A geodetic terrestrial reference system (TRS) is a 3D coordinate system centered at the Earth\u2019s center of mass. It rotates with the Earth and uses standard meters for measuring distance. Its three Cartesian axes (X, Y, and Z) stay fixed in a stable orientation, even though the Earth\u2019s tectonic plates move. In short, a TRS provides an Earth\u2011centered, Earth\u2011fixed coordinate system used for mapping, navigation, and positioning.<\/p>\n
A terrestrial reference system is just an abstract idea \u2013 it doesn\u2019t give us actual points on the Earth that we can measure or use directly. To make it usable, we need to connect the TRS to the real Earth. This is done by measuring and defining a set of physical reference points on Earth\u2019s surface that follow the rules of the TRS. Today, this is done using various geodetic observing methods, one of which is with a global set of permanent GNSS tracking stations.<\/p>\n
When we create this real-world version of the TRS, it becomes a terrestrial reference frame (TRF), also known as a geodetic datum. The TRF is what anchors the abstract system to the Earth so that geospatial professionals, including GIS users, can accurately measure positions.<\/p>\n
A widely used TRS\/TRF is WGS 84. WGS 84 is a reference system that becomes tied to the real Earth by using the measured coordinates of 17 Global Positioning System (GPS) tracking stations around the world. When WGS 84 is anchored this way, it becomes a realization, known as the WGS 84 terrestrial reference frame (TRF).<\/p>\n"},{"acf_fc_layout":"image","image":{"ID":2966167,"id":2966167,"title":"GPS","filename":"GPS.png","filesize":616278,"url":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS.png","link":"https:\/\/www.esri.com\/arcgis-blog\/products\/arcgis-pro\/data-management\/datum-ensembles-a-choreography-of-geodetic-datums\/gps-3","alt":"","author":"346932","description":"","caption":"Ground tracking network for the U.S. GPS satellite navigation system.\n\n","name":"gps-3","status":"inherit","uploaded_to":2966147,"date":"2026-05-14 20:51:33","modified":"2026-05-14 20:53:03","menu_order":0,"mime_type":"image\/png","type":"image","subtype":"png","icon":"https:\/\/www.esri.com\/arcgis-blog\/wp-includes\/images\/media\/default.png","width":1800,"height":1245,"sizes":{"thumbnail":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS-213x200.png","thumbnail-width":213,"thumbnail-height":200,"medium":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS.png","medium-width":377,"medium-height":261,"medium_large":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS.png","medium_large-width":768,"medium_large-height":531,"large":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS.png","large-width":1561,"large-height":1080,"1536x1536":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS-1536x1062.png","1536x1536-width":1536,"1536x1536-height":1062,"2048x2048":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS.png","2048x2048-width":1800,"2048x2048-height":1245,"card_image":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS-672x465.png","card_image-width":672,"card_image-height":465,"wide_image":"https:\/\/www.esri.com\/arcgis-blog\/app\/uploads\/2026\/05\/GPS-1561x1080.png","wide_image-width":1561,"wide_image-height":1080}},"image_position":"center","orientation":"horizontal","hyperlink":""},{"acf_fc_layout":"content","content":"
As technology improves with better instruments, better processing methods, and more tracking stations, we can create more accurate realizations (or versions) of a reference system. For WGS 84, there have been eight different realizations so far. Each realization is tagged with a \u201cG\u2011number,\u201d which refers to the GPS week during which satellite observations were collected to define that version (e.g. the current realization of WGS 84 is WGS 84 (G2296)).<\/p>\n
Even though the WGS 84 TRS (the abstract definition) stays the same, each new realization becomes a slightly updated WGS 84 TRF, with small differences in coordinates. The underlying reason for this is that WGS 84 is a dynamic datum, meaning that coordinates change slightly over time \u2013 something we can dive into in another blog post!<\/p>\n
Because all eight slightly different realizations of WGS 84 are closely related and based on the same TRS, they together represent the WGS 84 datum ensemble. As new data and better technology become available, we can expect more updated WGS 84 realizations in the future.<\/p>\n
<\/p>\n
Why do we need ensembles?<\/h2>\n
The widespread adoption of WGS 84 Web Mercator basemaps allow users to easily overlay their own GIS data on top of online mapping layers. However, this convenience came with an important requirement: any dataset not originally referenced to WGS 84 needed to be transformed into WGS 84 to align with the basemap.<\/p>\n
As more datasets were combined \u2013 or \u201cmashed up\u201d \u2013 on these WGS 84 Web Mercator basemaps, small positional misalignments began to appear. These discrepancies were typically subtle, but noticeable when overlaying multiple layers. Three primary issues contributed to this problem:<\/p>\n
- \n
- Most datasets did not specify which realization of WGS 84 they used.<\/li>\n
- Transformations to specific WGS 84 realizations were often unavailable.<\/li>\n
- Transformations between different WGS 84 realizations were also unavailable.<\/li>\n<\/ol>\n
Without this information GIS software had no reliable way to correctly align datasets tied to slightly different versions of WGS 84, even though the differences between those versions might only be a few centimeters.<\/p>\n
The long-term solution to this challenge was the introduction of the datum ensemble. A datum ensemble allows multiple closely related realizations of a reference system, such as WGS 84, to be treated as a single group for most GIS applications.<\/p>\n
In practice, new realizations of terrestrial or vertical reference systems often differ only by a small amount, typically on the order of centimeters. For many GIS use cases, especially those involving lower-accuracy data, these differences are negligible.<\/p>\n
Datum ensembles simplify workflows by allowing users to group these nearly equivalent realizations together. As a result, datasets based on any of the included member realizations can be combined without explicitly transforming coordinates between them, because they are treated as effectively the same.<\/p>\n
In summary, datum ensembles provide a practical way to manage consistency across datasets when high precision is not required. However, for high-accuracy applications, such as surveying, geodesy, or engineering, users should still work with the specific reference frame realization under which the data was collected and apply precise transformations where necessary.<\/p>\n
<\/p>\n
Where did the datum ensemble come from?<\/h2>\n
The datum ensemble was introduced in the ISO 19111:2019<\/a> standard and adopted by the OGC<\/a>. In these standards, datum ensemble is defined as<\/p>\n
\u201c\u2026a collection of two or more reference frames that are realizations of one Terrestrial or Vertical Reference System and which for all but the highest accuracy requirements may be considered to be insignificantly different from each other.\u201d<\/em><\/strong><\/p><\/blockquote>\n
Therefore, GIS data that uses slightly different versions of the same reference system can all be assigned to one common, representative datum \u2013 the ensemble. This makes it much easier to combine and manage data without having to transform coordinates between very similar frames.<\/p>\n
In short: datum ensembles let users treat very similar geodetic datums as the same one, provided high\u2011precision accuracy isn\u2019t required.<\/p>\n
On February 13, 2020, the EPSG Geodetic Parameter Dataset<\/a> (Version 10.0) officially added the new concept of a datum ensemble following the ISO 19111:2019<\/a> and ISO 19162:2019 (WKT2)<\/a> standards. At the same time, EPSG redefined WGS 84 as a datum ensemble rather than a single datum. This ensemble is identified as EPSG::6326<\/a>.<\/p>\n
This made the WGS 84 ensemble the very first datum ensemble in the EPSG geodetic registry. From that point on, Web Mercator became referenced to the WGS 84 datum ensemble instead of an individual WGS 84 realization.<\/p>\n
<\/p>\n
How are datum ensembles constructed?<\/h2>\n
When building a Coordinate Reference System (CRS) \u2013 or a Geographic Coordinate System (GCS) in Esri terms \u2013 you normally assign it a single geodetic datum. However, modern standards allow a datum ensemble to be used in place of an individual datum.<\/p>\n
A datum ensemble is treated just like a datum inside a CRS, but instead of representing one specific realization, it represents a group of closely related datums. All datums in the ensemble share the same ellipsoid and prime meridian, meaning they are almost identical at the basic geometric level.<\/p>\n
Standards such as ISO 19111:2019<\/a> and OGC Topic 2<\/a> describe datum ensembles conceptually, and formats like ISO 19162:2019 (WKT2)<\/a> show how real examples, such as the WGS 84 datum ensemble, are defined.<\/p>\n
It is important to note that members of a datum ensemble do not need to share the same area of use, and their individual extents may also differ from that of the ensemble as a whole. For example, in the WGS 84 ensemble (EPSG::6326<\/a>), each member applies globally, so their areas of use are identical to one another and to the ensemble itself. In contrast, the British Isles height ensemble (EPSG::1288<\/a>) presented in the next section includes members with distinct, localized areas of use that fall within the overall spatial extent of the ensemble.<\/p>\n
<\/p>\n
Datum ensemble examples<\/h2>\n
Standards such as ISO 19162:2019<\/a> and OGC WKT2 2.1.11<\/a> include real-world examples of how datum ensembles are written in Well\u2011Known Text (WKT) format. In WKT2, a CRS that uses the WGS 84 datum ensemble includes an ENSEMBLE element instead of a single DATUM. This element identifies:<\/p>\n
- \n
- the name of the datum ensemble (e.g., \u201cWorld Geodetic System 1984 ensemble\u201d),<\/li>\n
- the members of the ensemble (the different WGS 84 realizations, such as WGS 84 (G2296)), and<\/li>\n
- the accuracy associated with the ensemble.<\/li>\n<\/ul>\n
So instead of pointing to one exact WGS 84 realization, the CRS points to the entire group, because they are similar enough for most GIS uses.<\/p>\n
The WGS 84 ensemble, when used within a CRS, in WKT2 format is shown below:<\/p>\n
GEOGCRS[\"WGS 84\",\r\n ENSEMBLE[\"World Geodetic System 1984 ensemble\",\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (Transit)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G730)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G873)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G1150)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G1674)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G1762)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G2139)\"],\r\n \u00a0\u00a0 MEMBER[\"World Geodetic System 1984 (G2296)\"],\r\n \u00a0\u00a0 ELLIPSOID[\"WGS 84\",6378137,298.257223563,\r\n \u00a0\u00a0\u00a0\u00a0 LENGTHUNIT[\"metre\",1]],\r\n \u00a0\u00a0 ENSEMBLEACCURACY[2.0]]<\/strong>,\r\n PRIMEM[\"Greenwich\",0,\r\n \u00a0\u00a0 ANGLEUNIT[\"degree\",0.0174532925199433]],\r\n CS[ellipsoidal,2],\r\n \u00a0\u00a0 AXIS[\"geodetic latitude (Lat)\",north,\r\n \u00a0\u00a0\u00a0\u00a0 ORDER[1],\r\n \u00a0\u00a0\u00a0\u00a0 ANGLEUNIT[\"degree\",0.0174532925199433]],\r\n \u00a0\u00a0 AXIS[\"geodetic longitude (Lon)\",east,\r\n \u00a0\u00a0\u00a0\u00a0 ORDER[2],\r\n \u00a0\u00a0\u00a0\u00a0 ANGLEUNIT[\"degree\",0.0174532925199433]],\r\n USAGE[\r\n \u00a0\u00a0 SCOPE[\"Horizontal component of 3D system.\"],\r\n \u00a0\u00a0 AREA[\"World.\"],\r\n \u00a0\u00a0 BBOX[-90,-180,90,180]],\r\n\u00a0 ID[\"EPSG\",4326]]<\/pre>\n"},{"acf_fc_layout":"content","content":"What many GIS users casually call \u201cWGS 84\u201d isn\u2019t just one fixed datum. Instead, it is an ensemble of eight different realizations of the WGS 84 Terrestrial Reference System. The first version, called Transit, was created in 1984 using early satellite technologies. Over time, WGS 84 has been updated several times to improve accuracy.<\/p>\n
Since 1994, each new realization of WGS 84 has been aligned to different ITRF realizations, usually within 10 cm or less, and each new WGS 84 version is generally within a few centimeters of the previous one.<\/p>\n
Even with these differences, the WGS 84 ensemble continues to be the most widely used global reference frame. This is mainly because it is easy to access and is deeply built into most GPS devices, GIS software, and geospatial workflows.<\/p>\n
For completeness, an example of a vertical reference system (VRS) datum ensemble is the British Isles height ensemble:<\/p>\n
VERTCRS[\"BI height\",\r\n ENSEMBLE[\"British Isles height ensemble\",\r\n \u00a0\u00a0 MEMBER[\"Malin Head\"],\r\n \u00a0\u00a0 MEMBER[\"Belfast Lough\"],\r\n \u00a0\u00a0 MEMBER[\"Ordnance Datum Newlyn\"],\r\n \u00a0\u00a0 MEMBER[\"Ordnance Datum Newlyn (Offshore)\"],\r\n \u00a0\u00a0 MEMBER[\"Ordnance Datum Newlyn (Orkney Isles)\"],\r\n \u00a0\u00a0 MEMBER[\"Lerwick\"],\r\n \u00a0\u00a0 MEMBER[\"Stornoway\"],\r\n \u00a0\u00a0 MEMBER[\"Douglas\"],\r\n \u00a0\u00a0 MEMBER[\"St. Marys\"],\r\n \u00a0\u00a0 ENSEMBLEACCURACY[0.4]]<\/strong>,\r\n CS[vertical,1],\r\n \u00a0\u00a0 AXIS[\"gravity-related height (H)\",up,\r\n \u00a0\u00a0\u00a0\u00a0 LENGTHUNIT[\"metre\",1]],\r\n USAGE[\r\n \u00a0\u00a0 SCOPE[\"Spatial referencing.\"],\r\n \u00a0\u00a0 AREA[\"United Kingdom (UK) - offshore to boundary of UKCS within 49\u00b045'N to 61\u00b0N and 9\u00b0W to 2\u00b0E; onshore Great Britain (England, Wales and Scotland) and Northern Ireland. Ireland onshore. Isle of Man onshore.\"],\r\n \u00a0\u00a0 BBOX[49.75,-10.85,61.01,2.01]],\r\n ID[\"EPSG\",9451]]<\/pre>\n"},{"acf_fc_layout":"content","content":"The table below lists geographic and vertical datum ensembles currently available in ArcGIS Pro 3.7 software.<\/p>\n
"},{"ID":11262,"user_firstname":"Bojan","user_lastname":"Savric","nickname":"Bojan Savric","user_nicename":"bsavric","display_name":"Bojan \u0160avri\u010d","user_email":"BSavric@esri.com","user_url":"","user_registered":"2020-02-20 21:14:46","user_description":"Bojan \u0160avri\u010d is a Senior Software Development Engineer on the Projection Engine team at Esri, specializing in coordinate reference systems, datum transformations, and map projections. Bojan is the coauthor of several map projections for world maps and is an enthusiastic lover of the math behind maps.","user_avatar":"
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