Geojson Ogc Knowledge Patch

GeoJSON / OGC JSON-FG Knowledge Patch

Use this skill when producing, validating, transforming, or serving JSON-FG, especially where an RFC 7946 fallback must coexist with alternate coordinate reference systems, measures, extended geometry types, or typed features.

Treat the reference files as normative implementation guidance. Start with the quick rules below, then open the task-specific reference before editing data, schemas, validators, or HTTP behavior.

Reference index

Breaking and compatibility-sensitive rules

Keep the media type stable

Use application/geo+json for JSON-FG representations. Treat application/fg+json and application/vnd.ogc.fg+json only as compatibility values for earlier implementations.

Do not infer the representation profile from a legacy media type. Use a profile link and, for Web APIs, the profile query parameter.

Separate RFC 7946 geometry from extended geometry

Use geometry only for an ordinary RFC 7946 geometry in WGS 84. It cannot carry coordRefSys or measures.

Use place for a geometry that:

• uses an alternate CRS;
• has a measure coordinate; or
• uses a JSON-FG extended geometry type.

When place is present, geometry may be null or a distinct WGS 84 fallback for GeoJSON-only readers. Do not copy alternate-CRS coordinates into geometry.

An ordinary Point, LineString, Polygon, or collection in CRS84 without measures does not belong in place; keep it in geometry and omit or null place.

Put CRS metadata in the right scope

coordRefSys applies to place and to geometries inside properties, never to geometry. In core documents, declare it only on the root object, so all primary geometries use the same CRS.

Do not assume longitude-latitude ordering after selecting another CRS. Coordinate order follows the axis order defined by the closest applicable CRS.

Declare conformance once, completely

Only the root object has conformsTo. Always include the core conformance URI:

http://www.opengis.net/spec/json-fg-1/1.0/conf/core

If a capability occurs anywhere in the document, also add its class URI. The recognized suffixes are:

• polyhedra
• prisms
• circular-arcs
• measures
• types-schemas

Nested geometries do not repeat conformsTo.

Common authoring patterns

Publish an alternate-CRS feature with a fallback

{
  "type": "Feature",
  "conformsTo": [
    "http://www.opengis.net/spec/json-fg-1/1.0/conf/core"
  ],
  "coordRefSys": "http://www.opengis.net/def/crs/EPSG/0/27700",
  "geometry": {
    "type": "Point",
    "coordinates": [-6.2580609, 55.6824121]
  },
  "place": {
    "type": "Point",
    "coordinates": [132440.63, 651435.92]
  },
  "properties": {}
}

The two coordinate arrays describe the same feature for different readers; they are not required to be bytewise copies.

Select a CRS representation

Set coordRefSys to one of:

• a CRS URI string;
• a Reference object with href, and optionally epoch for a dynamic CRS; or
• an array of references for an ad hoc compound CRS.

If omitted, two-dimensional positions default to OGC:CRS84 and three-dimensional positions default to OGC:CRS84h. Do not count an appended M value when choosing between those defaults.

For a local CRS whose definition is unknown, use the OGC Engineering2D or Engineering3D CRS URI as appropriate.

Add a temporal extent

time may be absent, null, or an extensible object containing any of date, timestamp, and interval.

{
  "time": {
    "interval": ["2014-04-24T10:50:18Z", ".."]
  }
}

Use RFC 3339 full-date values for date and RFC 3339 date-time values with Z for timestamp. An interval is a closed two-item range. Both endpoints must be dates or both must be timestamps; .. makes an endpoint unbounded. When multiple temporal forms occur, their represented values must intersect.

Append measure values consistently

Declare a measures object with required Boolean enabled; optionally add unit and description. When enabled, append one M value after every position's CRS coordinates:

• a 2D CRS produces three-element positions;
• a 3D CRS produces four-element positions.

The declaration scopes downward. Directly embedded geometries in geometries or base cannot override it. Absence means that positions have no M value.

{
  "type": "LineString",
  "coordRefSys": "http://www.opengis.net/def/crs/OGC/0/CRS84",
  "measures": {
    "enabled": true,
    "unit": "km",
    "description": "distance from route origin"
  },
  "coordinates": [[8.0, 50.0, 0.0], [8.1, 50.1, 15.7]],
  "conformsTo": [
    "http://www.opengis.net/spec/json-fg-1/1.0/conf/core",
    "http://www.opengis.net/spec/json-fg-1/1.0/conf/measures"
  ]
}

Describe feature collections

Under the types-schemas conformance class, give a root feature a string featureType. For a feature collection, either set it once at collection level or set it on every feature.

For a homogeneous collection, declare its shared type and a geometryDimension: 0 for points, 1 for curves, 2 for surfaces, and 3 for solids or prisms. Use null for a mixed or unknown dimension.

Use featureSchema for a logical feature schema conforming to OGC API Features Part 5. It is a URI string for one feature type or an object mapping type codes to schema URIs for multiple types. It is not directly a JSON validation schema; provide a describedby link for a downloadable JSON Schema.

Extended geometry checklist

Before emitting an extended geometry, verify all of the following:

• the root declares the corresponding conformance class;
• the geometry is in place when used as a feature's primary extended shape;
• the root CRS has the dimensions required by the geometry;
• every position has the CRS coordinate count plus an M value only when measures are enabled;
• rings, shells, curve joins, and bounds satisfy the type-specific rules in Extended geometry types.

For unknown members, readers should continue processing. For an unrecognized place.type, readers should treat place as null rather than rejecting the whole feature.

Web API checklist

For GeoJSON-producing GET and HEAD operations:

1. Support the profile parameter with at least rfc7946 and jsonfg.
2. Prefer rfc7946 when no profile is requested.
3. Return application/geo+json.
4. Add a Link with rel="profile" for the selected profile URI.
5. For jsonfg-plus, include a non-null RFC 7946 geometry fallback whenever place is non-null.

The rfc7946 profile has neither place nor a JSON-FG conformance URI. Non-WGS 84 coordinates in that profile require prior arrangement between the parties.