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GeoJSON Specification

Scott Chamberlain

2024-10-28

Source: vignettes/geojson_spec.Rmd
geojson_spec.Rmd

In geojsonio we follow the below guidelines (RFC7946) for GeoJSON, and try to preserve CRS information, and bbox information when converting formats.

The following are the guidelines for CRS and bounding boxes for geojson, annotated as needed, get complete guidelines at https://tools.ietf.org/html/rfc7946

The Structure of GeoJSON

See https://tools.ietf.org/html/rfc7946#section-3 for further information.

GeoJSON text: a JSON text and consists of a single GeoJSON object.

GeoJSON object: represents a Geometry, Feature, or collection of Features (i.e., FeatureCollection).

  • has a member with the name “type”. The value of the member MUST be one of the GeoJSON types.
  • MAY have a “bbox” member, the value of which MUST be a bounding box array (see Section 5).
  • MAY have other members (see Section 6).

Geometry object: represents points, curves, and surfaces in coordinate space. Every Geometry object is a GeoJSON object no matter where it occurs in a GeoJSON text.

  • The value of a Geometry object’s “type” member MUST be one of the seven geometry types.
  • A GeoJSON Geometry object of any type other than “GeometryCollection” has a member with the name “coordinates”. The value of the “coordinates” member is an array. The structure of the elements in this array is determined by the type of geometry.

Position: the fundamental geometry construct. The “coordinates” member of a Geometry object is composed of either:

  • one position in the case of a Point geometry,
  • an array of positions in the case of a LineString or MultiPoint geometry,
  • an array of LineString or linear ring coordinates in the case of a Polygon or MultiLineString geometry, or
  • an array of Polygon coordinates in the case of a MultiPolygon geometry

Type of Geometries:

  • Point
  • MultiPoint
  • LineString
  • MultiLineString
  • Polygon
  • MultiPolygon
  • GeometryCollection

Feature Object: A Feature object represents a spatially bounded thing. Every Feature object is a GeoJSON object no matter where it occurs in a GeoJSON text. A Feature object has a “type” member with the value “Feature”; has a member with the name “geometry”, the value of which geometry member as defined above or a JSON null value. A Feature object has a member with the name “properties”; the value of the properties member is an object (any JSON object or a JSON null value).

FeatureCollection Object: A GeoJSON object with the type “FeatureCollection” is a FeatureCollection object. A FeatureCollection object has a member with the name “features”. The value of “features” is a JSON array. Each element of the array is a Feature object as defined above. It is possible for this array to be empty.

CRS (Coordinate Reference System)

See https://tools.ietf.org/html/rfc7946#page-12 for further information.

  • The coordinate reference system for all GeoJSON coordinates is a geographic coordinate reference system, using the World Geodetic System 1984 (WGS84) datum, with longitude and latitude units of decimal degrees. This is equivalent to the coordinate reference system identified by the Open Geospatial Consortium (OGC) URN urn:ogc:def:crs:OGC::CRS84. An OPTIONAL third-position element SHALL be the height in meters above or below the WGS 84 reference ellipsoid. In the absence of elevation values, applications sensitive to height or depth SHOULD interpret positions as being at local ground or sea level.
  • The crs member has been removed.
  • RFC7946 does not that “where all involved parties have a prior arrangement, alternative coordinate reference systems can be used without risk of data being misinterpreted.”

Bounding Boxes

See https://tools.ietf.org/html/rfc7946#page-12 for further information.

To include information on the coordinate range for Geometries, Features, or FeatureCollections, a GeoJSON object may have a member named bbox. The value of the bbox member must be a 2*n array where n is the number of dimensions represented in the contained geometries, with the lowest values for all axes followed by the highest values. The axes order of a bbox follows the axes order of geometries.

Example of a 2D bbox member on a Feature:

{
  "type": "Feature",
  "bbox": [-10.0, -10.0, 10.0, 10.0],
  "geometry": {
    "type": "Polygon",
    "coordinates": [[
      [-10.0, -10.0], [10.0, -10.0], [10.0, 10.0], [-10.0, -10.0]
    ]]
  }
  ...
}

Example of a 2D bbox member on a FeatureCollection:

{
  "type": "FeatureCollection",
  "bbox": [100.0, 0.0, 105.0, 1.0],
  "features": [
    ...
  ]
}

Example of a 3D bbox member with a depth of 100 meters on a FeatureCollection:

{
  "type": "FeatureCollection",
  "bbox": [100.0, 0.0, -100.0, 105.0, 1.0, 0.0],
  "features": [
    ...
  ]
}

Coordinate Precision

See https://tools.ietf.org/html/rfc7946#page-18 for further information.

The size of a GeoJSON text in bytes is a major interoperability consideration, and precision of coordinate values has a large impact on the size of texts. A GeoJSON text containing many detailed Polygons can be inflated almost by a factor of two by increasing coordinate precision from 6 to 15 decimal places. For geographic coordinates with units of degrees, 6 decimal places (a default common in, e.g., sprintf) amounts to about 10 centimeters, a precision well within that of current GPS systems. Implementations should consider the cost of using a greater precision than necessary.