[Esip-documentation] ACDD 1.3 issue: geospatial_bounds, geospatial_vertical_units - x y vs. y x ordering

Fri Oct 10 13:34:49 EDT 2014

Hi Bob:

Unfortunately, the MySql references you quote below are poorly worded. WKT contains no specification of "Point". Nor does it specify that the first axis (the "x" axis) of a coordinate system is "longitude". WKT only defines <axis>, which is a part of <coordinate system>. The first number in a "point" is assigned to the first <axis> in the WKT string. So, if your WTK looks like

        GEOGCS["OSGB 1936",
            DATUM["OSGB_1936",
                SPHEROID["Airy 1830",6377563.396,299.3249646,AUTHORITY["EPSG","7001"]],
                TOWGS84[375,-111,431,0,0,0,0],
                AUTHORITY["EPSG","6277"]],
            PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
            UNIT["DMSH",0.0174532925199433,AUTHORITY["EPSG","9108"]],
            AXIS["Lat",NORTH],
            AXIS["Long",EAST],
            AUTHORITY["EPSG","4277"]],

and if you have point P = (20, 30), then because the first AXIS is defined as "Lat", then the latitude of P is 20 seconds (notice WKT units in this example are DMSH) and the longitude of P is 30 seconds. 

Regards,
--greg

On Oct 10, 2014, at 12:31 PM, Bob Simons - NOAA Federal via Esip-documentation <esip-documentation at lists.esipfed.org> wrote:

> I am thoroughly confused about the x y vs. y x (or lon lat vs lat lon) ordering
> within WKT Geometries. Some people assert that the ordering is CRS-dependent.
> I'm not disputing that CRS's, in other situations, may have a preferred ordering 
> (can someone please tell me where they are defined?),
> but I have never read anything that says that that ordering applies to WKT.
> Indeed, all of the documentation I can find for WKT only refers to 
> x y (lon lat) ordering, but maybe I'm misreading or overlooking the 
> important sections.
> 
> Below are quotes from three documents that refer only to x y (lon lat) ordering.
> Can anyone refer me to authoritative documentation that indicates the 
> ordering in WKT is CRS-dependent and thus sometimes x y (lon lat) 
> and sometimes y x (lat lon)?
> 
> Thank you.
> 
> ---
> Seeking the most authoritative documentation, I see:
> 
> 1) http://en.wikipedia.org/wiki/Well-known_text says 
> "The formats were originally defined by the Open Geospatial Consortium (OGC) and described in their Simple Feature Access ... specifications"
> So that leads to ...
> 
> 2) http://en.wikipedia.org/wiki/Simple_Features refers to 
> "OpenGIS Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture (05-126, 06-103r3, 06-103r4), current version 1.2.1"
> So that leads to ...
> 
> 3) I think this is the most authoritative documentation:
> OGC's 06-103r4_Implementation_Specification_for_Geographic_Information_-_Simple_feature_access_-_Part_1_Common_Architecture_v1.2.1.pdf
> (downloadable at http://www.opengeospatial.org/standards/sfa )
> Here are some quotes from it:
> 
> "6.1.4.1 Description 
> A Point is a 0-dimensional geometric object and represents a single location in coordinate space. A Point has an 
> x-coordinate value, a y-coordinate value. If called for by the associated Spatial Reference System, it may also 
> have coordinate values for z and m."
> 
> "7.2.2 BNF Productions for Two-Dimension Geometry WKT 
> The following BNF defines two-dimensional geometries in (x, y) coordinate spaces. With the exception of the 
> addition of polyhedral surfaces, these structures are unchanged from earlier editions of this standard. 
> <point> ::= <x> <y>"
> 
> "7.2.3 BNF Productions for Three-Dimension Geometry WKT 
> The following BNF defines geometries in 3 dimensional (x, y, z) coordinates. 
> <point z> ::= <x> <y> <z>"
> 
> "7.2.5 BNF Productions for Three-Dimension Measured Geometry WKT 
> The following BNF defines three-dimensional geometries in (x, y, z) coordinate spaces. In addition, each 
> coordinate carries an "m" ordinate value that is part of some linear reference system. 
> <point zm> ::= <x> <y> <z> <m>"
> 
> "8.2.8 Description of WKBGeometry representations 
> The Well-known Binary Representation for Geometry is described below. The basic building block is the 
> representation for a Point, which consists of a number Doubles, depending on the coordinate referece system in 
> use for the geometry. The representations for other geometric objects are built using the representations for 
> geometric objects that have already been defined. 
> // Basic Type definitions 
> // byte : 1 byte 
> // uint32 : 32 bit unsigned integer (4 bytes) 
> // double : double precision number (8 bytes) 
> // Building Blocks : Coordinate, LinearRing 
> Point { 
>  double x; 
>  double y} 
> PointZ { 
>  double x; 
>  double y; 
>  double z} "
> 
> The document does not contain these text strings: "y, x" or "<y> <x>"
> 
> -----
> The book "Pro Spatial with SQL Server 2012" By Alastair Aitchison
> http://books.google.com/books?id=pHeWWBY3cPEC&pg=PA25&lpg=PA25&dq=wkt+point+syntax&source=bl&ots=STQk8kv__q&sig=aigFxz103N15G4yOCu0lCoXXvKI&hl=en&sa=X&ei=GQM4VPWLPMf7iwK454FA&ved=0CFcQ6AEwCTgK#v=onepage&q=wkt%20point%20syntax&f=false
> the very bottom of page 25 says
> "Defining a Point from geographic coordinates follows the same convention,
> but with one thing to watch out for: whereas in everyday language it is common
> to refer to coordinates of "latitude and longitude" (in that order), when you 
> write geographic coordinates in WKT the longitude coordinate always comes
> first, then the latitude coordinate. The WKT syntax for a geography
> Point located at a latitude of 40º and longitude of 60º is therefore:
>   POINT(60 40)
> "
> 
> -----
> The MySql 5.7 documentation at
> http://dev.mysql.com/doc/refman/5.7/en/gis-data-formats.html#gis-wkt-format says
> 
> mysql> SELECT X(POINT(15, 20));
> +------------------+
> | X(POINT(15, 20)) |
> +------------------+
> |               15 |
> +------------------+
> 
> mysql> SELECT X(GeomFromText('POINT(15 20)'));
> +---------------------------------+
> | X(GeomFromText('POINT(15 20)')) |
> +---------------------------------+
> |                              15 |
> +---------------------------------+
> 
> ---
> 
> 
> -- 
> Sincerely,
> Bob Simons 
> IT Specialist 
> Environmental Research Division 
> NOAA Southwest Fisheries Science Center 
> 99 Pacific St, Suite 255A 
> Monterey, CA 93940 
> Phone: (831)333-9878 (Changed 2014-08-20) 
> Fax: (831)648-8440 
> Email: bob.simons at noaa.gov 
> 
> The contents of this message are mine personally and 
> do not necessarily reflect any position of the 
> Government or the National Oceanic and Atmospheric 
> Administration. 
> <>< <>< <>< <>< <>< <>< <>< <>< <>< <><
> 
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