What kind of geometric objects can I store?
You can store point, line, polygon, multipoint, multiline, multipolygon, and geometrycollections. These are specified in the Open GIS Well Known Text Format (with XYZ,XYM,XYZM extensions).
How do I insert a GIS object into the database?
First, you need to create a table with a column of type
"geometry" to hold your GIS data. Connect to your database with
CREATE TABLE gtest ( ID int4, NAME varchar(20) ); SELECT AddGeometryColumn('', 'gtest','geom',-1,'LINESTRING',2);
If the geometry column addition fails, you probably have not loaded the PostGIS functions and objects into this database. See the installation instructions.
Then, you can insert a geometry into the table using a SQL insert statement. The GIS object itself is formatted using the OpenGIS Consortium "well-known text" format:
INSERT INTO gtest (ID, NAME, GEOM) VALUES ( 1, 'First Geometry', ST_GeomFromText('LINESTRING(2 3,4 5,6 5,7 8)', -1) );
For more information about other GIS objects, see the object reference.
To view your GIS data in the table:
SELECT id, name, ST_AsText(geom) AS geom FROM gtest;
The return value should look something like this:
id | name | geom ----+----------------+----------------------------- 1 | First Geometry | LINESTRING(2 3,4 5,6 5,7 8) (1 row)
How do I construct a spatial query?
The same way you construct any other database query, as an SQL combination of return values, functions, and boolean tests.
For spatial queries, there are two issues that are important to keep in mind while constructing your query: is there a spatial index you can make use of; and, are you doing expensive calculations on a large number of geometries.
In general, you will want to use the "intersects operator" (&&) which tests whether the bounding boxes of features intersect. The reason the && operator is useful is because if a spatial index is available to speed up the test, the && operator will make use of this. This can make queries much much faster.
You will also make use of spatial functions, such as Distance(), ST_Intersects(), ST_Contains() and ST_Within(), among others, to narrow down the results of your search. Most spatial queries include both an indexed test and a spatial function test. The index test serves to limit the number of return tuples to only tuples that might meet the condition of interest. The spatial functions are then use to test the condition exactly.
SELECT id, the_geom FROM thetable WHERE ST_Contains(the_geom,'POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))');
How do I speed up spatial queries on large tables?
Fast queries on large tables is the raison d'etre of spatial databases (along with transaction support) so having a good index is important.
To build a spatial index on a table with a
CREATE INDEX [indexname] ON [tablename] USING GIST ( [geometrycolumn] );
The "USING GIST" option tells the server to use a GiST (Generalized Search Tree) index.
You should also ensure that the PostgreSQL query planner has enough information about your index to make rational decisions about when to use it. To do this, you have to "gather statistics" on your geometry tables.
For PostgreSQL 8.0.x and greater, just run the VACUUM ANALYZE command.
For PostgreSQL 7.4.x and below, run the SELECT UPDATE_GEOMETRY_STATS() command.
Why aren't PostgreSQL R-Tree indexes supported?
Early versions of PostGIS used the PostgreSQL R-Tree indexes. However, PostgreSQL R-Trees have been completely discarded since version 0.6, and spatial indexing is provided with an R-Tree-over-GiST scheme.
Our tests have shown search speed for native R-Tree and GiST to be comparable. Native PostgreSQL R-Trees have two limitations which make them undesirable for use with GIS features (note that these limitations are due to the current PostgreSQL native R-Tree implementation, not the R-Tree concept in general):
Why should I use the
If you do not want to use the OpenGIS support functions, you do
not have to. Simply create tables as in older versions, defining your
geometry columns in the CREATE statement. All your geometries will
have SRIDs of -1, and the OpenGIS meta-data tables will
not be filled in properly. However, this will
cause most applications based on PostGIS to fail, and it is generally
suggested that you do use
MapServer is one application which makes use of the
What is the best way to find all objects within a radius of another object?
To use the database most efficiently, it is best to do radius queries which combine the radius test with a bounding box test: the bounding box test uses the spatial index, giving fast access to a subset of data which the radius test is then applied to.
For example, to find all objects with 100 meters of POINT(1000 1000) the following query would work well:
SELECT * FROM geotable WHERE ST_DWithin(geocolumn, 'POINT(1000 1000)', 100.0);
How do I perform a coordinate reprojection as part of a query?
To perform a reprojection, both the source and destination coordinate systems must be defined in the SPATIAL_REF_SYS table, and the geometries being reprojected must already have an SRID set on them. Once that is done, a reprojection is as simple as referring to the desired destination SRID. The below projects a geometry to NAD 83 long lat. The below will only work if the srid of the_geom is not -1 (not undefined spatial ref)
SELECT ST_Transform(the_geom,4269) FROM geotable;
I did an ST_AsEWKT and ST_AsText on my rather large geometry and it returned blank field. What gives?
You are probably using PgAdmin or some other tool that doesn't output large text. If your geometry is big enough, it will appear blank in these tools. Use PSQL if you really need to see it or output it in WKT.
--To check number of geometries are really blank SELECT count(gid) FROM geotable WHERE the_geom IS NULL;
When I do an ST_Intersects, it says my two geometries don't intersect when I KNOW THEY DO. What gives?
This generally happens in two common cases. Your geometry is invalid -- check ST_IsValid or you are assuming they intersect because ST_AsText truncates the numbers and you have lots of decimals after it is not showing you.