OGC GeoPackage WKT for Coordinate Reference Systems Extension

Open Geospatial Consortium

Submission Date: <yyyy-mm-dd>

Approval Date: <yyyy-mm-dd>

Publication Date: <yyyy-mm-dd>

External identifier of this OGC® document: http://www.opengis.net/doc/{doc-type}/{standard}/{m.n}

Internal reference number of this OGC® document: 21-057

Version: 1.1.0

Category: OGC® Implementation Standard

Editor: Jeff Yutzler

OGC GeoPackage WKT for Coordinate Reference Systems Extension

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Warning

This document is not an OGC Standard. This document is distributed for review and comment. This document is subject to change without notice and may not be referred to as an OGC Standard.

Recipients of this document are invited to submit, with their comments, notification of any relevant patent rights of which they are aware and to provide supporting documentation.

Document type: OGC® Standard

Document subtype: Encoding Standard

Document stage: Draft

Document language: English

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i. Abstract

This document is a revision to the GeoPackage WKT for Coordinate Reference Systems Extension that is currently published as Annex F.10 of GeoPackage Encoding Standard 1.3.0 (OGC 12-128r17). This document is designed to replace that annex. The extension defines how to encode coordinate reference systems in GeoPackage using the CRS WKT2 standard. This revision adds coordinate epochs to the encoding of coordinate reference systems in a GeoPackage.

ii. Keywords

The following are keywords to be used by search engines and document catalogues.

ogcdoc, OGC document, geopackage, crs, srs, wkt, crs_wkt, crs_wkt2

iii. Preface

The OGC GeoPackage Encoding Standard was adopted prior to the adoption of "12-063r5 OGC Well known text representation of Coordinate Reference Systems" in 13 August, 2014. As a result, the OGC GeoPackage Encoding Standard references an older document (OpenGIS® 01-009 Implementation Specification: Coordinate Transformation Services Revision 1.0) which has known ambiguities that are being encountered in the field. This extension establishes a new column in gpkg_spatial_ref_sys to contain values that conform to the new standard. In 2019, OGC adopted "18-010r7 OGC Well known text representation of Coordinate Reference Systems" as a revision to 12-063r5. The core (unextended) GeoPackage Encoding Standard supports the older CRS WKT standard. Through this extension, both encodings may be used. However, use of the newer encoding is strongly encouraged. GeoPackage clients are expected to use the newer encoding if both encodings are present.

In a dynamic CRS, coordinates of a point on the surface of the Earth may change with time. To be unambiguous, the coordinates must always be qualified with the epoch at which they are valid. (The coordinate epoch is not necessarily the epoch at which the observation was collected.) In 2021, the GeoPackage SWG added coordinate epochs to gpkg_spatial_ref_sys. Because this was the only substantive change to the GeoPackage Encoding Standard being considered at the time, the decision was made to split this extension into a separate document so that it could be considered separately.

iv. Security Considerations

No security considerations have been made for this Standard.

v. Submitting organizations

The following organizations submitted this Document to the Open Geospatial Consortium (OGC):

Image Matters
Spatialys

vi. Submitters

All questions regarding this submission should be directed to the editor or the submitters:

Name

Affiliation

Jeff Yutzler

Image Matters LLC

Even Rouault

Spatialys

1. Scope

This extension applies to the gpkg_spatial_ref_sys table and modifies the existing requirement in clause Requirement 10.

2. Conformance

This standard defines requirements for encoding coordinate reference systems and coordinate epochs in a GeoPackage.

Conformance with this standard shall be checked using all the relevant tests specified in Annex A (normative) of this document. The framework, concepts, and methodology for testing, and the criteria to be achieved to claim conformance are specified in the OGC Compliance Testing Policies and Procedures and the OGC Compliance Testing web site^[1].

3. References

The following normative documents contain provisions that, through reference in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.

OGC: OGC 12-128r17 OGC GeoPackage Encoding Standard 1.3.0, http://www.geopackage.org/spec130/ (2020)

OGC: OGC 19-005r4 OGC Abstract Specification Topic 2: Referencing by coordinates, https://docs.opengeospatial.org/as/18-005r4/18-005r4.html (2019)

OGC: OGC 18-010r7 Geographic information — Well-known text representation of coordinate reference systems 2.0.6, https://docs.opengeospatial.org/is/18-010r7/18-010r7.html (2019)

4. Terms and Definitions

This document used the terms defined in OGC Policy Directive 49, which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard and OGC documents do not use the equivalent phrases in the ISO/IEC Directives, Part 2.

This document also uses terms defined in the OGC Standard for Modular specifications (OGC 08-131r3), also known as the 'ModSpec'. The definitions of terms such as standard, specification, requirement, and conformance test are provided in the ModSpec.

For the purposes of this document, the following additional terms and definitions apply.

coordinate epoch: A property of a dynamic coordinate reference system indicating the date at which the coordinates are valid. (SOURCE: OGC 18-005r4)
Coordinate Reference System (CRS): A coordinate reference system, or spatial reference system (SRS), is a set of mathematical rules for specifying how coordinates are to be assigned to points that is related to an object by a datum. (SOURCE: ISO 19111:2019)

Note	The GeoPackage Encoding Standard (GES) uses the term "spatial reference system." In the GES, a spatial reference system is realized through a `gpkg_spatial_ref_sys` row.

well-known text (WKT): A compact machine- and human-readable representation of geometric objects that may also be used for succinctly describing the critical elements of CRS definitions. (SOURCE: OGC 18-020r7)

Note	CRS WKT2 specifically refers to a document conforming to OGC 18-010r7.

5. Conventions

This section provides details and examples for any conventions used in the document. Examples of conventions are symbols, abbreviations, use of XML schema, or special notes regarding how to read the document.

5.1. Identifiers

The normative provisions in this Standard are denoted by the URI

http://www.opengis.net/spec/{standard}/{m.n}

All requirements and conformance tests that appear in this document are denoted by partial URIs which are relative to this base.

6. WKT for Coordinate Reference Systems Extension (Informative)

Following is the information required by the GeoPackage Extension Template.

6.1. Extension Author

GeoPackage SWG, author_name gpkg.

6.2. Extension Name or Template

gpkg_crs_wkt

6.3. Extension Type

Extension of Existing Requirement 10.

6.4. Applicability

Applies to the gpkg_spatial_ref_sys table.

6.5. Scope

Read-write

7. Requirements (Normative)

7.1. Table Definition

7.1.1. gpkg_spatial_ref_sys

Requirement 115

For GeoPackages conforming to this extension, the gpkg_spatial_ref_sys table SHALL have an additional columns called definition_12_063 and epoch as per Table 1 and gpkg_spatial_ref_sys Table Definition SQL (CRS WKT Extension).

Table 1. Spatial Ref Sys Table Definition
Column Name	Column Type	Column Description	NOT NULL flag	Key
`srs_name`	TEXT	Human readable name of this SRS	true
`srs_id`	INTEGER	Unique identifier for each Spatial Reference System within a GeoPackage	true	PK
`organization`	TEXT	Case-insensitive name of the defining organization e.g. EPSG or epsg	true
`organization_coordsys_id`	INTEGER	Numeric ID of the Spatial Reference System assigned by the organization	true
`definition`	TEXT	Well-known Text [I32] Representation of the Spatial Reference System	true
`description`	TEXT	Human readable description of this SRS	false
`definition_12_063`	TEXT	Well-known Text [I34] or [I36] Representation of the Spatial Reference System	true
`epoch`	DOUBLE	Coordinate epoch as per https://docs.opengeospatial.org/as/18-005r4/18-005r4.html#68	false

Note	Previous versions of this extension specified default values for `definition` and `definition_12_063`. Those defaults have been removed for interoperability reasons but implementers should be aware that some GeoPackages may have these defaults in place.

7.2. Table Data Values

7.2.1. gpkg_extensions

Requirement 145

GeoPackages with a row in the gpkg_extensions table with an extension_name of "gpkg_crs_wkt" SHALL comply with this extension. GeoPackages complying with this extension SHALL have a row in the gpkg_extensions table as described in Table 2 (below).

Warning

Requirement 145 has been updated as part of GeoPackage 1.2.1. In 1.1.0 and 1.2.0, the table_name and column_name column values of the required gpkg_extensions row were inadvertently left unspecified. While the executable test suite running on an older GeoPackage version will not generate a failure due to missing gpkg_extensions column values, it is recommended to update these values to comply with the updated requirement on older versions as well.

Table 2. Extension Table Records
table_name	column_name	extension_name	definition	scope
`gpkg_spatial_ref_sys`	`definition_12_063`	`gpkg_crs_wkt`	see note below	`read-write`
`gpkg_spatial_ref_sys`	`definition_12_063`	`gpkg_crs_wkt_1_1`	see note below	`read-write`
`gpkg_spatial_ref_sys`	`epoch`	`gpkg_crs_wkt_1_1`	see note below	`read-write`

Note

For the definition column, use a hyperlink that describes the current implementation of this extension. While a URL like http://www.geopackage.org/spec/#extension_crs_wkt is acceptable, permalinks to specific versions are provided upon publication using the URL pattern http://www.geopackage.org/specMmP/#extension_crs_wkt where M is the major version, m is the minor version, and P is the patch. For example http://www.geopackage.org/spec121/#extension_crs_wkt is the permalink for this extension for GeoPackage 1.2.1.

7.2.2. gpkg_spatial_ref_sys

Requirement 116

Values of the definition_12_063 column SHALL be constructed per the WKT syntax in [I34] and [I36].

Note	For backwards compatibility, only use common WKT values between 12-063 and 18-010. Additional WKT values in 18-010r7 can be used when required in the CRS definition.

Requirement 117

At least one definition column SHALL be defined with a valid definition unless the value of the srs_id column is 0 or -1. Both columns SHOULD be defined. If it is not possible to produce a valid [I32] definition then the value of the definition column MAY be undefined. If it is not possible to produce a valid [I34] or [I36] definition then the value of the definition_12_063 column MAY be undefined.

Note

If, for a particular row, both the definition and definition_12_063 columns are populated, the value in the definition_12_063 column takes priority.
The coordinate epoch SHOULD be provided if available. It is therefore possible that a GeoPackage will have multiple SRSs with the same organization and organization_coordsys_id but different epochs. While the epoch should theoretically go in dataset metadata (per ISO 19111 / OGC Topic 2 as approved by OGC in March 2021), there is not currently a standard metadata profile that would be suitable.

Annex A: Conformance Class Abstract Test Suite (Normative)

A.1. Table Definition

Table Definition

Test Case ID

/extension_crs_wkt/table_def

Test Purpose

Verify that the gpkg_spatial_ref_sys table exists and has the correct definition. Extends /base/core/gpkg_spatial_ref_sys/data/table_def.

Test Method

PRAGMA table_info('gpkg_spatial_ref_sys')
Fail if returns an empty result set
Fail if result set does not include a column named 'definition_12_063' or if the column is not of type 'TEXT', notnull 1, and dflt_value 'undefined'.
Fail if result set does not include a column named 'epoch' or if the column is not of type 'DOUBLE' and notnull 0.
Pass if no failures.

Reference

Req 115

Test Type

Basic

Table Data Values

Test Case ID

/extensions/crs_wkt/extensions/data_values

Test Purpose

Verify that the gpkg_extensions table has the required rows.

Test Method

SELECT table_name, column_name, scope FROM gpkg_extensions WHERE extension_name = 'gpkg_crs_wkt';
Not testable if returns an empty result set
Fail if there is not exactly one row
Fail if scope is not "read-write"
Fail if column_name is not "definition_12_063"
Fail if table_name is not "gpkg_spatial_ref_sys"
Pass if no fails

Reference

Req 145

Test Type:

Capabilities

Test Case ID

/extensions/crs_wkt/extensions/data_values_1_1

Test Purpose

Verify that the gpkg_extensions table has the required rows for the revised extension.

Test Method

SELECT table_name, column_name, scope FROM gpkg_extensions WHERE extension_name = 'gpkg_crs_wkt_1_1';
Not testable if returns an empty result set
Fail if there is not exactly two rows
Fail if scope for both rows is not "read-write"
Fail if column_name is not "definition_12_063" or "epoch"
Fail if table_name for both rows is not "gpkg_spatial_ref_sys"
Pass if no fails

Reference

Req 145

Test Type:

Capabilities

Test Case ID

/extension_crs_wkt/data_values_default

Test Purpose

Verify that the gpkg_spatial_ref_sys table contains the required default contents. Extends /base/core/gpkg_spatial_ref_sys/data_values_default.

Test Method

SELECT organization, organization_coordsys_id, definition, definition_12_063 FROM gpkg_spatial_ref_sys WHERE srs_id = -1
1. Confirm that this returns "NONE" -1 "undefined" "undefined"
SELECT srs_id, organization, organization_coordsys_id, definition, definition_12_063 FROM gpkg_spatial_ref_sys WHERE srs_id = 0
1. Confirm that this returns "NONE" 0 "undefined" "undefined"
SELECT definition FROM gpkg_spatial_ref_sys WHERE organization IN ("epsg","EPSG") AND organization_coordsys_id 4326
1. Confirm that this is a valid CRS
SELECT definition_12_063 FROM gpkg_spatial_ref_sys WHERE organization IN ("epsg","EPSG") AND organization_coordsys_id 4326
1. Confirm that this is a valid 12-063/18-010 CRS
Pass if tests 1-4 are met
Fail otherwise

Reference

Req 116

Test Type

Capability

Test Case ID

/extension_crs_wkt/data_values_required

Test Purpose

Verify that the spatial_ref_sys table contains rows to define all srs_id values used by features and tiles in a GeoPackage. Extends /base/core/gpkg_spatial_ref_sys/data_values_required.

Test Method

SELECT definition, definition_12_063 FROM gpkg_spatial_ref_sys WHERE srs_id NOT IN (0, -1)
For each result
1. Fail if both definition values are 'undefined'
Pass if no failures

Reference

Req 117

Test Type

Capability

Annex B: Table Definition SQL (Normative)

B.1. gpkg_spatial_ref_sys

gpkg_spatial_ref_sys Table Definition SQL (CRS WKT Extension)

CREATE TABLE gpkg_spatial_ref_sys (
  srs_name TEXT NOT NULL,
  srs_id INTEGER NOT NULL PRIMARY KEY,
  organization TEXT NOT NULL,
  organization_coordsys_id INTEGER NOT NULL,
  definition  TEXT NOT NULL,
  description TEXT,
  definition_12_063 TEXT NOT NULL,
  epoch DOUBLE
);

Annex C: Revision History

Date	Release	Editor	Primary clauses modified	Description
2021-08-20	0.1	J. Yutzler	all	initial version

Date

Release

Editor

Primary clauses modified

Description

2021-08-20

0.1

J. Yutzler

all

initial version