QGIS and PostGIS : Identifying direction of a vector

If using the dijkstra function with direction turned on it is important to identify the order in which the nodes of a vector line have been digitised. This is called the direction, dijkstra can use this with a reverse_cost attribute to handicap wrong movement along lines to such an extent that the correct path can be calculated around things like roundabouts.

Here is an example of the roundabout in Straiton in Edinburgh just North of the A720 bypass. While some of the lines have a correct anti clockwise orienation clearly some have been incorrectly digitised.

First we can see this by displaying the network in QGIS but using the styling to arrow the direction.

The function that can be used to reverse such inaccuracies if you can’t resort to buying a correct dataset try ST_REVERSE

017 Postgres command line : psql : Notices

RAISE NOTICE can provide the same function as Message Box in VBA ie you can use it to comment on the progress of a script. RAISE NOTICE is not supported by SQL so you can’t place it in scripts containing SQL they need to be in plpgsql scripts. This isn’t too much of a hassle as the way I am working at the moment I am calling the SQL anyway from plpgsql so I can place my message boxes in there.

No VBA Ok buttons.

CREATE OR REPLACE FUNCTION noticeexample() returns void as $$
BEGIN
RAISE NOTICE 'ONE FINE DAY IN THE MIDDLE OF THE NIGHT';
END;
$$
LANGUAGE PLPGSQL;

016 Postgres command line : psql : Strip out the Z coordinate from a geometry field

When creating a topology the geometry field cannot contain a Z coordinate.

OK but the Ordnance Survey Open Data highways layers containse a Z coordinate. Previously I had stripped this out using the latest version of QGIS which has a tick box in the front end that allows for import stripping of the z coordinate in the process. If you don’t have access to the latest QGIS version how can you strip out the z coordinates.

ST_FORCE2D

ALTER TABLE public.nuroadlink ADD COLUMN geom2(multilinestring,27700);
UPDATE public.nuroadlink SET geom2 = ST_FORCE2D(public.nuroadlink.geom);
ALTER TABLE public.nuroadlink drop column geom;
ALTER TABLE public.nuroadlink RENAME COLUMN geom2 TO geom;

015 Postgres command line : psql : Create functions and then script those functions

I had assumed after I had created a working SQL Script I would just be able to wrap the whole thing easily into a function and then bang it would be off to the races.
My script really needed to be run in order and for some as yet undefined reason I was getting particular errors where a table would be created and then a following query would add or alter that table. It looked like the second query was trying to adapt the table prior to its creation with an inevitable error.

I managed to get it working by making each SQL Query a function and then scripting the functions consecutively in a separate function using the PERFORM instruction.

I incorporate into this the check_function_bodies switch which just allows the creation of sql referring to objects that may not be in existence yet.

BEGIN;
SET LOCAL check_function_bodies TO FALSE;
CREATE OR REPLACE FUNCTION query01() returns void as $$
CREATE TABLE t001start 
(
pkid serial primary key,
geompkidt001 geometry(point,27700)
);
$$ LANGUAGE SQL;
CREATE OR REPLACE FUNCTION query02() returns void as $$
CREATE TABLE t002end 
(
pkid serial primary key,
geompkidt002 geometry(point,27700)
);
$$ LANGUAGE SQL;
COMMIT;

And then subsequently I create a function that runs the functions.

CREATE OR REPLACE FUNCTION runallthequeries() 
returns text as
$BODY$
BEGIN
PERFORM query01();
PERFORM query02();
RETURN 'process end';
END;
$BODY$
LANGUAGE plpgsql;

014 Postgres command line : psql : Create SQL function referring to a table or column that does not yet exist

I was trying to write a script that would allow me to measure distances to schools and my original script gradually built up tables that were subsequently deleted. Worked fine in one big sql script but when I tried to convert this into a function so that it could be more easily stored with the database I kept on getting error messages stating that it was not possible to create sql that referred to objects that did not exist. Postgres validates functions and will at default prevent creation of functions containing SQL that refers to objects not yet in existence.

Postgres does not however save dependencies for code in the function body. So although once the function is created the tables and views can be dropped (and the function still exists) in default you need a set of tables in place with default settings before the function can be created. One workaround would be to create dummy tables and views in advance and later drop them but this if often clunky and awkward. Luckily this validation can be turned off.

BEGIN;
SET LOCAL check_function_bodies TO FALSE;
CREATE or REPLACE FUNCTION examplefunction() Returns void AS $$
  CREATE TABLE t001 (pkid serial primary key, field1 varchar(20));
$$ LANGUAGE sql;
COMMIT;

Documentation says

check_function_bodies (boolean)

This parameter is normally on. When set to off, it disables validation of the function body string during CREATE FUNCTION. Disabling validation avoids side effects of the validation process and avoids false positives due to problems such as forward references. Set this parameter to off before loading functions on behalf of other users; pg_dump does so automatically.

see here
Documentation

Totally invaluable when you write scripts like I do.

011 : Postgres amalgamate consecutive lines into a single line in a table

Here we take much of the work covered in post 010 and take the parts and user st_union to merge into a single record and place it in a table created by transforming a view into a table

Firstly go to your psql line and ensure that you are logged in with a username that you wish to be the owner of the table. In my case general

logging into edinburgh routing database

Now same measurement as before but this time we shall make a view out of the measurements then load that into a new table before deleting the view leaving us with the table with a combined measurement.

CREATE VIEW v001firstmeasurement AS SELECT seq,  id1 AS node, id2 AS edge, cost, geom, agg
  FROM pgr_dijkstra( 'SELECT id, source, target, st_length(geom) as cost FROM public.t01roadnetwork', 15883, 10967, false, false  ) as di
  JOIN public.t01roadnetwork pt ON di.id2 = pt.id ;

CREATE TABLE t003 as select sum(cost), st_union(geom) from v001firstmeasurement;

DROP VIEW v001firstmeasurement;

It is important in notepad to remove the blank spaces in the editor this looks as follows.

We then should then get some kind of confirmation that the view and table are created before the view is then dropped again. There might be a more efficient way of doing this but this was my first experiment. And we can go back to QGIS 3.4 and display the now single line in our project. Complete with now accurate measurement. It should be noted that if you were wanting to do multiple line measurements you would need to step out of the create statement and use an insert statement for all subsequent insertions as follows.
insert into t003(sum,st_union) select sum(cost),st_union(geom) from v001firstmeasurement;
This would allow you to do multiple measurments. I haven’t added up the measurement but it looks about right.

010 Postgres command line : psql : Getting started with pgrouting using open data from Ordnance Survey to identify and measure the shortest route between two points.

Objective here is to write a series of queries that can be used to measure the shortest distance between selected paired locations on a network such that the geometry of the routes can be calculated and displayed on a map.

For this particular tutorial you will need – QGIS 3 or higher and a version of Postgres I am using version 11.0 here (I have upgraded since my former posts). I believe this tutorial will work with previous versions but if you are following along now might be a good time to upgrade.

QGIS 3.4 or higher – needed as the Ordnance Survey road network geometry contains a z coordinate which will prevent the creation of the required geometry for measurement. QGIS 3 introduced the ability to save geometry excluding z coordinate. If you have a network without z coordinates you should not require this.

So let us first get the data. Here you tick the option in the top right hand corner – scroll to the bottom and submit your request after which you will be asked a few basic questions along with email address you wish the download to be sent to after a few minutes you should be sent the download link through your email – follow the instructions and you should be able to get the information

Ordnance Survey Open Data

The information you are downloading is a block framework for the whole of the uk. When you unzip the download into a folder you will see multiple files. We will be using a section of the national dataset relating to Edinburgh – NT. Choose the block or selection that you are interested in. More blocks may take more time however.

Open QGIS
Create a new project : eg EdinburghRouting.qgz
Load in your chosen network block : eg NT_RoadLink.shp

Select the layer you just loaded in : eg NT_RoadLink.shp

and navigate to the following in the menu settings
Layer / Save As

Fill out the Save Vector Layer as … dialog box
IMPORTANT – ensure within the Geometry section
Geometry type is set to LineString
Include z-dimension is unticked

Give the new file a name : eg ntosroutingnetwork.shp

Hit ok

Within the layer dialog of QGIS your new layer should appear you can now remove the for NT_RoadLink shape file from the project

Next go to your version of PostgreSQL and using a superuser account create a new database : eg edinburghrouting

I would suggest you use lower casing as well

As a superuser ensure you add the postgis and pgrouting extensions.

Next I set up the following connection between the QGIS project and PostgreSQL

Personal tastes may vary but I like like to select
Also list tables with no geometry
Allow saving/loading QGIS projects in the database

OK the selection and you should now have a connection to the new database you just created.

QGIS has an excellent dbmanager window which we will use to load our new shape file which excludes the z layer into the new database we created in PostgreSQL

Ensuring that you have a connection to your localpostgis database hit the

ImportLayerFile

Here I load the information into a new table t01roadnetwork

On pressing OK there will be delay after which if things go well you will receive the following message.

As ever it is good to check that things appear to be going well.
Add the layer to your project and determine approximately whether import was successful.

Next back in psql command line and in an editor we are going to run 4 queries
The first 2 add columns that are required in the shortest distance algorithm we shall use, the third will allow anyone to write an aggregation function to see the total cost of the route and the last creates a topology for the road network.

alter table public.t01roadnetwork add column source integer;
alter table public.t01roadnetwork add column target integer;
alter table public.t01roadnetwork add column agg smallint default 1;
select pgr_createTopology('public.t01roadnetwork', 0.0001, 'geom', 'id');

If things go correctly you should see the database engine start to create the topology and what I see is it gradually stepping through the creation process.

and on completion you should have something like the following:

A new table has been added to the edinburghrouting database and next step is to display the network and its vertices. In QGIS.

In QGIS we should see something like

The next thing that I like to do is to label the nodes so that for quick identification.

And look to the t01roadnetwork table and see if the columns are clear and present.

We are now ready to make a measurement. Here I choose the nodes 15883 and 10967

SELECT seq, id1 AS node, id2 AS edge, cost, geom , agg
  FROM pgr_dijkstra(
    'SELECT id, source, target, st_length(geom) as cost FROM public.t01roadnetwork',
    15883, 10967, false, false
  ) as di
  JOIN public.t01roadnetwork pt
  ON di.id2 = pt.id ;

Now we can load this as a new layer and then improve the symbology

Doing this we get.

It should be noted that the line you see is a collection of lines. In my next post I will go through and indicate how we can amalgamate that into a single line for storage in a table.

Congratulations if you have got this far you should be able to measure the shortest distance between any two points on a valid network by altering the numbers.