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SDW BP Glossary Issue 197: WKT and Axis Order

From: Little, Chris <chris.little@metoffice.gov.uk>
Date: Thu, 4 May 2017 16:42:37 +0000
To: Linda van den Brink <l.vandenbrink@geonovum.nl>, "Tandy, Jeremy" <jeremy.tandy@metoffice.gov.uk>
CC: "public-sdw-wg@w3.org" <public-sdw-wg@w3.org>
Message-ID: <3DAD8A5A545D7644A066C4F2E82072883E2CA60E@EXXCMPD1DAG4.cmpd1.metoffice.gov.uk>
1. Currently:
"Well Known Text (WKT): A text markup language for representing vector geometry objects on a map, spatial reference systems of spatial objects and transformations between spatial reference systems. wikipedia)"

2. ISO definition is ISO 19162:2015, "Geographic information – Well-known text representation of coordinate reference systems", but is not in the ISO TC211 spreadsheet.

3. There are also an OGC documents such as OGC Implementation Specification for geographic information – Simple Feature Access, section 6.1.11.1. http://www.opengeospatial.org/standards/sfa for vector geometry. AKA OGC 12-063r5 and WKT2.

4. I propose adding both ISO and OGC references as they cover different parts of the WKT, and leave in the general Wikipedia introduction.
"Well Known Text (WKT): A text mark-up language for representing vector geometry objects on a map, spatial reference systems of spatial objects and transformations between spatial reference systems. [[ISO 19162:2015]] [[http://www.opengeospatial.org/standards/sfa|Simple Feature Access]] [[https://en.wikipedia.org/wiki/Well-known_text|Wikipedia]]"



5. Axis Order
From Andrea: In the [SIMPLE-FEATURES] definition of WKT, the coordinate axis-order is by default longitude / latitude, irrespective of the coordinate reference system used. The same applies to EWKT (Extended WKT) - a PostGIS extension to WKT supported also by other GIS tools - which includes a parameter (SRID) for specifying the coordinate reference system. For this reason, whenever using WKT to encode geometries, it is important that the reference WKT specification can be unambiguously determined.

6. OSGEO has a very apposite analysis of the problem at https://wiki.osgeo.org/wiki/Axis_Order_Confusion .

7. There is no ISO definition of the term, just specifications of an order in various standards. 

8. OGC has a definition ("The order of the axes as specified in the Coordinate System. The order of the coordinates in a coordinate tuple in a content payload, API, service interface, and so forth corresponds to that order"). OGC also has a policy http://www.ogcnetwork.net/axisorder which is being updated.

9. I propose the following definition, without references:
"The order in which coordinates are presented. For example, some systems use (latitude, longitude) rather than (longitude, latitude). The latter is more similar to the mathematical convention of (x, y) ordering. The order used may differ from the order used to define the coordinate system."

Chris

Chris Little
Co-Chair, OGC Meteorology & Oceanography Domain Working Group

IT Fellow - Operational Infrastructures
Met Office  FitzRoy Road  Exeter  Devon  EX1 3PB  United Kingdom
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E-mail: chris.little@metoffice.gov.uk  http://www.metoffice.gov.uk


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