is maintained by the US National Geospatial-Intelligence Agency. In the US, the National Geodetic Survey (geodesy.noaa.gov) is preparing new frames to replace NAD83 and NAVD 88, so check which datum and epoch each project specifies.
What are the different coordinate systems used in surveying and GIS?
The different coordinate systems fall into three families: geographic (latitude, longitude, height), geocentric (Earth-centred X, Y, Z) and projected grid systems (easting, northing), plus local site grids.
- Lat long coordinate system (geographic): angles on the ellipsoid, e.g. 31.2304° N, 121.4737° E. Universal, but degrees are not a convenient unit for distances and areas.
- Geocentric XYZ: spatial coordinates in metres from the Earth's centre; used internally by GNSS processing.
- Projected grid coordinate systems: a map projection turns the curved surface into a flat x y coordinate system in metres or feet. Examples are UTM, state plane, British National Grid and national transverse Mercator grids.
- Local or site grids: an arbitrary origin such as 1000, 5000, often tied to GNSS by a site calibration.
UTM
Universal Transverse Mercator divides the world into 60 zones, each 6° of longitude wide. Each zone has a central meridian scale factor of 0.9996 and a false easting of 500,000 m; southern hemisphere zones add a false northing of 10,000,000 m.
State plane
The State Plane Coordinate System divides the United States into more than 100 zones, using Lambert conformal conic projections for states that extend east to west and transverse Mercator for those that extend north to south. Zones are small, so distortion is low. Watch units: the US survey foot was officially deprecated at the end of 2022, and it differs from the international foot by 2 ppm, about 2 ft on a northing of 1,000,000 ft.
These coordinate system examples show why the coordinate system meaning on a drawing must always include the datum, projection, zone and units.
What is a grid coordinate vs a ground coordinate?
A grid coordinate lies on the flat map projection, while ground distances are measured on the Earth's surface at site elevation; the two differ by a combined scale factor that can reach hundreds of ppm.
The combined scale factor is the grid scale factor of the projection at that location multiplied by the elevation factor, which shrinks distances from site height down to the ellipsoid. At 1,000 m elevation the elevation factor alone is about 157 ppm, or 15.7 cm per kilometre. Total stations measure ground distances; GNSS gives grid coordinates. If you mix them without applying the scale factor, staked lines will not close.
| Situation | Recommended approach |
|---|---|
| Large or linear projects, GIS deliverables | Work on the official grid and apply scale factors to total station distances |
| Small building sites, high elevation | Low-distortion projection or scaled ground system documented in the job |
| Mixed GNSS and total station crews | One agreed system and scale factor in every controller |
How does coordinate transformation work?
A coordinate transformation converts coordinates from one datum to another, while a conversion of coordinates changes how a position is expressed within the same datum, such as converting latitude and longitude to UTM.
- Helmert 7-parameter transformation: three translations, three rotations and a scale. Time-dependent 14-parameter versions add rates for frames that move with plate tectonics.
- Grid-based transformations: published shift grids (for example NTv2 format files) that model irregular distortions in older national datums.
- Site calibration (localisation): a best-fit transformation computed in the field from GNSS measurements on known local control points.
Accuracy depends on the parameters. Published national transformations are often good to a few centimetres; generic WGS84 to local datum parameters may only be good to a metre or worse. When you convert datum on a project, record which transformation and version was used, so that the next crew gets the same result.
Converting x and y coordinates to latitude and longitude: step by step
Converting x and y coordinates to latitude and longitude requires knowing the exact projection, datum, zone and units of the grid; with those, any reliable coordinate conversion tool or field software performs an inverse projection.
- Identify the CRS. Find the datum, projection, zone and units on the drawing, in the metadata or from the EPSG code. Never guess the zone.
- Check units. Confirm metres, international feet or US survey feet.
- Choose the tool. Use survey or GIS software, or the national geodetic agency's online converter; a UTM zone converter or state plane converter is fine for single points if it states the datum.
- Convert xy to latitude longitude on the same datum first.
- Transform datum if needed, for example NAD83 to WGS84, using the agreed transformation.
- Check a known point. Plot it on imagery or compare it with a published control coordinate.
Coordinates read from consumer maps are only metre-level; for boundaries, see our guide on finding property lines with GPS. The same steps in reverse convert coordinates to x y. If the result is hundreds of metres off, the zone or datum is usually wrong; if it is a few metres off, suspect the datum transformation or units.
How to set up the coordinate system on a GNSS survey job
Set the coordinate system in the controller before the first point is measured, and make the base, rover and office software use exactly the same definition.
- Select the datum, projection and zone, or the project's EPSG code, and the geoid model for heights.
- Set up the base on a known control point using a tribrach with GNSS adapter to centre accurately, as described in our RTK base station setup guide, or connect to a CORS network in the correct frame, as explained in our NTRIP and CORS guide.
- Run a site calibration if the project uses a local grid, using at least four well-distributed control points, and check the residuals.
- Measure an independent check point before and after the work.
Our E1 GNSS RTK receiver works as radio base, radio rover or CORS/NTRIP rover, and the T8 Pro rugged GNSS tablet is an 8-inch Android 10 controller with a high-precision GNSS module, IP67 sealing and 4G for network corrections. For a complete set with the right accessories, request a quote.
Frequently asked questions
What is the datum definition in surveying?
In surveying, a datum is the reference system to which positions and heights are referred. A horizontal or geodetic datum defines an ellipsoid and how it is fixed to the Earth, such as WGS84 or NAD83. A vertical datum defines zero height, usually related to mean sea level, such as NAVD 88. Every survey should state both.
What is the GPS map datum?
GPS satellites broadcast orbits in WGS84, so a receiver computes positions in WGS84 by default. The GPS map datum setting in a device or app controls which datum coordinates are displayed in. Survey receivers using RTK take the datum of their base or CORS network, so check that the network frame matches your project before measuring.
Is WGS84 the same as NAD83?
No. They used nearly identical ellipsoids, but NAD83 is fixed to the North American plate while WGS84 follows a global frame. In the conterminous US the two differ by roughly one to two metres, and the difference slowly grows with plate motion. For mapping-grade work they may look the same, but for surveying you must transform between them.
What is the coordinate system definition in GIS?
In GIS, a coordinate system definition, or coordinate reference system, is the full description of how coordinates relate to the Earth: the datum, ellipsoid, prime meridian, units and, for projected systems, the projection and its parameters. It is usually stored as an EPSG code or a well-known text string, and every layer needs one to overlay correctly.
How to find property coordinates?
You can read approximate property coordinates from online maps or a GPS coordinates device, but these are typically accurate only to a few metres and are not legal boundaries. Official corner coordinates, if they exist, come from recorded plats or a licensed surveyor. Our article on finding property lines with GPS explains the limits of consumer methods.
What is a spatial coordinate system?
A spatial coordinate system is any system that locates features in space using numbers, either on the Earth as latitude, longitude and height, as Earth-centred X, Y, Z, or on a projected grid as easting, northing and elevation. In surveying and GIS it always needs a datum so that coordinates from different sources line up correctly.
What is a reference datum for heights?
A reference datum for heights is the surface from which elevations are measured, usually a geoid-based model close to mean sea level, such as NAVD 88 in the US. GNSS gives ellipsoidal heights, so a geoid model is needed to convert them. Using different height datums on one project can create errors of decimetres or more.
Need equipment for this work?
Tell us the job, the country and the quantity. An engineer replies with a configuration and price within one business day. MOQ is one unit; OEM is available.
