Fitting Road Network Data to a Graph

The following guide provides step-by-step instructions to get started with fitting existing road network data to a graph. Since edges in graphs can only be composed of two nodes, the weight (or cost to travel) for complex WKT LINESTRINGs (e.g., more than two points) that typically define road networks must be assigned consistently to the segments composing the LINESTRING, as noted under Weights. There are two methods for calculating and assigning weights to a graph with edges derived from large WKT LINESTRINGs:

In-line Expressions — calculating weights using inline expressions and forcing the graph server to divide complex WKT LINESTRINGs during the graph creation
Expansion — using iteration to divide complex WKT LINESTRINGs and, using a subquery, calculate weights prior to the graph creation

Prerequisites

D.C. Shape data file
Access to GAdmin

Data File

The tutorial makes use of the dc_shape dataset, which can be ingested from the data file mentioned above. To ingest the file using GAdmin:

Navigate to GAdmin and log in (http://localhost:8080/)
In the top menu, click Data —> Import
In the top right corner, click Advanced CSV Import
Click Select File and select the data file from your local directory
Leave the default options and values for the rest of Step 1
Under Step 2, change Schema to graph
Under Step 3: Confirm, click Import CSV

The file will be validated and records will be inserted.

Fit Using Inline Expressions

Fitting road network data to a graph using inline expressions offers a simpler but less efficient approach to deriving the individual edge segments’ weight. The dc_shape_inline graph that will be created will calculate weight as distance inline and is based on the aforementioned dc_shape dataset.

Create Schema

Before starting with graph creation, a schema, graph_c_fit_data, will be created to contain the tables supporting graph creation & solving. To create the schema:

Still in GAdmin, on the top menu, click Query —> SQL to go to the SQL Tool page.
Copy the following CREATE SCHEMA statement into the SQL Statements text area on the top section of the page:
```
CREATE SCHEMA graph_c_fit_data
```
Highlight the statement and click Run Selected.

Create Graph

To create the graph, in GAdmin:

Copy the following CREATE GRAPH statement into the SQL Statements text area on the top section of the page:

CREATE DIRECTED GRAPH graph_c_fit_data.dc_shape_inline
(
    EDGES => INPUT_TABLE
    (
        SELECT
            link_id AS ID,
            shape AS WKTLINE,
            direction AS DIRECTION,
            ST_LENGTH(shape,1)/(ST_NPOINTS(shape)-1) AS WEIGHT_VALUESPECIFIED
        FROM graph.dc_shape
    ),
    OPTIONS => KV_PAIRS
    (
        'merge_tolerance' = '0.00001',
        'recreate' = 'true',
        'graph_table' = 'graph_c_fit_data.dc_shape_inline_graph'
    )
)

Highlight the statement and click Run Selected.

This creates the dc_shape_inline graph with the following characteristics:

It is DIRECTED because the roads in the graph have directionality (one-way and two-way roads).
It has no explicitly defined nodes, because the example relies on implicit nodes attached to the defined edges.
The EDGES are derived from WKT LINESTRINGs in the shape column of the dc_shape table (AS WKTLINE); the graph server will automatically divide each complex LINESTRING into simple edges. Each edge’s directionality is derived from the direction column of the dc_shape table (AS DIRECTION), and each edge will be assigned an ID that is the link_id column of the dc_shape table (AS ID).
The edge weights are represented as distance, which is calculated as the length of the entire shape column’s LINESTRING (in meters) divided by the number of points in the LINESTRING minus 1 (AS WEIGHT_VALUESPECIFIED).
It has no inherent restrictions for any of the nodes or edges in the graph.
It utilizes the following OPTIONS:
- It will be replaced with this instance of the graph if a graph of the same name exists (recreate).
- If nodes are within 0.00001 degrees (1 meter) of each other, they are merged together (merge_tolerance).
- The resulting graph’s information is placed into a table (graph_table) and an EDGE_WKTLINE column is included so the graph can be visualized.

To view the created graph:

On the top menu, click Data —> Graphs to go to the Graphs page.
Click on the dc_shape_inline entry in the list of graphs.
Click Visualize to bring up the map view.

Fit Using Expansion

Fitting road network data to a graph using expansion separates the creation of line segments and calculation of distance from graph creation. This increases the performance of the graph creation process, as most of the calculations are done within the database instead of the graph server. Expansion is achieved through iteration, wherein each source record with a WKT is expanded into a number of records equal to the number of edges in that corresponding WKT. For example, a single road record with a 10-segment WKT LINESTRING will become ten individual records, each containing one of the 10 segments. In this way, the weights can be pre-calculated and assigned directly to the road segments/edges, alleviating the need for the graph server to do this processing itself.

Create Expansion Table

To accomplish this, first create the expansion table, in GAdmin:

On the top menu, click Query —> SQL to go to the SQL Tool page.

Copy the following CREATE TABLE statement into the SQL Statements text area on the top section of the page:

CREATE TABLE graph_c_fit_data.dc_shape_expanded AS
SELECT
    link_id,
    direction,
    REMOVE_NULLABLE(shape) AS shape,
    REMOVE_NULLABLE(ST_LENGTH(shape,1)) AS distance
FROM
(
    SELECT
        direction,
        link_id,
        ST_MAKELINE(ST_POINTN(shape, i + 1), ST_POINTN(shape, i + 2)) AS shape
    FROM graph.dc_shape
    JOIN ITER ON i < ST_NUMPOINTS(shape) - 1
)

Highlight the statement and click Run Selected.

This CREATE TABLE … AS statement performs several actions necessary to expand the edges and prepare the data for use by the graph creation process. First the complex LINESTRINGs are divided into single edges via a subquery:

The existing dc_shape table is joined to the ITER virtual table, where each WKT LINESTRING is iterated through to extract line segments.
A new WKT LINESTRING is constructed (ST_MAKELINE) from each consecutive pair of points in the source LINESTRING (dc_shape.shape) column.
The direction and link_id columns are extracted from the dc_shape table to assist in graph creation later.

Next, an outer query is used to eliminate the column nullability introduced by join (since graphs cannot be created from columns with the nullable property), and also to calculate the weight as distance. Lastly, the result is saved to a table, which the graph creation will use as input.

The results of the query are placed in a new table (graph_c_fit_data.dc_shape_expanded).
The link_id and direction columns are passed through as they are.
The shape column has its nullability removed (REMOVE_NULLABLE).
The shape column is used to calculate a distance column (distance), and nullability is removed from that column as well.

Create Graph from Expansion Table

Finally, the dc_shape_expansion graph is created directly from the dc_shape_expanded table with no extra calculations necessary. The graph is created with the same characteristics as before, with the following changes:

The edges are derived from the line segments expanded from the original WKT LINESTRINGs in the shape column of the dc_shape_expanded table (WKTLINE).
The weights are pulled directly from the distance column of the dc_shape_expanded table (WEIGHT_VALUESPECIFIED), pre-calculated above.

To create the graph:

Copy the following CREATE GRAPH statement into the SQL Statements text area on the top section of the page:

CREATE DIRECTED GRAPH graph_c_fit_data.dc_shape_expansion
(
    EDGES => INPUT_TABLE
    (
        SELECT
            shape AS WKTLINE,
            link_id AS ID,
            direction AS DIRECTION,
            distance AS WEIGHT_VALUESPECIFIED
        FROM graph_c_fit_data.dc_shape_expanded
    ),
    OPTIONS => KV_PAIRS
    (
        'merge_tolerance' = '0.00001',
        'recreate' = 'true',
        'graph_table' = 'graph_c_fit_data.dc_shape_expanded_graph'
    )
)

Highlight the statement and click Run Selected.

To view the created graph:

On the top menu, click Data —> Graphs to go to the Graphs page.
Click on the dc_shape_expansion entry in the list of graphs.
Click Visualize to bring up the map view.

​Prerequisites

​Data File

​Fit Using Inline Expressions

​Create Schema

​Create Graph

​Fit Using Expansion

​Create Expansion Table

​Create Graph from Expansion Table