This document describes using a virtual layer in QGIS to validate overlay between geometries in a shapefile. It provides a zip file with a QGIS project, virtual layer XML file, and Python script. The virtual layer is defined to check for overlay between geometries using SQL. The QGIS project configures this layer. The Python script enables automatic updates to reload the validation layer whenever geometries in the source layer are edited.

1. Virtual layer for validate overlay in
shapefile
Luiz.motta@ibama.gov.br
http://www.ibama.gov.br
Luiz Motta

2. MMA - Ministry of the Environment – Brazil
IBAMA - Brazilian Institute of Environment and
Renewable Natural Resources
DIPRO - Directorship of Environmental Protection
CGMAM – General Coordination of Monitoring
COTIG – Coordination of Geospatial Technology *
Luiz Motta:
● Environment Analyst of IBAMA since 2003
● Expert in FOSS4G (contributor of QGIS)

3. ● Database example.
● Definition of virtual layer (VRT) for check overlay.
● QGIS project configuration.
● Python script for automatic update.
● Checking overlay
Presentation plan

4. ● Dowload:
https://www.dropbox.com/s/8efs5e8x054kiru/overlay_vrt.zip?dl=0
● overlay_vrt.zip:
├── overlay_vrt.qgs
├── validate overlay_inside.vrt.xml
├── py
│ └── update_vrt_by_edit.py
├── shp
├── test_overlay.dbf
├── test_overlay.prj
├── test_overlay.qml
├── test_overlay.qpj
├── test_overlay.shp
└── test_overlay.shx
Database example

5. ● validate overlay_inside.vrt.xml:
● OGRVRTDataSource: Define dataset (set of layers)
● OGRVRTLayer: Define a layer
● SrcDataSource: Source of data
● SrcSQL: Define SQL for check overlay using 'SrcDataSource'
● GeometryField: Geometry's field of layer
● GeometryType: Geometry's type
● LayerSRS: Spatial Reference System
Definition of VRT for check overlay
Sources:
● VRT format: http://www.gdal.org/drv_vrt.html
● Spatialite: http://www.gaia-gis.it/spatialite-2.4.0/spatialite-sql-2.4.html

6. QGIS project: overlay_vrt.qgs
● test_overlay properties:
Suppress attribute form pop-up after creation → On
QGIS project configuration

7. Script: update_vrt_by_edit.py
● Enable automatic update:
● test_overlay: Populate 'id' field
● overlay_area: Reload layer (remove and add again)
● Happening when save edition
Python script for automatic update.

8. Run 'update_vrt_by_edit.py' by console
Python script for automatic update.

9. Checking overlay
● Start edition of 'test_overlay'
● Create geometry with overlay
● Save edition
● Youtube: https://youtu.be/L78aOUwicBo