The GIS objects supported by PostGIS are all the vector types defined in the "Simple Features for SQL 1.2.1" standard defined by the OpenGIS Consortium (OGC), and the ISO "SQL/MM Part 3: Spatial" document. In addition, PostGIS supports a raster type (no standards exist to follow), and a topology model (following an early draft ISO standard for topology that has not been published as yet).
The OGC and ISO standards define 2D (x/y), 3D (x/y/z, x/y/m) and 4D (x/y/z/m) variants of points, lines, polygons, curved features, polyhedra, and TINS.
The OGC and ISO specifications define both text and binary representations for geometry objects, WKT and WKB. Both representations include information about the type of the object and the coordinates that form the object.
Examples of the text representations (WKT) of the spatial objects of the features are as follows:
The OpenGIS specification also requires that the internal storage format of spatial objects include a spatial referencing system identifier (SRID). The SRID is required when creating spatial objects for insertion into the database.
Input/Output of these formats are available using the following interfaces:
bytea WKB = ST_AsBinary(geometry); text WKT = ST_AsText(geometry); geometry = ST_GeomFromWKB(bytea WKB, SRID); geometry = ST_GeometryFromText(text WKT, SRID);
For example, a valid insert statement to create and insert an OGC spatial object would be:
INSERT INTO geotable ( the_geom, the_name ) VALUES ( ST_GeomFromText('POINT(-126.4 45.32)', 312), 'A Place');
OGC formats only support 2d geometries, and the associated SRID is *never* embedded in the input/output representations.
PostGIS extended formats are currently superset of OGC one (every valid WKB/WKT is a valid EWKB/EWKT) but this might vary in the future, specifically if OGC comes out with a new format conflicting with our extensions. Thus you SHOULD NOT rely on this feature!
PostGIS EWKB/EWKT add 3dm,3dz,4d coordinates support and embedded SRID information.
Examples of the text representations (EWKT) of the extended spatial objects of the features are as follows. The * ones are new in this version of PostGIS:
Input/Output of these formats are available using the following interfaces:
bytea EWKB = ST_AsEWKB(geometry); text EWKT = ST_AsEWKT(geometry); geometry = ST_GeomFromEWKB(bytea EWKB); geometry = ST_GeomFromEWKT(text EWKT);
For example, a valid insert statement to create and insert a PostGIS spatial object would be:
INSERT INTO geotable ( the_geom, the_name ) VALUES ( ST_GeomFromEWKT('SRID=312;POINTM(-126.4 45.32 15)'), 'A Place' )
The "canonical forms" of a PostgreSQL type are the representations you get with a simple query (without any function call) and the one which is guaranteed to be accepted with a simple insert, update or copy. For the postgis 'geometry' type these are:
- Output - binary: EWKB ascii: HEXEWKB (EWKB in hex form) - Input - binary: EWKB ascii: HEXEWKB|EWKT
For example this statement reads EWKT and returns HEXEWKB in the process of canonical ascii input/output:
=# SELECT 'SRID=4;POINT(0 0)'::geometry; geometry ---------------------------------------------------- 01010000200400000000000000000000000000000000000000 (1 row)
The SQL Multimedia Applications Spatial specification extends the simple features for SQL spec by defining a number of circularly interpolated curves.
The SQL-MM definitions include 3dm, 3dz and 4d coordinates, but do not allow the embedding of SRID information.
The well-known text extensions are not yet fully supported. Examples of some simple curved geometries are shown below:
PostGIS prior to 1.4 does not support compound curves in a curve polygon, but PostGIS 1.4 and above do support the use of Compound Curves in a Curve Polygon.
All floating point comparisons within the SQL-MM implementation are performed to a specified tolerance, currently 1E-8.
The geography type provides native support for spatial features represented on "geographic" coordinates (sometimes called "geodetic" coordinates, or "lat/lon", or "lon/lat"). Geographic coordinates are spherical coordinates expressed in angular units (degrees).
The basis for the PostGIS geometry type is a plane. The shortest path between two points on the plane is a straight line. That means calculations on geometries (areas, distances, lengths, intersections, etc) can be calculated using cartesian mathematics and straight line vectors.
The basis for the PostGIS geographic type is a sphere. The shortest path between two points on the sphere is a great circle arc. That means that calculations on geographies (areas, distances, lengths, intersections, etc) must be calculated on the sphere, using more complicated mathematics. For more accurate measurements, the calculations must take the actual spheroidal shape of the world into account, and the mathematics becomes very complicated indeed.
Because the underlying mathematics is much more complicated, there are fewer functions defined for the geography type than for the geometry type. Over time, as new algorithms are added, the capabilities of the geography type will expand.
One restriction is that it only supports WGS 84 long lat (SRID:4326). It uses a new data type called geography. None of the GEOS functions support this new type. As a workaround one can convert back and forth between geometry and geography types.
The new geography type uses the PostgreSQL 8.3+ typmod definition format so that a table with a geography field can be added in a single step. All the standard OGC formats except for curves are supported.
The geography type only supports the simplest of simple features. Standard geometry type data will autocast to geography if it is of SRID 4326. You can also use the EWKT and EWKB conventions to insert data.
CREATE TABLE testgeog(gid serial PRIMARY KEY, the_geog geography(POINT,4326) );Creating a table with z coordinate point
CREATE TABLE testgeog(gid serial PRIMARY KEY, the_geog geography(POINTZ,4326) );
The new geography fields don't get registered in the geometry_columns . They get registered in a new view called geography_columns which is a view against the system catalogs so is always automatically kept up to date without need for an AddGeom. like function.
Now, check the "geography_columns" view and see that your table is listed.
You can create a new table with a GEOGRAPHY column using the CREATE TABLE syntax. Unlike GEOMETRY, there is no need to run a separate AddGeometryColumns() process to register the column in metadata.
CREATE TABLE global_points ( id SERIAL PRIMARY KEY, name VARCHAR(64), location GEOGRAPHY(POINT,4326) );
Note that the location column has type GEOGRAPHY and that geography type supports two optional modifier: a type modifier that restricts the kind of shapes and dimensions allowed in the column; an SRID modifier that restricts the coordinate reference identifier to a particular number.
Allowable values for the type modifier are: POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON. The modifier also supports dimensionality restrictions through suffixes: Z, M and ZM. So, for example a modifier of 'LINESTRINGM' would only allow line strings with three dimensions in, and would treat the third dimension as a measure. Similarly, 'POINTZM' would expect four dimensional data.
The SRID modifier is currently of limited use: only 4326 (WGS84) is allowed as a value. If you do not specify an SRID, the a value 0 (undefined spheroid) will be used, and all calculations will proceed using WGS84 anyways.
In the future, alternate SRIDs will allow calculations on spheroids other than WGS84.
Once you have created your table, you can see it in the GEOGRAPHY_COLUMNS table:
-- See the contents of the metadata view SELECT * FROM geography_columns;
You can insert data into the table the same as you would if it was using a GEOMETRY column:
-- Add some data into the test table INSERT INTO global_points (name, location) VALUES ('Town', ST_GeographyFromText('SRID=4326;POINT(-110 30)') ); INSERT INTO global_points (name, location) VALUES ('Forest', ST_GeographyFromText('SRID=4326;POINT(-109 29)') ); INSERT INTO global_points (name, location) VALUES ('London', ST_GeographyFromText('SRID=4326;POINT(0 49)') );
Creating an index works the same as GEOMETRY. PostGIS will note that the column type is GEOGRAPHY and create an appropriate sphere-based index instead of the usual planar index used for GEOMETRY.
-- Index the test table with a spherical index CREATE INDEX global_points_gix ON global_points USING GIST ( location );
Query and measurement functions use units of meters. So distance parameters should be expressed in meters, and return values should be expected in meters (or square meters for areas).
-- Show a distance query and note, London is outside the 1000km tolerance SELECT name FROM global_points WHERE ST_DWithin(location, ST_GeographyFromText('SRID=4326;POINT(-110 29)'), 1000000);
You can see the power of GEOGRAPHY in action by calculating the how close a plane flying from Seattle to London (LINESTRING(-122.33 47.606, 0.0 51.5)) comes to Reykjavik (POINT(-21.96 64.15)).
-- Distance calculation using GEOGRAPHY (122.2km) SELECT ST_Distance('LINESTRING(-122.33 47.606, 0.0 51.5)'::geography, 'POINT(-21.96 64.15)':: geography);
-- Distance calculation using GEOMETRY (13.3 "degrees") SELECT ST_Distance('LINESTRING(-122.33 47.606, 0.0 51.5)'::geometry, 'POINT(-21.96 64.15)':: geometry);
The GEOGRAPHY type calculates the true shortest distance over the sphere between Reykjavik and the great circle flight path between Seattle and London.
Great Circle mapper The GEOMETRY type calculates a meaningless cartesian distance between Reykjavik and the straight line path from Seattle to London plotted on a flat map of the world. The nominal units of the result might be called "degrees", but the result doesn't correspond to any true angular difference between the points, so even calling them "degrees" is inaccurate.
The new GEOGRAPHY type allows you to store data in longitude/latitude coordinates, but at a cost: there are fewer functions defined on GEOGRAPHY than there are on GEOMETRY; those functions that are defined take more CPU time to execute.
The type you choose should be conditioned on the expected working area of the application you are building. Will your data span the globe or a large continental area, or is it local to a state, county or municipality?
Refer to Section�13.10, “PostGIS Function Support Matrix” for compare between what is supported for Geography vs. Geometry. For a brief listing and description of Geography functions, refer to Section�13.3, “PostGIS Geography Support Functions”
Do you calculate on the sphere or the spheroid?
By default, all distance and area calculations are done on the spheroid. You should find that the results of calculations in local areas match up will with local planar results in good local projections. Over larger areas, the spheroidal calculations will be more accurate than any calculation done on a projected plane.
All the geography functions have the option of using a sphere calculation, by setting a final boolean parameter to 'FALSE'. This will somewhat speed up calculations, particularly for cases where the geometries are very simple.
What about the date-line and the poles?
All the calculations have no conception of date-line or poles, the coordinates are spherical (longitude/latitude) so a shape that crosses the dateline is, from a calculation point of view, no different from any other shape.
What is the longest arc you can process?
We use great circle arcs as the "interpolation line" between two points. That means any two points are actually joined up two ways, depending on which direction you travel along the great circle. All our code assumes that the points are joined by the *shorter* of the two paths along the great circle. As a consequence, shapes that have arcs of more than 180 degrees will not be correctly modelled.
Why is it so slow to calculate the area of Europe / Russia / insert big geographic region here ?
Because the polygon is so darned huge! Big areas are bad for two reasons: their bounds are huge, so the index tends to pull the feature no matter what query you run; the number of vertices is huge, and tests (distance, containment) have to traverse the vertex list at least once and sometimes N times (with N being the number of vertices in the other candidate feature).
As with GEOMETRY, we recommend that when you have very large polygons, but are doing queries in small areas, you "denormalize" your geometric data into smaller chunks so that the index can effectively subquery parts of the object and so queries don't have to pull out the whole object every time. Just because you *can* store all of Europe in one polygon doesn't mean you *should*.
The OpenGIS "Simple Features Specification for SQL" defines standard GIS object types, the functions required to manipulate them, and a set of meta-data tables. In order to ensure that meta-data remain consistent, operations such as creating and removing a spatial column are carried out through special procedures defined by OpenGIS.
There are two OpenGIS meta-data tables: SPATIAL_REF_SYS and GEOMETRY_COLUMNS . The SPATIAL_REF_SYS table holds the numeric IDs and textual descriptions of coordinate systems used in the spatial database.
The spatial_ref_sys table is a PostGIS included and OGC compliant database table that lists over 3000 known spatial reference systems and details needed to transform/reproject between them.
Although the PostGIS spatial_ref_sys table contains over 3000 of the more commonly used spatial reference system definitions that can be handled by the proj library, it does not contain all known to man and you can even define your own custom projection if you are familiar with proj4 constructs. Keep in mind that most spatial reference systems are regional and have no meaning when used outside of the bounds they were intended for.
An excellent resource for finding spatial reference systems not defined in the core set is http://spatialreference.org/
Some of the more commonly used spatial reference systems are: 4326 - WGS 84 Long Lat, 4269 - NAD 83 Long Lat, 3395 - WGS 84 World Mercator, 2163 - US National Atlas Equal Area, Spatial reference systems for each NAD 83, WGS 84 UTM zone - UTM zones are one of the most ideal for measurement, but only cover 6-degree regions.
Various US state plane spatial reference systems (meter or feet based) - usually one or 2 exists per US state. Most of the meter ones are in the core set, but many of the feet based ones or ESRI created ones you will need to pull from spatialreference.org.
For details on determining which UTM zone to use for your area of interest, check out the utmzone PostGIS plpgsql helper function.
The SPATIAL_REF_SYS table definition is as follows:
CREATE TABLE spatial_ref_sys ( srid INTEGER NOT NULL PRIMARY KEY, auth_name VARCHAR(256), auth_srid INTEGER, srtext VARCHAR(2048), proj4text VARCHAR(2048) )
The SPATIAL_REF_SYS columns are as follows:
An integer value that uniquely identifies the Spatial Referencing System (SRS) within the database.
The name of the standard or standards body that is being cited for this reference system. For example, "EPSG" would be a valid AUTH_NAME .
The ID of the Spatial Reference System as defined by the Authority cited in the AUTH_NAME . In the case of EPSG, this is where the EPSG projection code would go.
The Well-Known Text representation of the Spatial Reference System. An example of a WKT SRS representation is:
PROJCS["NAD83 / UTM Zone 10N", GEOGCS["NAD83", DATUM["North_American_Datum_1983", SPHEROID["GRS 1980",6378137,298.257222101] ], PRIMEM["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["metre",1] ]
For a listing of EPSG projection codes and their corresponding WKT representations, see http://www.opengeospatial.org/. For a discussion of WKT in general, see the OpenGIS "Coordinate Transformation Services Implementation Specification" at http://www.opengeospatial.org/standards. For information on the European Petroleum Survey Group (EPSG) and their database of spatial reference systems, see http://www.epsg.org.
PostGIS uses the Proj4 library to provide coordinate transformation capabilities. The PROJ4TEXT column contains the Proj4 coordinate definition string for a particular SRID. For example:
+proj=utm +zone=10 +ellps=clrk66 +datum=NAD27 +units=m
For more information about, see the Proj4 web site at http://trac.osgeo.org/proj/. The spatial_ref_sys.sql file contains both SRTEXT and PROJ4TEXT definitions for all EPSG projections.
In versions of PostGIS prior to 2.0.0, geometry_columns was a table that could be directly edited, and sometimes got out of synch with the actual definition of the geometry columns. In PostGIS 2.0.0, GEOMETRY_COLUMNS became a view with the same front-facing structure as prior versions, but reading from database system catalogs Its structure is as follows:
\d geometry_columns
View "public.geometry_columns" Column | Type | Modifiers -------------------+------------------------+----------- f_table_catalog | character varying(256) | f_table_schema | character varying(256) | f_table_name | character varying(256) | f_geometry_column | character varying(256) | coord_dimension | integer | srid | integer | type | character varying(30) |
The column meanings have not changed from prior versions and are:
F_TABLE_CATALOG, F_TABLE_SCHEMA, F_TABLE_NAMEThe fully qualified name of the feature table containing the geometry column. Note that the terms "catalog" and "schema" are Oracle-ish. There is not PostgreSQL analogue of "catalog" so that column is left blank -- for "schema" the PostgreSQL schema name is used ( public is the default).
The name of the geometry column in the feature table.
The spatial dimension (2, 3 or 4 dimensional) of the column.
The ID of the spatial reference system used for the coordinate geometry in this table. It is a foreign key reference to the SPATIAL_REF_SYS .
The type of the spatial object. To restrict the spatial column to a single type, use one of: POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON, GEOMETRYCOLLECTION or corresponding XYM versions POINTM, LINESTRINGM, POLYGONM, MULTIPOINTM, MULTILINESTRINGM, MULTIPOLYGONM, GEOMETRYCOLLECTIONM. For heterogeneous (mixed-type) collections, you can use "GEOMETRY" as the type.
This attribute is (probably) not part of the OpenGIS specification, but is required for ensuring type homogeneity.
Creating a table with spatial data, can be done in one step. As shown in the following example which creates a roads table with a 2D linestring geometry column in WGS84 long lat
CREATE TABLE ROADS ( ID int4 , ROAD_NAME varchar(25), geom geometry(LINESTRING,4326) );
We can add additional columns using standard ALTER TABLE command as we do in this next example where we add a 3-D linestring.
ALTER TABLE roads ADD COLUMN geom2 geometry(LINESTRINGZ,4326);
For backwards compability, you can still create a spatial table in two stages using the management functions.
AddGeometryColumn( , , , , , )Or, using current schema:
AddGeometryColumn( , , , , )
Here is an example of SQL used to create a table and add a spatial column (assuming that an SRID of 128 exists already):
CREATE TABLE parks ( park_id INTEGER, park_name VARCHAR, park_date DATE, park_type VARCHAR ); SELECT AddGeometryColumn('parks', 'park_geom', 128, 'MULTIPOLYGON', 2 );
Here is another example, using the generic "geometry" type and the undefined SRID value of 0:
CREATE TABLE roads ( road_id INTEGER, road_name VARCHAR ); SELECT AddGeometryColumn( 'roads', 'roads_geom', 0, 'GEOMETRY', 3 );
The AddGeometryColumn() approach creates a geometry column and also registers the new column in the geometry_columns table. If your software utilizes geometry_columns, then any geometry columns you need to query by must be registered in this view. Starting with PostGIS 2.0, geometry_columns is no longer editable and all geometry columns are autoregistered.
However they may be registered as a generic geometry column if the column was not defined as a specific type during creation.
Two of the cases where this may happen, but you can't use AddGeometryColumn, is in the case of SQL Views and bulk inserts. For these cases, you can correct the registration in the geometry_columns table by constraining the column. Note in PostGIS 2.0+ if your column is typmod based, the creation process would register it correctly, so no need to do anything.
--Lets say you have a view created like this CREATE VIEW public.vwmytablemercator AS SELECT gid, ST_Transform(geom,3395) As geom, f_name FROM public.mytable; -- For it to register correctly in PostGIS 2.0+ -- You need to cast the geometry -- DROP VIEW public.vwmytablemercator; CREATE VIEW public.vwmytablemercator AS SELECT gid, ST_Transform(geom,3395)::geometry(Geometry, 3395) As geom, f_name FROM public.mytable; -- If you know the geometry type for sure is a 2D POLYGON then you could do DROP VIEW public.vwmytablemercator; CREATE VIEW public.vwmytablemercator AS SELECT gid, ST_Transform(geom,3395)::geometry(Polygon, 3395) As geom, f_name FROM public.mytable;
--Lets say you created a derivative table by doing a bulk insert SELECT poi.gid, poi.geom, citybounds.city_name INTO myschema.my_special_pois FROM poi INNER JOIN citybounds ON ST_Intersects(citybounds.geom, poi.geom); --Create 2d index on new table CREATE INDEX idx_myschema_myspecialpois_geom_gist ON myschema.my_special_pois USING gist(geom); -- If your points are 3D points or 3M points, -- then you might want to create an nd index instead of a 2d index -- like so CREATE INDEX my_special_pois_geom_gist_nd ON my_special_pois USING gist(geom gist_geometry_ops_nd); --To manually register this new table's geometry column in geometry_columns -- Note that this approach will work for both PostGIS 2.0+ and PostGIS 1.4+ -- For PostGIS 2.0 it will also change the underlying structure of the table to -- to make the column typmod based. -- For PostGIS prior to 2.0, this technique can also be used to register views SELECT populate_geometry_columns('myschema.my_special_pois'::regclass); --If you are using PostGIS 2.0 and for whatever reason, you -- you need the old constraint based definition behavior -- (such as case of inherited tables where all children do not have the same type and srid) -- set new optional use_typmod argument to false SELECT populate_geometry_columns('myschema.my_special_pois'::regclass, false);
Although the old-constraint based method is still supported, a constraint-based geomentry column used directly in a view, will not register correctly in geometry_columns, as will a typmod one. In this example we define a column using typmod and another using constraints.
CREATE TABLE pois_ny(gid SERIAL PRIMARY KEY , poi_name text, cat varchar(20) , geom geometry(POINT,4326) ); SELECT AddGeometryColumn('pois_ny', 'geom_2160', 2160, 'POINT', 2, false);
If we run in psql
\d pois_ny;
We observe they are defined differently -- one is typmod, one is constraint
Table "public.pois_ny" Column | Type | Modifiers -----------+-----------------------+------------------------------------------------------ gid | integer | not null default nextval('pois_ny_gid_seq'::regclass) poi_name | text | cat | character varying(20) | geom | geometry(Point,4326) | geom_2160 | geometry | Indexes: "pois_ny_pkey" PRIMARY KEY, btree (gid) Check constraints: "enforce_dims_geom_2160" CHECK (st_ndims(geom_2160) = 2) "enforce_geotype_geom_2160" CHECK (geometrytype(geom_2160) = 'POINT'::text OR geom_2160 IS NULL) "enforce_srid_geom_2160" CHECK (st_srid(geom_2160) = 2160)
In geometry_columns, they both register correctly
SELECT f_table_name, f_geometry_column, srid, type FROM geometry_columns WHERE f_table_name = 'pois_ny';
f_table_name | f_geometry_column | srid | type -------------+-------------------+------+------- pois_ny | geom | 4326 | POINT pois_ny | geom_2160 | 2160 | POINT
However -- if we were to create a view like this
CREATE VIEW vw_pois_ny_parks AS SELECT * FROM pois_ny WHERE cat='park'; SELECT f_table_name, f_geometry_column, srid, type FROM geometry_columns WHERE f_table_name = 'vw_pois_ny_parks';
The typmod based geom view column registers correctly, but the constraint based one does not.
f_table_name | f_geometry_column | srid | type ------------------+-------------------+------+---------- vw_pois_ny_parks | geom | 4326 | POINT vw_pois_ny_parks | geom_2160 | 0 | GEOMETRY
This may change in future versions of PostGIS, but for now To force the constraint based view column to register correctly, we need to do this:
DROP VIEW vw_pois_ny_parks; CREATE VIEW vw_pois_ny_parks AS SELECT gid, poi_name, cat , geom , geom_2160::geometry(POINT,2160) As geom_2160 FROM pois_ny WHERE cat='park'; SELECT f_table_name, f_geometry_column, srid, type FROM geometry_columns WHERE f_table_name = 'vw_pois_ny_parks';
f_table_name | f_geometry_column | srid | type ------------------+-------------------+------+------- vw_pois_ny_parks | geom | 4326 | POINT vw_pois_ny_parks | geom_2160 | 2160 | POINT
PostGIS is compliant with the Open Geospatial Consortium’s (OGC) OpenGIS Specifications. As such, many PostGIS methods require, or more accurately, assume that geometries that are operated on are both simple and valid. For example, it does not make sense to calculate the area of a polygon that has a hole defined outside of the polygon, or to construct a polygon from a non-simple boundary line.
According to the OGC Specifications, a simple geometry is one that has no anomalous geometric points, such as self intersection or self tangency and primarily refers to 0 or 1-dimensional geometries (i.e. [MULTI]POINT, [MULTI]LINESTRING ). Geometry validity, on the other hand, primarily refers to 2-dimensional geometries (i.e. [MULTI]POLYGON) and defines the set of assertions that characterizes a valid polygon. The description of each geometric class includes specific conditions that further detail geometric simplicity and validity.
A POINT is inheritably simple as a 0-dimensional geometry object.
MULTIPOINT s are simple if no two coordinates ( POINT s) are equal (have identical coordinate values).
A LINESTRING is simple if it does not pass through the same POINT twice (except for the endpoints, in which case it is referred to as a linear ring and additionally considered closed).