a Python library to assist in realigning (multi-)polygons (OGC Simple Features) to reference borders

brdr is a Python package that assists in aligning geometric boundaries to reference boundaries. This is an important task in geographic data management to enhance data quality.

• In the context of geographic data management, it is important to have accurate and consistent boundaries for a variety of applications such as calculating areas, analyzing spatial relationships, and visualizing and querying geographic information.
• When creating geographic data, it is often more efficient to derive boundaries from existing reference data rather than collecting new data in the field.
• brdr can be used to align boundaries from new data to reference data, ensuring that the boundaries are accurate and consistent.

The figure below shows:

• the original thematic geometry (blue),
• A reference layer (yellow-black).
• The resulting geometry after alignment with brdr (green)

brdr provides a variety of side-functionalities to assist in aligning boundaries, including:

• Loading thematic data ((Multi-)Polygons): as a dict, geojson or Web Feature Service (WFS-url)
• Loading reference data ((Multi-)Polygons): as a dict, geojson or Web Feature Service (WFS-url)
• (Flanders-specific) Download reference data from GRB-Flanders
• Align thematic boundaries to reference boundaries
• Calculating a descriptive formulation of a thematic boundary based on a reference layer

• Geodata-management:
  • Implementation of brdr in business-processes and tooling
  • Bulk geodata-alignment
  • Alignment after reprojection of data
  • Cleaning data: In a postprocessing-phase, the algorithm executes sliver-cleanup and validity-cleaning on the resulting geometries
  • ...
• Data-Analysis: Investigate the pattern in deviation and change between thematic and reference boundaries
• Update-detection: Investigate the descriptive formulation before and after alignment to check for (automatic) alignment of geodata
• ...

An implementation of brdr for QGIS can be found at GitHub-brdrQ. This QGIS- script provides a User Interface to align thematic data to a reference layer, showing the results in the QGIS Table of Contents.

You can install the latest release of brdr from GitHub or PyPi:

pip install brdr

PIP_COMPILE_ARGS="-v --strip-extras --no-header --resolver=backtracking --no-emit-options --no-emit-find-links"
pip-compile $PIP_COMPILE_ARGS
pip-compile $PIP_COMPILE_ARGS -o requirements-dev.txt --all-extras

from brdr.aligner import Aligner
from shapely import from_wkt
from brdr.enums import OpenbaarDomeinStrategy
from examples import show_results

#CREATE AN ALIGNER
aligner = Aligner(relevant_distance=1, od_strategy=OpenbaarDomeinStrategy.SNAP_SINGLE_SIDE,
                  threshold_overlap_percentage=50, crs='EPSG:31370')
#ADD A THEMATIC POLYGON TO THEMATIC DICTIONARY
thematic_dict= {"theme_id_1": from_wkt('POLYGON ((0 0, 0 9, 5 10, 10 0, 0 0))')}
#ADD A REFERENCE POLYGON TO REFERENCE DICTIONARY
reference_dict = {"ref_id_1": from_wkt('POLYGON ((0 1, 0 10,8 10,10 1,0 1))')}
#LOAD THEMATIC DICTIONARY
aligner.load_thematic_data_dict(thematic_dict)
#LOAD REFERENCE DICTIONARY
aligner.load_reference_data_dict(reference_dict)
#EXECUTE THE ALIGNMENT
r, rd, rd_plus, rd_min, sd, si = aligner.process_dict_thematic(relevant_distance=1)
#SHOW RESULTING GEOMETRY AND CHANGES
show_results(r, rd_plus,rd_min,thematic_dict,reference_dict)

The resulting figure shows:

• the reference polygon (yellow-black)
• the original geometry (blue)
• the resulting geometry (green line)
• the added zone (green squares)
• the removed zone (red squares)

More examples can be found in Examples

(see also Basic example)

To use brdr, follow these steps:

• Create a Aligner-class with specific parameters:
  • relevant_distance (m) (default: 1): Distance-parameter used to decide which parts will be aligned, and which parts remain unchanged.
  • od_strategy (enum) (default: SNAP_SINGLE_SIDE): Strategy to align geodata that is not covered by reference-data
  • treshold_overlap_percentage (%)(0-100) (default 50)
  • crs: The Coordinate Reference System (CRS) (default: EPSG:31370 - Belgian Lambert72)
• Load thematic data
• Load reference data
• Process (align) the thematic data
• Results are returned:
  • Resulting geometry
  • Differences: parts that are 'different' from the original geometry (positive or negative)
  • Positive differences: parts that are added to the original geometry
  • Negative differences: parts that are removed form the original geometry
  • Relevant intersections: relevant intersecting parts of the reference geometries
  • Relevant differences: relevant differences of the reference geometries

The algorithm for alignment is based on 2 main principles:

• Principle of intentionality: Thematic boundaries can consciously or unconsciously deviate from the reference borders. The algorithm should keep notice of that.
• Selective spatial conservation of shape: The resulting geometry should re-use the shape of the reference borders where aligned is of relevance.

The algorithm can be split into 3 main phases:

• Initialisation:
  • Deciding which reference polygons are candidate-polygons to re-use its shape. The reference candidate polygons are selected based on spatial intersection with the thematic geometry.
• Processing:
  • Process all candidate-reference polygons one-by-one
  • Calculate relevant zones for each candidate-reference-polygon
    • relevant intersections: zones that must be present in the final result
    • relevant differences: zones that must be excluded from the final result
  • Evaluate each candidate based on their relative zones: which parts must be kept and which parts must be excluded
  • Union all kept parts to recompose a resulting geometry
• Post-processing:
  • Technical validation on inner holes and multipolygons
  • Clean-up slivers
  • Make the resulting geometry valid

The figure below shows a schematic overview of the algorithm (in dutch):

A more in-depth description of the algorithm can be found in the following article (in dutch):

ADD LINK to report

• Please report any issues or bugs here: https://github.com/OnroerendErfgoed/brdr/issues.