Simple Features
Simple Feature Access is a specification of the Open Geospatial Consortium, which defines a general architecture for geographic data and their geometries.
The specification describes on the one hand the storage and access to different spatial geometries and the other hand operators.
Geometry class model
Folgende instantiable classes includes the model:
- Points (Point)
- Lines ( Line String)
- Polygons (polygon )
- Several points ( multipoint)
- Several lines (multi-line string )
- Several polygons (multi- polygon )
- Collection of these geometries ( GeometryCollection )
All geometries are derived from the abstract class Geometry.
Methods on geometry objects
The Simple Feature Access specification distinguishes three different groups of methods:
Representation of the geometry
Another part of the specification includes the representation of the geometry. These include the so-called Well-known text (WKT ) or the well-known binary ( WKB ) format.
Well-known text
The Well-known text representation is mainly used to represent the geometry can be alphanumeric. Examples of Well-known text:
- Point
Point ( 10 10 ) LineString (line with " inflection points " )
Linestring (10 10, 20 20, 30 40) Polygon (area )
A polygon is enclosed by two clamps. Polygon without holes:
Polygon ( (10 10, 10 20, 20 20, 20 15, 10 10) ) with an outer ring and an inner ring (hole)
Polygon ( (0 0, 0 20, 20 20, 20 0, 0 0), ( 5 5, 5 15, 15 15, 15 5, 5 5) ) Multiple polygon
Two polygons
Multi Polygon ( ( (10 10, 10 20, 20 20, 20 15, 10 10) ), ( (60 60, 70 70, 80 60, 60 60 ))) two polygons, the first polygon with hole:
MULTIPOLYGON ((( 0 0, 0 20, 20 20, 20 0, 0 0), ( 5 5, 5 15, 15 15, 15 5, 5 5) ), ( (30 30, 30 40, 40 40, 40 30, 30 30 ))) The first bracket encases the complete multi polygon. The following two brackets enclose the respective polygon. Is this polygon is a hole so a clamp is closed and separated by a comma, the second polygon. If the second polygon geometry within the first, so is this a hole is, it lies geometrically outside the polygon, then it is in this an exclave.
Well- known binary
The well-known binary representation is a representation of the geometries transmitted as a continuous byte data string. The data types used WKB unsigned integers from one or four bytes and double precision numbers of eight bytes. Example of well-known binary:
- Point with coordinate 1.1
0101000000000000000000F03F000000000000F03F This data string is stripped into parts as follows:
01: byte order 01000000: geometry type 000000000000F03F: X 000000000000F03F: Y Representation of coordinate systems
Simple Feature Access also standardizes the representation of geodetic systems with geographic, projected or geocentric coordinate in an alphanumeric form as well -known text. Examples of the representation of coordinate systems:
- UTM zone 10 with the North American date NAD27
PROJCS [" UTM Zone 10, Northern Hemisphere " GEOGCS [" clark66 " DATE [" North_American_Datum_1927 " SPHEROID [" clark66 " 6378206.4,294.9786982 ] ], Primers [" Greenwich ", 0 ], UNIT [ "degree ", 0.0174532925199433 ] ], PROJECTION [" Transverse_Mercator " ], PARAMETER [ " latitude_of_origin ", 0 ], PARAMETER [ " central_meridian ", -123 ], PARAMETER [ " scale_factor ", 0.9996 ], PARAMETER [ " false_easting ", 500000 ], PARAMETER [ " false_northing ", 0 ], UNIT [" Meter", 1] ] Geographic coordinate system WGS84
GEOGCS [" wgs84 " DATE [" WGS_1984 " SPHEROID [" wgs84 " 6378137,298.257223563 ] TOWGS84 [ 0.000,0.000,0.000 ] ], Primers [" Greenwich ", 0 ], UNIT [ "degree ", 0.0174532925199433 ] ] use
The Simple Feature Access specification has in Geoinformatics in both open source projects and proprietary programs in a wide distribution. Two well-known LGPL licensed program libraries are the JTS Topology Suite and GEOS. JTS represents an API for Java prepared to GEOS, however, is an API for C . By using GEOS in PostGIS WKT / WKB can be used directly in the database PostgreSQL.