Network Working Group T. Clarke
Internet-Draft BATC
Intended status: Informational March 22, 2019
Expires: September 23, 2019

Concise Binary Object Representation (CBOR) Tag for Coordinate Reference System (CRS) Specification


The Concise Binary Object Representation (CBOR, RFC 7049) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation.

In CBOR, one point of extensibility is the definition of CBOR tags. An existing CBOR tag, 103, allows for the representation of geographic coordinates. Proper exploitation of geographic coordinates requires an associated reference frame. The present document defines a CBOR tag for referencing the coordinate reference system (CRS) for a geographic coordinate. It is intended as the reference document for the IANA registration of the CBOR tag defined.

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Table of Contents

1. Introduction

Specifying geographic coordinates for a location on Earth requires the definition of a coordinate reference system (CRS). A CRS is made up of several components: [EDS1]

2. Objectives

This document aims to address the specification of coordinate reference systems in CBOR [RFC7049] encoded data. This is accomplished using complete CRS specification or through a well-known spatial reference identifier.

3. Applicability

This tag is designed for use with the Geographic Coordinates CBOR Tag 103 [CBOR-GC]. A CRS may be attached to a Geographic Coordinate or it may specify a default CRS for an entire scope. This is the expected application but this tag may be used in any relevant context.

4. Semantics

The CBOR CRS tag shall be associated with one of multiple CBOR data types. Each allowed type is associated with a different method of specifying a CRS.

4.1. Well-known Text

OGC [OGC] Well-known Text (WKT) [WKT] is a standarized format for CRS specification. When the CRS tag is associated with a Text String (CBOR Major type 3), the data shall be interpreted as OGC WKT. This allows for complete CRS specifcation of and subtype of CRS.

4.2. EPSG Spatial Reference Identifier

A spatial reference identifier (SRID) is a unique value which unambiguously identifies a CRS. Many vendors and registries provice SRIDs. This association is not intended to allow specification of an arbitrary SRID, but provides a way to reference an SRID in the European Pertroleum Survey Group’s (EPSG) SRID database. EPSG numbers are a de-facto standard for CRS reference and are very commonly used. EPSG numbers can be searched and referenced in a number of places including [] and []. When the CRS tag is associated with a basic numeric type (CBOR Major type 0), the data shall be interpreted as an EPSG SRID.

5. IANA Considerations

IANA is requested to allocate a tag from the Specification Required space, with the present document as the specification reference.

Tag Data Item Semantics
TBD multiple Geographic Coordinate Reference System WKT or EPSG number

RFC editor note: Please replace TBD by the tag number allocated by IANA throughout the document and delete this note.

6. Security Considerations

The security considerations of [RFC7049] apply; the tag introduced here are not expected to raise security considerations beyond those.

7. References

7.1. Normative References

[CBOR-GC] "CBOR Geographic Coordinates", n.d..
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[WKT] Open Geospatial Consortium, "Geographic information - Well-known text representation of coordinate reference systems", May 2015.

7.2. Informative References

[EDS1] Wasser, L., "Earthlab/Earth-Analytics-R-Course: Earth Analytics Course In The R Programming Language", Zenodo article, DOI 10.5281/zenodo.1326873, August 2018.
[] "MapTiler EPSG Reference Website", n.d..
[OGC] "The Open Geospatial Consortium", n.d..
[] "Spatial Reference Website", n.d..
[SR-ORG-7428] "SR-ORG:7428 WGS 84 (3D EGM96 geoid height)", n.d..

Appendix A. Examples

CRS for EPSG:4326, the World Geodetic System 1984 horizontal coordinate system used by GPS satellites, specified using an EPSG SRID.

    D8 68      # Geographic Coordinate System - tag(104)
       19 10E6 # EPSG:4326                    - unsigned(4326)

    # Diagnostic notation: 104(4326)

CRS for WGS 84 3D EGM96 geoid height [SR-ORG-7428] specified using WKT

    D8 68             # Geographic Coordinate System - tag(104)
       79 0191        # OGC WKT                      - text(401)

    # Diagnostic notation:
        104("GEOGCS[\"WGS 84 (3D EGM96 geoid height)\",
             DATUM[\"World Geodetic System 1984\",
             SPHEROID[\"WGS 84\",6378137.0,298.257223563,
             AXIS[\"Geodetic latitude\",NORTH],
             AXIS[\"Geodetic longitude\",EAST],
             AXIS[\"Gravity-related height\",UP,

Author's Address

Trevor R.H. Clarke Ball Aerospace and Technologies Corp. 2862 Presidential Dr. Fairborn, OH, 45324 United States Phone: +1-937-320-4000 EMail: