Identify Relevant Networks in Spatial Data
- How to identify a sub-network that represents only the area relevant to your route
- How to create a circle to identify a relevant area in your spatial data
- How to flag nodes of the transportation network located in a circle
- You have completed the tutorial Determine the Distance to a Target POI.
- You have access to the sample spatial data used in this tutorial group.
- Make sure your database instance is running before you start.
In this tutorial, you will learn how to identify a relevant area in your spatial data.
Now that you know the distance from your location to the target destination you want to go to, your next task is to find a route in the transportation network. Instead of finding the path while considering the whole transportation network, it is more useful to first select a meaningful sub-network that represents only the area of the network relevant to your route.
We will show you how to do that in these two steps:
- Create a Circle for the Relevant Area
- Add Flags for all Nodes in the Circle
- Step 1
First, you need to construct a circle to identify the relevant area for the transportation network. The circle should have a minimal size and should contain origin and destination with a buffer of 500 meters around it. To illustrate how the query is being built we will use step-by-step queries with sub-SELECT statements.
Note, that generally, the center point of the smallest circle, which includes two points, is the center point of their direct connection line.
You can see in this image the circle you will create:
First, select the two points from the previous tutorials using this statement:SQLCopy
SELECT ST_GeomFromText('POINT (706327.107445 5710259.94449)', 32630) AS START_PT, SHAPE AS TARGET_PT FROM LONDON_POI lp WHERE "osmid" = 6274057185;
To create a line geometry, which is connecting both points, you can then use the function
SELECT ST_MakeLine(START_PT, TARGET_PT) AS CONN_LINE FROM ( -- previous statement );
To retrieve an arbitrary point on this line, we need to use function
ST_LineInterpolatePoint(*), which takes a fraction of the line as argument. For retrieving the center point of the line, we pass the value 0.5.SQLCopy
SELECT CONN_LINE.ST_LineInterpolatePoint(0.5) AS CENTER_PT FROM ( -- previous statement );
As a final step, we would like to draw a circle with radius distance (start, target)/2 + 500 around
CENTER_PT. The respective function, which takes the radius as an input, is called
SELECT CENTER_PT.ST_Buffer(4835) AS AREA FROM ( -- previous statement );
The above steps can be combined into a single select using method chaining.SQLCopy
SELECT ST_MakeLine( ST_GeomFromText('POINT (706327.107445 5710259.94449)', 32630), SHAPE ) .ST_LineInterpolatePoint(0.5) .ST_Buffer(5000) AS AREA FROM LONDON_POI WHERE "osmid" = 6274057185;
- Step 2
Next, you need to flag all nodes of the transportation network located in the circle, by setting
First, we need to enhance the existing vertex table by column
ALTER TABLE LONDON_VERTICES ADD (IN_SCOPE INTEGER);
Second, we fill the column with 0 or 1 based on the intersection with the identified area. We can use function
ST_Intersects(*) to determine if two geometries intersect or not.
Instead of using an UPDATE statement, we will use MERGE INTO, which allows a more complex update logic.SQLCopy
MERGE INTO LONDON_VERTICES lv USING ( -- previous statement begin -- SELECT ST_MakeLine( ST_GeomFromText('POINT (706327.107445 5710259.94449)', 32630), SHAPE ) .ST_LineInterpolatePoint(0.5) .ST_Buffer(5000) AS AREA FROM LONDON_POI WHERE "osmid" = 6274057185 -- previous statement end -- ) circle ON 1=1 WHEN MATCHED THEN UPDATE SET lv.IN_SCOPE = CIRCLE.AREA.ST_Intersects(SHAPE);
You can confirm that you properly selected the nodes by using your preferred tool for visualizing the result set with this query:SQLCopy
SELECT SHAPE FROM LONDON_VERTICES WHERE IN_SCOPE = 1;
By creating a circle in your relevant area and flagging nodes in this circle, you have successfully identified a sub-network for transportation, which you will consider for finding your path through London in the following exercises.
Learn in the next tutorial how to check if this area is suitable for riding a bike by visualizing edges and using
- Step 3
You want to draw a circle around the center point of the line created between two POIs. What is the required function that takes the radius as an input?