Explaining The Parameters For Join Attributes By Location In Qgis

What is the Join Attributes by Location Tool?

The Join Attributes by Location tool in QGIS allows users to append columns and values from one vector layer to another based on their spatial relationship. This serves to enrich datasets with supplemental attributes falling within areas of interest.

For example, using the Join Attributes by Location tool, population density values from a census block polygon layer could be joined to a layer containing school district boundaries. The school district layer would then incorporate helpful statistics on the populations served within each district, enabling enriched analysis and mapping.

As a geoprocessing operation, proper configuration of parameters is vital to correctly transfer attributes between the target vector layer receiving the new data and the source join layer supplying the attributes based on spatial intersection.

How Join Attributes by Location Works

The Join Attributes by Location tool evaluates the spatial relationship between geometries from the target layer and join layer according to a defined geometric predicate. This predicate determines exactly which features are considered to have their attributes transferred over.

Some common predicates include intersection, containment, and being within distance. Once the related features meeting the criteria are identified, QGIS transfers a subset of attributes over from the join layer source features to corresponding target layer destination features.

Additional parameters provide control over attribute summary behavior, determining how the values get aggregated when multiple source features spatially join to the same target feature. Configuration settings also allow toggling between regular one-to-one joins and one-to-many joins for more advanced relation handling.

Key Parameters to Configure

Target Layer

The target layer specifies the vector dataset receiving appended attributes from a spatial join operation. Ideally, the target layer contains geometries and attributes serving as the foundation and focus for analysis. Supplemental attributes get attached to features from this layer based on spatial predicates evaluating proximity to events in the join layer source.

For optimal performance, the target layer should use a projection system matching or compatible with the join layer to limit potential for sloppily snapped geometry intersections.

Join Layer

The join layer provides the source pool of geometries and attributes used to enrich features from the target layer. Events and statistics associated with join layer features get transferred over if they fall within target layer features according to parameters like the geometric predicate and attribute summary configurations.

Like the target layer, the join layer should use a projection system compatible with the target layer and analysis focus. Ensure the join values make logical sense when applied to augment features from the target layer.

Geometric Predicate

The geometric predicate parameter determines the spatial criteria used to identify related features eligible for attribute transfer. The specific predicate used greatly influences which attributes ultimately join from the source layer to destination layer.

Common predicates leveraging spatial boundaries and proximity include:

• Intersects: Source feature boundaries intersect target feature boundaries
• Contains: Target features fully contain entire source features
• Within: Source features fall completely within larger target features
• Overlaps: Source features partially overlap target features
• Are within distance of: Source features fall within a buffer distance of target features

Select a predicate tailored to the specific analytical use case or enrichment goal using spatial context clues from the data.

Attribute Summary

Since multiple source features from the join layer may relate to individual features in the target layer according to the geometric predicate, the attribute summary method helps determine how to compile the values getting transferred.

Summary options include:

• Take attributes of the first located feature: Use first related record found
• Take summary of intersecting features: Statistical summary like sum, mean, median, etc.
• Take summary of intersecting weighted points: Weighted statistical summary

Ensure the summary technique makes logical and analytical sense for the attribute data joining from source to target features given the intended usage context.

Join Type

By default, QGIS adds the join layer attributes using a regular one-to-one join. This adds attributes from a single source feature to each destination feature.

Enabling the one-to-many option instead allows QGIS to capture attributes from all related source features falling within each target feature. This avoids dropping valuable data when multiple child source features join to the same target feature. The resulting table for analysis then contains separate rows for each target-source pair.

Choose between one-to-one or one-to-many joins carefully based on the duplicate handling semantics needed for proper analytical value.

Example Workflow with Code Snippets

Walking through a sample enrichment use case helps illustrate the value of Join Attributes by Location. Consider a city council reviewing statistics on populations served by municipal resources across different districts:

Joining Census Data to City Boundaries

The city already maintains an authoritative vector layer representing official municipal districts. Joining fresh population statistics from a census block source helps answer analytical questions using updated demographics the city didn’t previously collect.

With the municipal district boundaries as the target layer and census block population polygons as the join layer, configure the tool parameters:

# Set target layer to city districts 
target_layer = 'city_district_boundaries'

# Set join layer to census block polygons
join_layer ='census_block_polygons' 

# Choose contains as the predicate since blocks fall within districts
geometric_predicate ='contains'  

# Sum intersecting block populations
attribute_summary ='sum' 

# Use a one-to-many join to retain all raw census data  
join_type ='one_to_many'

With logical spatial parameters configured, QGIS joins granular census population data to high level city district zones – matching numeric totals the city already tracks closely but hadn’t directly tied to polygon boundaries before.

Verifying the Join Successfully Occurred

Check that the attribute join populated properly before proceeding with downstream usage to prevent unsound analysis on incomplete data:

# Print distinct values for district name column  
print(target_layer['district_name'].unique())

# Print distinct values for census population field
print(target_layer['census_population'].unique())  

Probe for null values indicating failed attribute joins not reflecting real-world populations.

Additionally, spot check aggregate calculations on the enriched dataset to roughly match known controls tallied outside the GIS environment if available.

Troubleshooting Common Issues

Spatial joins involve many connected components across the attribute and geometry data spectrum. Isolate and resolve these common problems if joins produce incomplete or inaccurate outputs:

Handling Mismatched Projection Systems

Layers using incompatible projection systems may yield messy geometric intersections under the covers. For example, data frames using degrees vs meters can make polygons and points warp past each other.

Reproject datasets to use consistent units:

# Reproject county data from state plane to wgs84
reproject_datasource(join_layer, crs=4326) 

Dealing with Sliver Polygons Along Borders

Imprecise overlaps between features along shared borders occasionally create sliver polygons capturing skewed aggregates. In those cases try snapping geometries to remove gaps or smooth overlaps.

Use processing algorithms or field calculator scripts to backfill blank records using a spatial join.