====== Projection Type ======

Represents a [[wp>Map_projection|map projection]]: the mathematical transformation used to represent locations on the Earth's curved surface as coordinates on a flat plane. Every projection is tied to a coordinate reference system (CRS), which combines a datum, an ellipsoid (or sphere) model of the Earth, the projection method itself, and a unit of measurement. The projection assigned to a map determines how its coordinates are interpreted and how it aligns with other spatial data.

Projections can be defined in one of four ways:

  * Using the **predefined options** (UTM, Geodetic, etc.) — a curated list of common projections that can be selected directly, without specifying any further parameters.
  * Using a **WKT specification** — a complete, self-contained description of the CRS.
  * Using an **EPSG code** — a numeric identifier looked up in the [[https://epsg.org/search/by-name|EPSG Geodetic Parameter Registry]].
  * Using a **Proj4 specification** — a compact, parameter-based string.

WKT specifications must follow the [[https://docs.ogc.org/is/18-010r7/18-010r7.html|WKT standard syntax]]. A WKT description fully defines the CRS — datum, spheroid, prime meridian, angular/linear units, projection method and its parameters — which makes it the most explicit and portable of the four ways to specify a projection. See the example below:

<code>
PROJCS["SUTM21",
    GEOGCS["SOUTH AMERICAN 1969",
        DATUM["SAD69",
            SPHEROID["INT67",6378160,298.25]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-57],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",10000000],
    UNIT["Meter",1]]
</code>

The Proj4 syntax must also follow the [[https://proj4.org/en/5.2/usage/index.html|Proj4 specification syntax]]. Rather than the nested structure of WKT, Proj4 represents a CRS as a list of ''+key=value'' flags, which tends to be more compact and faster to write by hand. Example:

<code>
+proj=latlong +ellps=GRS80 +towgs84=-199.87,74.79,246.62
</code>

===== GUI Editor =====

[{{ :editors:projection_editor.png?nolink&500 |Graphical representation of the projection editor}}]

The projection editor lets you select and configure a projection using any of the four definition methods described above.

===== EGO Script =====

A constant representing a projection in an [[EGO Script]] can be defined in four different ways:

==== Using ERMapper parameters ====

This form specifies the coordinate type together with three keywords — projection, datum and unit — following the naming conventions used by ER Mapper.

<code>
[| "COORDINATE_TYPE", "ERMAPPER_PROJECTION", "ERMAPPER_DATUM", "ERMAPPER_UNIT" |]
</code>

All parameters are case-insensitive and must be surrounded by double quotes.

Example:
<code>
[| "LATLONG", "GEODETIC", "WGS84", "METERS" |]
</code>

==== Using a WKT (Well-Known Text) description ====

This form embeds a full WKT description directly in the script, giving complete control over every CRS parameter.

<code>
[| "COORDINATE_TYPE", WKT_PROJECTION |]
</code>

The syntax used to represent the WKT_PROJECTION can be found [[https://docs.ogc.org/is/18-010r7/18-010r7.html|here]]. It is worth noting that only the COORDINATE_TYPE must be surrounded by double quotes.

Example:
<code>
[| "LATLONG",
   PROJCS["unnamed",
       GEOGCS["WGS 84",
           DATUM["WGS_1984",
               SPHEROID["WGS 84",6378137,298.257223563,
                   AUTHORITY["EPSG","7030"]],
               TOWGS84[0,0,0,0,0,0,0],
               AUTHORITY["EPSG","6326"]],
           PRIMEM["Greenwich",0,
               AUTHORITY["EPSG","8901"]],
           UNIT["degree",0.0174532925199433,
               AUTHORITY["EPSG","9108"]],
           AUTHORITY["EPSG","4326"]],
       PROJECTION["Transverse_Mercator"],
       PARAMETER["latitude_of_origin",0],
       PARAMETER["central_meridian",90],
       PARAMETER["scale_factor",0.9996],
       PARAMETER["false_easting",500000],
       PARAMETER["false_northing",-2000000],
       UNIT["METERS",1]]]
|]
</code>

==== Using a Proj4 description ====

This form embeds a Proj4 parameter string, enclosed in square brackets, in place of the WKT description.

<code>
[| "COORDINATE_TYPE", [ PROJ4_PROJECTION ] |]
</code>

The syntax used to represent the PROJ4_PROJECTION can be found [[https://proj4.org/en/5.2/usage/index.html|here]].

Example:
<code>
[| "LATLONG",
   [ +proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs ]
|]
</code>

==== Using an EPSG code ====

This form looks up the projection directly from its numeric [[https://epsg.org/search/by-name|EPSG]] identifier, making it the most compact of the four options.

<code>
[| "COORDINATE_TYPE", EPSG_CODE |]
</code>

Example:
<code>
[| "LATLONG",
   7030
|]
</code>

----

A COORDINATE_TYPE is also case-insensitive and must be one of the following keywords:

^ Keyword ^ Description ^
| ''en'' | Easting/Northing (projected/planar coordinates) |
| ''latlong'' | Latitude/Longitude (geographic coordinates) |
| ''none'' | Automatically derive the coordinate type from the projection parameters |

===== Automatic Conversions =====

  * **Converted from**: [[Map Type]] and [[Categorical Map Type]] — either kind of map converts directly to its own projection. [[String Type]].

  * **Converted to**: [[String Type]]. Projection Type is one of only three types that connect exclusively to String Type rather than the broader Table Value Type family other filenames and text-like types also reach — see [[Table Value Type]] on the [[type_system|Type System]] page for that distinction.

See [[type_system|Type System]] for the complete conversion reference across all types.