Skip to content

Proposal Circles and Ellipses Geoms

Brian Panulla edited this page May 10, 2013 · 5 revisions

I'd like to see Ellipses and/or Circle geometries added to the spec.

Based on this theoretical draft spec with Ellipses (mentioned a while back on the list, apparently Richard's original creation), I created this potential spec which is included below as well.

The idea is basic - just add native support for Ellipse and Circle shapes. I could see removing Circles and just using the same major/minor axis to make an 'ellipse circle', but that seems a bit of a pain. Right now we are storing Point geometries and just adding major/minor/rotation properties. The goal of Ellipse/Circle is obviously to make that easier.

Notable Changes

A geometry is a GeoJSON object where the type member's value is one of the
following strings: "Point", "MultiPoint", "LineString", "MultiLineString",
"Polygon", "MultiPolygon", "Circle", "Ellipse" or "GeometryCollection"...


2.1.9 Circle
.........................

For type "Circle", the "coordinates" member must be a single point. A "Circle"
object must have a member with the name "radius". The value corresponding to
"radius" is a single value. A properties value of "radius_units" is optional but
may be used to specify units. A default of kilometers is otherwise assumed.

 2.1.10 Ellipse
 .........................

 For type "Ellipse", the "coordinates" member must be a single point. An
 "Ellipse" object must have three additional members: "maj", "min" and 
 "rotation", indicating "major axis", "minor axis" and degrees of rotation,
 respectively. A properties value of "axis_units" is optional and may be used to
 specify units. A default of kilometers is otherwise assumed. A properties value 
 of "rotation_units" is optional and may be used to specify units. A default of 
 decimal degrees is otherwise assumed...



 Circle
 ------------------

 { "type": "Circle",
   "coordinates": [ 100.0, 0.0 ],
   "radius": 0.5,
   "properties": {
          "radius_units": "km"
   }
}

Ellipse
------------------

 { "type": "Ellipse",
   "coordinates": [ 100.0, 0.0 ],
   "maj": 20.0,
   "min": 10.0,
   "rot": 45.0,
   "properties": {
     "axis_units": "km",
     "rot_units": "decimal degrees"
   }
 }

``

Discussion notes

There are 2 arguments against Circles and Ellipses:

  1. Coordinates in longitude and latitude and radius in meters require out of band information: ellipsoid and datum dependent meters per degree.
  2. Simple operations on ellipses and circles produce not representable/not encodable geometries.

Entire Potentially New Spec

================================ The GeoJSON Format Specification

:Abstract: GeoJSON is a geospatial data interchange format based on JavaScript Object Notation (JSON).

:Authors: Howard Butler (Hobu Inc.), Martin Daly (Cadcorp), Allan Doyle (MIT), Sean Gillies (UNC-Chapel Hill), Tim Schaub (OpenGeo), Christopher Schmidt (MetaCarta)

:Revision: 1.0 :Date: 16 June 2008

:Copyright: Copyright |copy| 2008 by the Authors. This work is licensed under a Creative Commons Attribution 3.0 United States License__.

.. |copy| unicode:: 0xA9 .. copyright sign .. __: http://creativecommons.org/licenses/by/3.0/us/

.. contents::

  1. Introduction ===============

GeoJSON is a format for encoding a variety of geographic data structures. A GeoJSON object may represent a geometry, a feature, or a collection of features. GeoJSON supports the following geometry types: Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection. Features in GeoJSON contain a geometry object and additional properties, and a feature collection represents a list of features.

A complete GeoJSON data structure is always an object (in JSON terms). In GeoJSON, an object consists of a collection of name/value pairs -- also called members. For each member, the name is always a string. Member values are either a string, number, object, array or one of the literals: "true", "false", and "null". An array consists of elements where each element is a value as described above.

1.1. Examples

A GeoJSON feature collection::

{ "type": "FeatureCollection", "features": [ { "type": "Feature", "geometry": {"type": "Point", "coordinates": [102.0, 0.5]}, "properties": {"prop0": "value0"} }, { "type": "Feature", "geometry": { "type": "LineString", "coordinates": [ [102.0, 0.0], [103.0, 1.0], [104.0, 0.0], [105.0, 1.0] ] }, "properties": { "prop0": "value0", "prop1": 0.0 } }, { "type": "Feature", "geometry": { "type": "Polygon", "coordinates": [ [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ] ] }, "properties": { "prop0": "value0", "prop1": {"this": "that"} } } ] }

1.2. Definitions

  • JavaScript Object Notation (JSON), and the terms object, name, value, array, and number, are defined in IETF RTC 4627, at http://www.ietf.org/rfc/rfc4627.txt.

  • The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in IETF RFC 2119, at http://www.ietf.org/rfc/rfc2119.txt.

  1. GeoJSON Objects ==================

GeoJSON always consists of a single object. This object (referred to as the GeoJSON object below) represents a geometry, feature, or collection of features.

  • The GeoJSON object may have any number of members (name/value pairs).

  • The GeoJSON object must have a member with the name "type". This member's value is a string that determines the type of the GeoJSON object.

  • The value of the type member must be one of: "Point", "MultiPoint", "LineString", "MultiLineString", "Polygon", "MultiPolygon", "Circle", "Ellipse", "GeometryCollection", "Feature", or "FeatureCollection". The case of the type member values must be as shown here.

  • A GeoJSON object may have an optional "crs" member, the value of which must be a coordinate reference system object (see 3. Coordinate Reference System Objects_).

  • A GeoJSON object may have a "bbox" member, the value of which must be a bounding box array (see 4. Bounding Boxes_).

2.1 Geometry Objects

A geometry is a GeoJSON object where the type member's value is one of the following strings: "Point", "MultiPoint", "LineString", "MultiLineString", "Polygon", "MultiPolygon", "Circle", "Ellipse" or "GeometryCollection".

A GeoJSON geometry object of any type other than "GeometryCollection" must have a member with the name "coordinates". The value of the coordinates member is always an array. The structure for the elements in this array is determined by the type of geometry.

2.1.1. Positions ................

A position is the fundamental geometry construct. The "coordinates" member of a geometry object is composed of one position (in the case of a Point geometry), an array of positions (LineString or MultiPoint geometries), an array of arrays of positions (Polygons, MultiLineStrings), or a multidimensional array of positions (MultiPolygon).

A position is represented by an array of numbers. There must be at least two elements, and may be more. The order of elements must follow x, y, z order (easting, northing, altitude for coordinates in a projected coordinate reference system, or longitude, latitude, altitude for coordinates in a geographic coordinate reference system). Any number of additional elements are allowed -- interpretation and meaning of additional elements is beyond the scope of this specification.

Examples of positions and geometries are provided in Appendix A. Geometry Examples_.

2.1.2. Point ............

For type "Point", the "coordinates" member must be a single position.

2.1.3. MultiPoint .................

For type "MultiPoint", the "coordinates" member must be an array of positions.

2.1.4. LineString .................

For type "LineString", the "coordinates" member must be an array of two or more positions.

A LinearRing is closed LineString with 4 or more positions. The first and last positions are equivalent (they represent equivalent points). Though a LinearRing is not explicitly represented as a GeoJSON geometry type, it is referred to in the Polygon geometry type definition.

2.1.5. MultiLineString ......................

For type "MultiLineString", the "coordinates" member must be an array of LineString coordinate arrays.

2.1.6. Polygon ..............

For type "Polygon", the "coordinates" member must be an array of LinearRing coordinate arrays. For Polygons with multiple rings, the first must be the exterior ring and any others must be interior rings or holes.

2.1.7. MultiPolygon ...................

For type "MultiPolygon", the "coordinates" member must be an array of Polygon coordinate arrays.

2.1.8 Geometry Collection .........................

A GeoJSON object with type "GeometryCollection" is a geometry object which represents a collection of geometry objects.

A geometry collection must have a member with the name "geometries". The value corresponding to "geometries" is an array. Each element in this array is a GeoJSON geometry object.

2.1.9 Circle .........................

For type "Circle", the "coordinates" member must be a single point. A "Circle" object must have a member with the name "radius". The value corresponding to "radius" is a single value. A properties value of "radius_units" is optional but may be used to specify units. A default of kilometers is otherwise assumed.

2.1.10 Ellipse .........................

For type "Ellipse", the "coordinates" member must be a single point. An "Ellipse" object must have three additional members: "maj", "min" and "rotation", indicating "major axis", "minor axis" and degrees of rotation, respectively. A properties value of "axis_units" is optional and may be used to specify units. A default of kilometers is otherwise assumed. A properties value of "rotation_units" is optional and may be used to specify units. A default of decimal degrees is otherwise assumed.

2.2. Feature Objects

A GeoJSON object with the type "Feature" is a feature object.

  • A feature object must have a member with the name "geometry". The value of the geometry member is a geometry object as defined above or a JSON null value.

  • A feature object must have a member with the name "properties". The value of the properties member is an object (any JSON object or a JSON null value).

  • If a feature has a commonly used identifier, that identifier should be included as a member of the feature object with the name "id".

2.3. Feature Collection Objects

A GeoJSON object with the type "FeatureCollection" is a feature collection object.

An object of type "FeatureCollection" must have a member with the name "features". The value corresponding to "features" is an array. Each element in the array is a feature object as defined above.

  1. Coordinate Reference System Objects ======================================

The coordinate reference system (CRS) of a GeoJSON object is determined by its "crs" member (referred to as the CRS object below). If an object has no crs member, then its parent or grandparent object's crs member may be acquired. If no crs member can be so acquired, the default CRS shall apply to the GeoJSON object.

  • The default CRS is a geographic coordinate reference system, using the WGS84 datum, and with longitude and latitude units of decimal degrees.

  • The value of a member named "crs" must be a JSON object (referred to as the CRS object below) or JSON null. If the value of CRS is null, no CRS can be assumed.

  • The crs member should be on the top-level GeoJSON object in a hierarchy (in feature collection, feature, geometry order) and should not be repeated or overridden on children or grandchildren of the object.

  • A non-null CRS object has two mandatory members: "type" and "properties".

  • The value of the type member must be a string, indicating the type of CRS object.

  • The value of the properties member must be an object.

  • CRS shall not change coordinate ordering (see 2.1.1. Positions_).

3.1. Named CRS

A CRS object may indicate a coordinate reference system by name. In this case, the value of its "type" member must be the string "name". The value of its "properties" member must be an object containing a "name" member. The value of that "name" member must be a string identifying a coordinate reference system. OGC CRS URNs such as "urn:ogc:def:crs:OGC:1.3:CRS84" shall be preferred over legacy identifiers such as "EPSG:4326"::

"crs": { "type": "name", "properties": { "name": "urn:ogc:def:crs:OGC:1.3:CRS84" } }

3.2. Linked CRS

A CRS object may link to CRS parameters on the Web. In this case, the value of its "type" member must be the string "link", and the value of its "properties" member must be a Link object (see 3.2.1. Link Objects_).

3.2.1. Link Objects ...................

A link object has one required member: "href", and one optional member: "type".

The value of the required "href" member must be a dereferenceable URI.

The value of the optional "type" member must be a string that hints at the format used to represent CRS parameters at the provided URI. Suggested values are: "proj4", "ogcwkt", "esriwkt", but others can be used::

"crs": { "type": "link", "properties": { "href": "http://example.com/crs/42", "type": "proj4" } }

Relative links may be used to direct processors to CRS parameters in an auxiliary file::

"crs": { "type": "link", "properties": { "href": "data.crs", "type": "ogcwkt" } }

  1. Bounding Boxes =================

To include information on the coordinate range for geometries, features, or feature collections, 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. In addition, the coordinate reference system for the bbox is assumed to match the coordinate reference system of the GeoJSON object of which it is a member.

Example of a bbox member on a feature::

{ "type": "Feature", "bbox": [-180.0, -90.0, 180.0, 90.0], "geometry": { "type": "Polygon", "coordinates": [[ [-180.0, 10.0], [20.0, 90.0], [180.0, -5.0], [-30.0, -90.0] ]] } ... }

Example of a bbox member on a feature collection::

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

Appendix A. Geometry Examples

Each of the examples below represents a complete GeoJSON object. Note that unquoted whitespace is not significant in JSON. Whitespace is used in the examples to help illustrate the data structures, but is not required.

Point

Point coordinates are in x, y order (easting, northing for projected coordinates, longitude, latitude for geographic coordinates)::

{ "type": "Point", "coordinates": [100.0, 0.0] }

LineString

Coordinates of LineString are an array of positions (see 2.1.1. Positions_)::

{ "type": "LineString", "coordinates": [ [100.0, 0.0], [101.0, 1.0] ] }

Polygon

Coordinates of a Polygon are an array of LinearRing coordinate arrays. The first element in the array represents the exterior ring. Any subsequent elements represent interior rings (or holes).

No holes::

{ "type": "Polygon", "coordinates": [ [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ] ] }

With holes::

{ "type": "Polygon", "coordinates": [ [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ], [ [100.2, 0.2], [100.8, 0.2], [100.8, 0.8], [100.2, 0.8], [100.2, 0.2] ] ] }

MultiPoint

Coordinates of a MultiPoint are an array of positions::

{ "type": "MultiPoint", "coordinates": [ [100.0, 0.0], [101.0, 1.0] ] }

MultiLineString

Coordinates of a MultiLineString are an array of LineString coordinate arrays::

{ "type": "MultiLineString", "coordinates": [ [ [100.0, 0.0], [101.0, 1.0] ], [ [102.0, 2.0], [103.0, 3.0] ] ] }

MultiPolygon

Coordinates of a MultiPolygon are an array of Polygon coordinate arrays::

{ "type": "MultiPolygon", "coordinates": [ [102.0, 2.0], [103.0, 2.0], [103.0, 3.0], [102.0, 3.0], [102.0, 2.0], [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0, 100.2, 0.2], [100.8, 0.2], [100.8, 0.8], [100.2, 0.8], [100.2, 0.2] ] }

GeometryCollection

Each element in the geometries array of a GeometryCollection is one of the geometry objects described above::

{ "type": "GeometryCollection", "geometries": [ { "type": "Point", "coordinates": [100.0, 0.0] }, { "type": "LineString", "coordinates": [ [101.0, 0.0], [102.0, 1.0] ] } ] }

Circle

{ "type": "Circle", "coordinates": [ 100.0, 0.0 ], "radius": 0.5, "properties": { "radius_units": "km" } }

Ellipse

{ "type": "Ellipse", "coordinates": [ 100.0, 0.0 ], "maj": 20.0, "min": 10.0, "rot": 45.0, "properties": { "axis_units": "km", "rot_units": "decimal degrees" } }

Appendix B. Contributors

The GeoJSON format specification is the product of discussion on the GeoJSON list:

http://lists.geojson.org/listinfo.cgi/geojson-geojson.org

You can’t perform that action at this time.