Docs in progress for ‘QGIS testing’. Visit https://docs.qgis.org/2.18 for QGIS 2.18 docs and translations.

17.5. Lesson: Geometry Construction

In this section we are going to delve a little deeper into how simple geometries are constructed in SQL. In reality, you will probably use a GIS like QGIS to create complex geometries using their digitising tools; however, understanding how they are formulated can be handy for writing queries and understanding how the database is assembled.

The goal of this lesson: To better understand how to create spatial entities directly in PostgreSQL/PostGIS.

17.5.1. Creating Linestrings

Going back to our address database, let’s get our streets table matching the others; i.e., having a constraint on the geometry, an index and an entry in the geometry_columns table.

17.5.2. Try Yourself moderate

  • Modify the streets table so that it has a geometry column of type ST_LineString.
  • Don’t forget to do the accompanying update to the geometry columns table!
  • Also add a constraint to prevent any geometries being added that are not LINESTRINGS or null.
  • Create a spatial index on the new geometry column

Check your results

Now let’s insert a linestring into our streets table. In this case we will update an existing street record:

update streets
set the_geom = 'SRID=4326;LINESTRING(20 -33, 21 -34, 24 -33)'
where streets.id=2;

Take a look at the results in QGIS. (You may need to right-click on the streets layer in the ‘Layers’ panel, and choose ‘Zoom to layer extent’.)

Now create some more streets entries - some in QGIS and some from the command line.

17.5.3. Creating Polygons

Creating polygons is just as easy. One thing to remember is that by definition, polygons have at least four vertices, with the last and first being co-located:

insert into cities (name, the_geom)
values ('Tokyo', 'SRID=4326;POLYGON((10 -10, 5 -32, 30 -27, 10 -10))');

Note

A polygon requires double brackets around its coordinate list; this is to allow you to add complex polygons with multiple unconnected areas. For instance

insert into cities (name, the_geom)
values ('Tokyo Outer Wards',
        'SRID=4326;POLYGON((20 10, 20 20, 35 20, 20 10),
                           (-10 -30, -5 0, -15 -15, -10 -30))'
        );

If you followed this step, you can check what it did by loading the cities dataset into QGIS, opening its attribute table, and selecting the new entry. Note how the two new polygons behave like one polygon.

17.5.4. Exercise: Linking Cities to People

For this exercise you should do the following:

  • Delete all data from your people table.
  • Add a foreign key column to people that references the primary key of the cities table.
  • Use QGIS to capture some cities.
  • Use SQL to insert some new people records, ensuring that each has an associated street and city.

Your updated people schema should look something like this:

\d people

Table "public.people"
   Column   |         Type          |                      Modifiers
 -----------+-----------------------+--------------------------------------------
  id        | integer               | not null
            |                       | default nextval('people_id_seq'::regclass)
  name      | character varying(50) |
  house_no  | integer               | not null
  street_id | integer               | not null
  phone_no  | character varying     |
  the_geom  | geometry              |
  city_id   | integer               | not null
Indexes:
  "people_pkey" PRIMARY KEY, btree (id)
  "people_name_idx" btree (name)
Check constraints:
  "people_geom_point_chk" CHECK (st_geometrytype(the_geom) =
                       'ST_Point'::text OR the_geom IS NULL)
Foreign-key constraints:
  "people_city_id_fkey" FOREIGN KEY (city_id) REFERENCES cities(id)
  "people_street_id_fkey" FOREIGN KEY (street_id) REFERENCES streets(id)

Check your results

17.5.5. Looking at Our Schema

By now our schema should be looking like this:

../../../_images/final_schema.png

17.5.6. Try Yourself hard

Create city boundaries by computing the minimum convex hull of all addresses for that city and computing a buffer around that area.

17.5.7. Access Sub-Objects

With the SFS-Model functions, you have a wide variety of options to access sub-objects of SFS Geometries. When you want to select the first vertex point of every polygon geometry in the table myPolygonTable, you have to do this in this way:

  • Transform the polygon boundary to a linestring:

    select st_boundary(geometry) from myPolygonTable;
    
  • Select the first vertex point of the resultant linestring:

    select st_startpoint(myGeometry)
    from (
        select st_boundary(geometry) as myGeometry
        from myPolygonTable) as foo;
    

17.5.8. Data Processing

PostGIS supports all OGC SFS/MM standard conform functions. All these functions start with ST_.

17.5.9. Clipping

To clip a subpart of your data you can use the ST_INTERSECT() function. To avoid empty geometries, use:

where not st_isempty(st_intersection(a.the_geom, b.the_geom))
../../../_images/qgis_001.png
select st_intersection(a.the_geom, b.the_geom), b.*
from clip as a, road_lines as b
where not st_isempty(st_intersection(st_setsrid(a.the_geom,32734),
  b.the_geom));
../../../_images/qgis_002.png

17.5.10. Building Geometries from Other Geometries

From a given point table, you want to generate a linestring. The order of the points is defined by their id. Another ordering method could be a timestamp, such as the one you get when you capture waypoints with a GPS receiver.

../../../_images/qgis_006.png

To create a linestring from a new point layer called ‘points’, you can run the following command:

select ST_LineFromMultiPoint(st_collect(the_geom)), 1 as id
from (
  select the_geom
  from points
  order by id
) as foo;

To see how it works without creating a new layer, you could also run this command on the ‘people’ layer, although of course it would make little real-world sense to do this.

../../../_images/qgis_007.png

17.5.11. Geometry Cleaning

You can get more information for this topic in this blog entry.

17.5.12. Differences between tables

To detect the difference between two tables with the same structure, you can use the PostgreSQL keyword EXCEPT:

select * from table_a
except
select * from table_b;

As the result, you will get all records from table_a which are not stored in table_b.

17.5.13. Tablespaces

You can define where postgres should store its data on disk by creating tablespaces:

CREATE TABLESPACE homespace LOCATION '/home/pg';

When you create a database, you can then specify which tablespace to use e.g.:

createdb --tablespace=homespace t4a

17.5.14. In Conclusion

You’ve learned how to create more complex geometries using PostGIS statements. Keep in mind that this is mostly to improve your tacit knowledge when working with geo-enabled databases through a GIS frontend. You usually won’t need to actually enter these statements manually, but having a general idea of their structure will help you when using a GIS, especially if you encounter errors that would otherwise seem cryptic.