17.1. Configurar PostGIS ILSI¶
Setting up PostGIS functions will allow you to access spatial functions from within PostgreSQL.
El objetivo de esta lección: instalar funciones espaciales y una breve demostración de sus efectos.
We will assume the use of PostGIS version 2.1 in this exercise. The installation and database configuration are different for older versions, but the rest of this material in this module will still work. Consult the documentation for your platform for help with installation and database configuration.
17.1.1. Instalando bajo Ubuntu¶
Postgis is easily installed from apt.
$ sudo apt install postgis $ sudo apt install postgresql-9.1-postgis
En serio, es así de fácil…
Depending on which version of Ubuntu you are using, and which
repositories you have configured, these commands will install PostGIS 1.5,
or 2.x. You can find the version installed by issuing a
PostGIS_full_version(); query with psql or another tool.
To install the absolute latest version of PostGIS, you can use the following commands.
$ sudo apt-add-repository ppa:sharpie/for-science $ sudo apt-add-repository ppa:sharpie/postgis-nightly $ sudo apt update $ sudo apt install postgresql-9.1-postgis-nightly
17.1.2. Instalando bajo Windows¶
Installing on Windows is a little more complicated, but still not hard. Note that you need to be online to install the postgis stack.
First Visit the download page.
Then follow this guide.
More information about installing on Windows can be found on the PostGIS website.
17.1.3. Installing on Other Platforms¶
The PostGIS website download has information about installing on other platforms including macOS and on other linux distributions
17.1.4. Configuring Databases to use PostGIS¶
Once PostGIS is installed, you will need to configure your database to use the extensions. If you have installed PostGIS version > 2.0, this is as simple as issuing the following command with psql using the address database from our previous exercise.
$ psql -d address -c "CREATE EXTENSION postgis;"
If you are using PostGIS 1.5 and a version of PostgreSQL lower than 9.1, you will need to follow a different set of steps in order to install the postgis extensions for your database. Please consult the PostGIS Documentation for instructions on how to do this. There are also some instructions in the previous version of this manual.
17.1.5. Looking at the installed PostGIS functions¶
PostGIS can be thought of as a collection of in-database functions that extend the core capabilities of PostgreSQL so that it can deal with spatial data. By “deal with”, we mean store, retrieve, query and manipulate. In order to do this, a number of functions are installed into the database.
address database is now geospatially enabled, thanks to PostGIS.
We are going to delve a lot deeper into this in the coming sections, but let’s
give you a quick little taster. Let’s say we want to create a point from text.
First we use the psql command to find functions relating to point. If you are not
already connected to the
address database, do so now. Then run:
This is the command we’re looking for: st_pointfromtext. To page through the list, use the down arrow, then press Q to quit back to the psql shell.
Intente ejecutar este comando:
select st_pointfromtext('POINT(1 1)');
st_pointfromtext -------------------------------------------- 0101000000000000000000F03F000000000000F03F (1 row)
Three things to note:
We defined a point at position 1,1 (EPSG:4326 is assumed) using POINT(1 1),
We ran an sql statement, but not on any table, just on data entered from the SQL prompt,
The resulting row does not make much sense.
The resulting row is in the OGC format called “Well Known Binary” (WKB). We will look at this format in detail in the next section.
To get the results back as text, we can do a quick scan through the function list for something that returns text:
The query we’re looking for now is
st_astext. Let’s combine it with the
select st_astext(st_pointfromtext('POINT(1 1)'));
st_astext ------------ POINT(1 1) (1 row)
Here, we entered the string POINT(1,1), turned it into a point using st_pointfromtext(), and turned it back into a human-readable form with st_astext(), which gave us back our original string.
One last example before we really get into the detail of using PostGIS:
select st_astext(st_buffer(st_pointfromtext('POINT(1 1)'),1.0));
What did that do? It created a buffer of 1 degree around our point, and returned the result as text.
17.1.6. Spatial Reference Systems¶
In addition to the PostGIS functions, the extension contains a collection of spatial reference system (SRS) definitions as defined by the European Petroleum Survey Group (EPSG). These are used during operations such as coordinate reference system (CRS) conversions.
We can inspect these SRS definitions in our database as they are stored in normal database tables.
First, let’s look at the schema of the table by entering the following command in the psql prompt:
The result should be this:
Table "public.spatial_ref_sys" Column | Type | Modifiers -----------+-------------------------+----------- srid | integer | not null auth_name | character varying(256) | auth_srid | integer | srtext | character varying(2048) | proj4text | character varying(2048) | Indexes: "spatial_ref_sys_pkey" PRIMARY KEY, btree (srid)
You can use standard SQL queries (as we have learned from our introductory sections), to view and manipulate this table - though its not a good idea to update or delete any records unless you know what you are doing.
One SRID you may be interested in is EPSG:4326 - the geographic / lat lon reference system using the WGS 84 ellipsoid. Let’s take a look at it:
select * from spatial_ref_sys where srid=4326;
srid | 4326 auth_name | EPSG auth_srid | 4326 srtext | GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],TOWGS84[0, 0,0,0,0,0,0],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328, AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]] proj4text | +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs
The srtext is the projection definition in well known text (you may recognise this from .prj files in your shapefile collection).
17.1.7. In Conclusion¶
You now have PostGIS functions installed in your copy of PostgreSQL. With this you’ll be able to make use of PostGIS” extensive spatial functions.
17.1.8. What’s Next?¶
Next you’ll learn how spatial features are represented in a database.