Ch01: Spatial Reference Systems (SRS)

• Spatial Reference Systems - coordinate system to identify particular points on the earth (e.g. WGS84)
• geoid - the word used to describe the shape of the earth.
• reference ellipsoid - a smooth model used by scientists to approximate the shape of the earth.
  • oblate spheroid - most commonly used reference ellipsoid
  • Most reference ellipsoids aren't meant to model the entire earth, but some such as the World Geodetic System 1984 do.
• SQL Server 2012 recognizes spatial reference systems based on a number of different reference ellipsoids.

The above table demonstrates the Size/Shape characteristics of a SRS.

To complete the package we need reference points to place the ellipsoid in the correct position relative to the earth.

Definition [Spatial Reference System] A geodetic datum is made up of a reference ellipsoid and a set of reference points to position the ellipsoid relative to the earth.

Geographic Coordinate Systems

• Latitude - 0 degrees at equator, 90 degrees at the north pole, -90 degrees at the south pole.
• Longitude - 0-180 degrees east or west of the prime meridian

Projected Coordinate Systems

Because we carry maps on flat surfaces (instead of geoids), the area of the world that we want to see on the map needs to be projected onto the Cartesian coordinate plane.

• Conformal Projection - any projection that preserves the local shape of objects on the resulting map.

• Mercador projection is such a map projection and is used by Bing and Google maps.

Universal Transverse Mercator Projection is a grid composed of many projections laid side by side. Globe is sliced (north/south) into 60 strips (of 6 degrees each). Each strip is divided at the equator. E.g. Chattanooga, TN is in Zone 16N.

Projection Parameters

• Easting - X Coordinate (measured east of origin)
• Northing - Y Coordinate (measured north of origin)

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Components of a Spatial Reference System

(a) Projection parameters are only defined for spatial reference systems based on projected coordinate systems.

Spaitial Reference Identifiers (SRIDs)

SRIDs identify all the data like a pointer. E.g. 4326 = WGS84; 32136 = Tennessee State Plane. See details at http://www.epsg-registry.org/.

Well-Known Text (WKT) format is an industry standard format for expressing spatial information defined by the Open Geospatial Consortium (OGC).

Try the following:

SELECT well_known_text
FROM sys.spatial_reference_systems
WHERE authority_name = 'EPSG' AND authorized_spatial_reference_id = 4326;

Results in:

GEOGCS["WGS 84", DATUM["World Geodetic System 1984", ELLIPSOID["WGS 84", 6378137, 298.257223563]], PRIMEM["Greenwich", 0], UNIT["Degree", 0.0174532925199433]]

Another example for projected is the Tennessee state plane:

PROJCS["NAD83 / Tennessee",
    GEOGCS["NAD83",
        DATUM["North_American_Datum_1983",
            SPHEROID["GRS 1980",6378137,298.257222101,
                AUTHORITY["EPSG","7019"]],
            AUTHORITY["EPSG","6269"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.01745329251994328,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4269"]],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    PROJECTION["Lambert_Conformal_Conic_2SP"],
    PARAMETER["standard_parallel_1",36.41666666666666],
    PARAMETER["standard_parallel_2",35.25],
    PARAMETER["latitude_of_origin",34.33333333333334],
    PARAMETER["central_meridian",-86],
    PARAMETER["false_easting",600000],
    PARAMETER["false_northing",0],
    AUTHORITY["EPSG","32136"],
    AXIS["X",EAST],
    AXIS["Y",NORTH]]

■ Note The Well-Known Text format in which SQL Server stores the properties of spatial reference systems in the sys.spatial_reference_systems table is exactly the same format as used in the .PRJ file used to describe the spatial reference in which the data in an ESRI shapefile are stored.

David Mercer and Thomas Olsen presented this chapter. See "Still waiting for file"...