CRS Authorities
Defining a CRS from scratch can be cumbersome, with many parameters that must be specified. But for a commonly used CRS, it should be enough to just refer to it by a standard name like WGS84, right?
In theory, yes, but not in practice, because it is hard to standardize names. As a horrible example, here are some names for the old Swedish national CRS:
Name | Comment |
|---|---|
Rikets nät | Colloquial Swedish name. |
Rikets triangelnät | Another Swedish name. |
Rikets koordinatsystem 1990 | A third Swedish name. |
Swedish Grid | Colloquial English name. |
RT 90 2,5 gon V 0:−15 | Complete technical notation. The central meridian of the map projection goes 2.5 gon west ("väst") of Stockholm. The 0:−15 indicates the national values for false easting and false northing, as opposed to local alternatives. |
RT90 2,5 gon V | Partial abbreviation. Note that the space between RT and 90 is volatile. |
RT90 | Compact abbreviation. In Swedish usage, "RT90 coordinates" means projected coordinates, not latitude and longitude. |
RT90 2.5 gon W | An early English translation, using the English abbreviation for "west" and a dot instead of the comma. |
RT90 2.5 gon V | The organization that made the previous translation decided to keep the dot but revert to V. |
RT90_25_gon_V | Name used by an American GIS company that dislikes blanks – they solved the dot/comma dilemma in a radical way. |
So, how can we refer to this CRS in a simple, standardized and machine-readable way? That is easy, actually:
EPSG:3021
The "EPSG" is the name of an authority that publishes a database of CRS definitions that have unique code numbers, which can be used in many GIS standards.
The earliest versions of the EPSG database were published in the 1990s by the European Petroleum Survey Group. The survey group has now become a part of IOGP, the International Organization of Oil & Gas Producers, and the database is maintained by the Geodesy Subcommittee of the IOGP Geomatics Committee. While the EPSG trademark has been kept for the database, some of the documentation is now known as IOGP Guidance Notes. The EPSG database does not and cannot contain every regional or local CRS definition in the world, nor is that a stated goal, but it does contain a vast amount of CRS information.
EPSG codes are integers less than 32767, and there are about twenty types of entities that are assigned codes, not just CRS instances. So there are EPSG codes for geodetic datums, length units, projection methods, naming systems, and whatnot. (Actually, the codes for change requests are not integers but on a format like 2021.123.) Note that each type of entity has its own separate codespace, so an EPSG code can be ambiguous if you do not know what type of entity is meant. Code 6135 is an example: the CRS with that EPSG code is CIGD11 which is used on the Cayman Islands, but the Geodetic Datum with that EPSG code is the Old Hawaiian datum.
While EPSG is dominant, there are also some alternative authorities (for external links, see table further down):
"ESRI": an authority based on the Esri Projection Engine Database.
"IGNF": a database published by the French mapping institute IGN. The variant spelling "IGN-F" is sometimes seen, but Carmenta does not support it.
"ISO": the ISO geodetic registry, maintained by a control body appointed by ISO/TC 211/WG 9.
"CRS": a tiny codespace with a peculiar name, defined by OGC, the Open Geospatial Consortium.
"OGC": a renamed and recoded version of the "CRS" authority.
Mistakes happen. Sometimes CRS parameters from a national mapping agency have been lost, corrupted or misunderstood on their way to EPSG, resulting in an erroneous CRS definition in the EPSG database. When alerted, the EPSG committee will publish a correction: since July 2001, the policy is to keep the bad definition in the database but tag it as deprecated, and then make a new definition with a new code. Minor amendments in an EPSG definition can be made without changing the code, although the definition of minor has changed over the years – a modified name for a CRS was once considered enough reason for deprecation and replacement, but that is no longer the case. For the ISO geodetic registry, the workflow has been designed to reduce the risk of errors: only national mapping agencies are allowed to submit new CRS definitions to ISO, and only by using a standardized spreadsheet template.
In Carmenta Engine, one can construct a new CRS instance by specifying an authority and a code, although the ISO authority is not yet supported; see the Crs class for details. Carmenta considers the authority names to be case insensitive, and the default Crs.Authority in Carmenta Studio is "epsg", but the Carmenta Engine knowledge about authorities comes from the SQLite database bin/data/gdal/proj.db, where authority names are uppercase.
In various web mapping standards by OGC (the Open Geospatial Consortium), "EPSG" is the most important authority. The authorities "ESRI" and "IGNF" are not supported (as of 2021), the "CRS" authority is mostly used in older standards and is replaced by "OGC" in newer ones, while "ISO" is allowed in some standards but not all. If a CRS you want to use is missing in all authorities allowed by a web mapping standard, there is not much you can do. As a workaround, people sometimes invent their own EPSG codes; for example, people may pretend that 34003 and 34005 are EPSG codes for the two zones of Denmark's System 34. But apart from the limited interoperability, such pretense could violate the EPSG terms of use, clause 6.7: "No data that has been modified other than as permitted in these Terms of Use shall be attributed to the EPSG Dataset."
Authority | Size¹ | Codes | Supported by Carmenta | Supported in OGC web mapping standards |
|---|---|---|---|---|
6821 | Integers | Yes | Yes | |
1247 | Integers | Yes | No | |
412 | CRS codes are strings, e.g. "EURO54UTM37S" | Yes | No | |
81 | Integers | No | In some of them | |
3 | 1, 27, 83, 84 and 88 | Yes | In older ones | |
3 | "CRS1", "CRS27", "CRS83", "CRS84" and "CRS88" | Yes | In newer ones |
¹Here, the size is defined (somewhat arbitrarily) as the number of non-deprecated CRS definitions, in the beginning of 2021, of the types geographic 2D or projected only.
The ISO codes start from 1.
EPSG has committed to use codes only in the range from 1024 to 32767 (IOGP Guidance Note 7-1, section 5.9).
For the "ESRI" authority, the Esri-specific code range goes from 33000 to 199999, but the Esri terminology blurs the distinction between ESRI codes and EPSG codes, calling them all WKIDs (well-known identifiers). So in some sense, it can be argued that the full Esri Projection Engine Database contains the EPSG database as a subset, but that is not how it works in Carmenta Engine, where "ESRI" cannot be used as the authority if the WKID is really an EPSG code. If you only have a WKID, you can usually figure out whether it is an EPSG code or an ESRI code by checking if it is less than 32767, but unfortunately there are exceptions. That is because in rare cases, ESRI has kept an old code that was deleted from the EPSG database before July 2001, when they stopped deleting codes. The deletion could in those days be caused by a simple name change (which today would not even trigger deprecation), so there could be a more up-to-date EPSG code for the CRS. For example, the code 24721 was deleted from EPSG in 1999 but remains as an ESRI code for "La_Canoa_UTM_Zone_21N"; a more up-to-date EPSG code is 24821 for "PSAD56 / UTM zone 21N", which has the remark: "within Venezuela (but not beyond) the names La Canoa and PSAD56 are synonymous" (only a tiny corner of Venezuela is in zone 21N).
Carmenta Engine does not use "CARMENTA" as an authority for CRS instances, with the single exception of our predefined Crs.NonGeoreferenced, which has the Carmenta code 51003. But other Carmenta codes are assigned to datum shifts designed by Carmenta, mostly in the range 51020 to 51500.
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