Leveraging Postgis Functions To Add Coordinate Fields In Qgis

The Problem: Displaying Spatial Data Without Coordinate Fields

Spatial data such as points, lines, and polygons often lack numeric coordinate fields that specify geographic locations. This poses challenges for QGIS users who need to calculate distances, create spatial indexes, or work with data in coordinate-based workflows.

Without longitude and latitude fields, spatial data layers have geometries but no numeric coordinates tied to features. Many analysis and data management tasks require these numeric fields in order to function properly.

PostGIS, the spatial database extender for PostgreSQL, provides over 500 spatial functions for processing geometry data. Among these are the ST_X and ST_Y functions which return numeric longitude and latitude values from spatial data.

By leveraging PostGIS functionality within QGIS, we can generate coordinate fields for spatial layers to enable coordinate-based analysis. This guides demonstrates workflows to:

• Set up PostGIS functions in QGIS
• Use ST_X and ST_Y to add longitude and latitude fields
• Join the new coordinate fields back to the spatial layer
• Apply advanced coordinate-based analysis with the enhanced data

Setting Up PostGIS Functions in QGIS

To utilize PostGIS functions in QGIS, we first need to:

1. Install the required plugins
2. Connect QGIS to a PostGIS database
3. Load a spatial data layer into the database

Installing Required Plugins

We will use two plugins for working with PostGIS data:

• DB Manager – provides database management tools and SQL query interface
• Virtual Layers – allows creating virtual layers from SQL queries

To install these plugins:

1. Open the Plugin Manager from the QGIS toolbar menu
2. Search for “DB Manager” and “Virtual Layers”
3. Install the plugins by clicking Install Plugin

Connecting to PostGIS Database

Next we will use the DB Manager to set up our database connection. In the DB Manager window:

1. Click New Connection
2. Select PostGIS connection type
3. Enter connection details for your PostGIS database
4. Click Test Connection then OK to save

Loading Data Layer

Now we can load our spatial data layer into the PostGIS database:

1. Right-click the database in DB Manager, choose Import GIS Data
2. Select the shapefile or other spatial data for import
3. In the dialog confirm the import parameters, keep defaults
4. Click OK, the data will be loaded into the database

We have now set up PostGIS functions and loaded spatial data in QGIS, ready for the next steps.

Adding Coordinate Fields with ST_X and ST_Y

With our PostGIS data in place, we can call ST_X and ST_Y functions to add new numeric longitude and latitude fields to the data.

Example Code for Generating Longitude Field

The ST_X function returns the longitude value for geometry features. We use it in SQL code:

  SELECT id, ST_X(geom) As lon 
  FROM spatial_layer

This selects the id field and generates a new field “lon” containing the longitude from the geom geometry field.

Example Code for Generating Latitude Field

Similarly, ST_Y returns the latitude values from geometry data:

  SELECT id, ST_Y(geom) As lat
  FROM spatial_layer  

Here we generate a new “lat” field to hold latitude coordinates.

With both SQL snippets, we now have numeric longitude and latitude values extracted from spatial geometries.

Joining Coordinate Fields to Spatial Layer

The longitude and latitude fields reside in the PostGIS database. To utilize them in QGIS, we need to join the fields back to the original spatial layer.

Performing Attribute Join

Using the Virtual Layers plugin:

1. Create a new virtual layer from the SQL query for longitude values
2. In layer properties, toggle ID Field as join field and target layer primary key
3. Repeat to create latitude virtual layer, toggling join fields
4. Join the virtual layers back to the parent spatial layer

The coordinate fields now append to the attribute table for the spatial layer.

Viewing Coordinate Data

To view the results:

1. Open attribute table for spatial layer, with joins enabled
2. Scroll to end of table to see added lon and lat fields
3. Values show coordinate locations for each feature

The spatial layer now has numeric longitude and latitude values ready for coordinate-based analysis.

Advanced Uses for PostGIS-Derived Fields

With coordinate fields added, we open up many possibilities for deeper geospatial analysis. Two valuable examples are:

• Calculating distances between points for proximity analysis
• Creating spatial indexes for performance enhancements

Calculating Distances Between Points

Using PostGIS functions, we can find distances between points based on coordinate values:

  SELECT ST_Distance(
    point1.geom, 
    point2.geom) 
  FROM spatial_layer As point1, spatial_layer As point2
  WHERE...

This provides the core logic to analyze distances, travel times, closest facility, clustering, and many other coordinate-based workflows.

Creating Spatial Indexes

Spatial indexes improve query performance by optimizing geom fields. The syntax leverages numeric coordinates:

  CREATE INDEX idx_spatial_layer_geom
  ON spatial_layer
  USING GIST (geom gist_geometry_ops_nd); 

For large data, spatial indexes can drastically speed up display, query, and analysis.

Summary

By integrating PostGIS functions into our QGIS workflow, we can augment spatial data with valuable coordinate fields. This opens up many deeper analysis options that would not be possible otherwise.

A key takeaway is how non-numeric geometry data can produce numeric longitude/latitude values using ST_X and ST_Y. Joining the results back to the layer enables advanced coordinate-aware processing.

With the power of PostGIS and QGIS combined, many new avenues of geospatial analysis are possible.