Show pageOld revisionsBacklinksBack to top This page is read only. You can view the source, but not change it. Ask your administrator if you think this is wrong. ===== Introduction ===== Two-dimensional points are elements composed of two ordinates, X and Y, often corresponding to longitude and latitude. Line strings are composed of one or more pairs of points that define line segments. Polygons are composed of connected line strings that form a closed ring, and the area of the polygon is implied. For example, a point might represent a building location, a line string might represent a road or flight path, and a polygon might represent a state, city, zoning district, or city block. Self-crossing polygons are not supported, although self-crossing line strings are supported. If a line string crosses itself, it does not become a polygon. A self-crossing line string does not have any implied area. ==== Data Model ==== The Spatial data model is a hierarchical structure consisting of elements, geometries, and layers. Layers are composed of geometries, which in turn are made up of elements. === Element === An element is the basic building block of a geometry. The supported spatial element types are points, line strings, and polygons. For example, elements might model star constellations (point clusters), roads (line strings), and county boundaries (polygons). Each coordinate in an element is stored as an X,Y pair. The exterior ring and zero or more interior rings (holes) of a complex polygon are considered a single element. Point data consists of one coordinate. Line data consists of two coordinates representing a line segment of the element. Polygon data consists of coordinate pair values, one vertex pair for each line segment of the polygon. Coordinates are defined in order around the polygon (counterclockwise for an exterior polygon ring, clockwise for an interior polygon ring). === Geometry === A geometry (or geometry object) is the representation of a spatial feature, modeled as an ordered set of primitive elements. A geometry can consist of a single element, which is an instance of one of the supported primitive types, or a homogeneous or heterogeneous collection of elements. A multipolygon, such as one used to represent a set of islands, is a homogeneous collection. A heterogeneous collection is one in which the elements are of different types, for example, a point and a polygon. An example of a geometry might describe the buildable land in a town. This could be represented as a polygon with holes where water or zoning prevents construction. === Layer === A layer is a collection of geometries having the same attribute set. For example, one layer in a GIS might include topographical features, while another describes population density, and a third describes the network of roads and bridges in the area (lines and points). The geometries and associated spatial index for each layer are stored in the database in standard tables. === Coordinate System === A coordinate system (also called a spatial reference system) is a means of assigning coordinates to a location and establishing relationships between sets of such coordinates. It enables the interpretation of a set of coordinates as a representation of a position in a real world space. Any spatial data has a coordinate system associated with it. The coordinate system can be georeferenced (related to a specific representation of the Earth) or not georeferenced (that is, Cartesian, and not related to a specific representation of the Earth). If the coordinate system is georeferenced, it has a default unit of measurement (such as meters) associated with it, but you can have Spatial automatically return results in another specified unit (such as miles). (For more information about unit of measurement support, see Section 2.10.) Spatial data can be associated with a Cartesian, geodetic (geographical), projected, or local coordinate system: * Cartesian coordinates are coordinates that measure the position of a point from a defined origin along axes that are perpendicular in the represented two-dimensional or three-dimensional space. If a coordinate system is not explicitly associated with a geometry, a Cartesian coordinate system is assumed. * Geodetic coordinates (sometimes called geographic coordinates) are angular coordinates (longitude and latitude), closely related to spherical polar coordinates, and are defined relative to a particular Earth geodetic datum. (A geodetic datum is a means of representing the figure of the Earth and is the reference for the system of geodetic coordinates.) * Projected coordinates are planar Cartesian coordinates that result from performing a mathematical mapping from a point on the Earth's surface to a plane. There are many such mathematical mappings, each used for a particular purpose. * Local coordinates are Cartesian coordinates in a non-Earth (non-georeferenced) coordinate system. Local coordinate systems are often used for CAD applications and local surveys. When performing operations on geometries, Spatial uses either a Cartesian or curvilinear computational model, as appropriate for the coordinate system associated with the spatial data. === EPSG Co-Ordinate Systems === There are two co-ordinate systems and projections commonly in use. ^Term^Description^ |EPSG: 4326|Uses a coordinate system on the surface of a sphere or ellipsoid of reference| |EPSG: 3857|Uses a coordinate system PROJECTED from the surface of the sphere or ellipsoid to a flat surface.| Easy way of thinking about it: EPSG 4326 uses a coordinate system the same as a GLOBE (curved surface). EPSG 3857 uses a coordinate system the same as a MAP (flat surface). So, store data in EPSG 4326, display in EPSG 3857. Open Street Map data is stored in 4326, Map tiles are presented using EPSG 3857. === Tolerance === Tolerance is used to associate a level of precision with spatial data. Tolerance reflects the distance that two points can be apart and still be considered the same (for example, to accommodate rounding errors). The tolerance value must be a positive number greater than zero. The significance of the value depends on whether or not the spatial data is associated with a geodetic coordinate system. (Geodetic and other types of coordinate systems are described in Section 1.5.4.) * For geodetic data (such as data identified by longitude and latitude coordinates), the tolerance value is a number of meters. For example, a tolerance value of 100 indicates a tolerance of 100 meters. The tolerance value for geodetic data should not be smaller than 0.05 (5 centimeters), and in most cases it should be larger. Spatial uses 0.05 as the tolerance value for geodetic data if you specify a smaller value with the following functions: SDO_GEOM.RELATE, SDO_GEOM.SDO_DIFFERENCE, SDO_GEOM.SDO_INTERSECTION, SDO_GEOM.SDO_UNION, and SDO_GEOM.SDO_XOR; for other functions, Spatial uses the smaller tolerance value that you specify. * For non-geodetic data, the tolerance value is a number of the units that are associated with the coordinate system associated with the data. For example, if the unit of measurement is miles, a tolerance value of 0.005 indicates a tolerance of 0.005 (that is, 1/200) mile (approximately 26 feet or 7.9 meters), and a tolerance value of 2 indicates a tolerance of 2 miles. In both cases, the smaller the tolerance value, the more precision is to be associated with the data. ===== Element Terminology ===== ^Term^Meaning^ |Point| Spatial reference consitisting of one unique locationGeometry contains one location or point| |Line or Curve| Geometry contains one line string that can contain straight or circular arc segments, or both. (LINE and CURVE are synonymous in this context.)| |Polygon or Surface|Geometry contains one polygon with or without holesor one surface consisting of one or more polygons. A hole is a polygon inside a polygon| |Collection|Geometry is a heterogeneous collection of elements. COLLECTION is a superset that includes all other types.| |Multipoint|Geometry has one or more points. (MULTIPOINT is a superset of POINT.)| |Multiline or Multicurve|Geometry has one or more line strings. (MULTILINE and MULTICURVE are synonymous in this context, and each is a superset of both LINE and CURVE.)| |Multipolygon or Multisurface| Geometry can have multiple, disjoint polygons (more than one exterior boundary). or surfaces (MULTIPOLYGON is a superset of POLYGON, and MULTISURFACE is a superset of SURFACE.)| The concepts are shown below: {{:gis:geom_types.gif|}} gis/start.txt Last modified: 11/24/21 @ 08:33:08by justin