Qgis Knowledge Patch

QGIS Knowledge Patch

Use this skill when changing QGIS projects, plugins, Processing workflows, providers, QGIS Server deployments, cartography, layouts, elevation profiles, or 3D and point-cloud workflows. Inspect the project's QGIS, Qt, GDAL, GEOS, PDAL, SFCGAL, and provider versions before applying version-dependent advice.

How to use this skill

1. Identify whether the task concerns desktop behavior, server behavior, Processing, Python/C++ APIs, plugin metadata, or provider capabilities.
2. Read the matching topic reference below. Consult more than one when a workflow crosses boundaries, such as a plugin invoking Processing or a server project backed by PostgreSQL.
3. Check dependency gates explicitly. Several features require particular GDAL, PDAL, wrench, SFCGAL, or GeoPandas versions.
4. Preserve changed defaults deliberately when upgrading existing projects.
5. Test provider- and rendering-dependent work against the actual data source, because advertised service capabilities and build options affect behavior.

Reference index

Breaking changes and migration traps

Declare QGIS 4 plugin compatibility with version bounds

Plugin compatibility is determined by qgisMinimumVersion and the optional qgisMaximumVersion. If the maximum is absent, compatibility extends only to the end of the minimum version's major line. A plugin retaining QGIS 3.22 support while declaring QGIS 4 compatibility can use:

[general]
qgisMinimumVersion=3.22
qgisMaximumVersion=4.99

The QGIS 4 Ready list accepts a plugin when either bound is at least 4.0. Remove supportsQt6=True: QGIS no longer recognizes it. Before widening the range, replace Qt 5-only APIs and direct PyQt5 imports with Qt 6 equivalents, prefer qgis.PyQt, test under QGIS 4, and run pyqgis4-checker. Its Qt6 Check findings identify files and lines but do not block upload or approval.

Target the separate QGIS 4 settings location

QGIS 4 settings are isolated from QGIS 3. First startup performs a one-time, lossless copy of the loaded QGIS 3 profile, but later changes do not synchronize. Update installation, profile-management, and enterprise scripts to target the QGIS 4 location.

Account for changed server and layout defaults

• The QGIS Server OAPIF root is /ogcapi, not /wfs3. Override it with QGIS_SERVER_API_WFS3_ROOT_PATH when a deployment needs another path.
• New layout legends default to Visible Layers. Choose All Project Layers or Manual Layer Selection when that is the intended contract; a global layout setting can restore the earlier default.
• WMS service resolution for scale-aware raster extraction defaults to 96 DPI.
• Merge Vector Layers still enables source layer and path fields by default for backward compatibility.

Replace deprecated hub-distance algorithms

Use the C++ Hub Distance algorithm. It replaces Distance to Nearest Hub (Points) and Distance to Nearest Hub (Line to Hub), exposes both optional outputs, and leaves the older pair deprecated.

High-value workflow guidance

Make Cloud Optimized GeoTIFF intent explicit

When GDAL 3.13 or later is present, raster Save and Export dialogs can request COG optimization and pyramids, and Processing can bulk-convert a directory. For Processing outputs pass -of COG; the .tif or .tiff extension cannot distinguish the COG and GTiff drivers.

Handle temporal raster and WMS behavior deliberately

Raster layers representing temporal values can accumulate pixels across time. WMS-T groups can derive a recursive time dimension from children, and a fixed raster date/time can define both temporal endpoints. Disabling time propagation on a group prevents child dimensions from reaching its parent.

Use the expanded geometry APIs and engines

• PyQGIS exposes QgsGeos directly.
• QgsGeometry.as_numpy() preserves XY, XYZ, XYM, or XYZM dimensionality.
• QgsGeometry.area3D() returns surface area for polygonal and surface types, and zero for points and lines.
• QgsVectorLayer.as_geopandas() creates a GeoPandas dataframe when GeoPandas is installed.
• SFCGAL is available through QgsSfcgalEngine and QgsSfcgalGeometry.

Choose OAPIF formats and paths intentionally

OGC API Features connections can select advertised formats, including GML and bulk-download choices, rather than always using GeoJSON. The server exports FlatGeobuf. Keep client format selection and the /ogcapi server root aligned with deployment routing.

Use durable OAuth2 and cloud authentication

OAuth2 can attach extra token-endpoint values as HTTP headers and refresh tokens automatically while connections remain in use. Authentication-manager SAS signing supports Microsoft Planetary Computer, while Pro GeoCatalogs can use SAS plus OAuth2; the authentication configuration travels in STAC, GDAL, and point-cloud data-source URIs.

Build reusable layouts instead of per-output workarounds

• Layout legends can wrap text in millimeters and synchronize using explicit layer-selection modes.
• Atlas polygons can reshape map frames for clipping and masking, and an atlas can render only its current coverage feature.
• Charts accept expression-driven series, filtering, ordering, pie plots, and renderer-derived categories and colors.
• Geospatial PDF export can preserve the project layer tree when the map has no locked layers, but does not support mutually exclusive groups.

Treat elevation profiles as project resources

Profiles can be saved, reopened, renamed, removed, and optionally synchronized to the main layer tree. Per-layer custom tolerance overrides the widget-wide value. Point clouds can render continuous profile lines, and a profile curve can be displayed in 3D with linked cursor feedback.

Validate point-cloud dependency gates

• M3C2 comparison requires a build with PDAL later than 2.10.
• PDAL TIN maximum-edge filtering requires PDAL 2.6+ and wrench 1.2.2+.
• VPC editing requires the VPC and every linked COPC file to be local.
• A .vpz may contain a zipped VPC, and multiple assets with the overview role can supply zoomed-out overviews.

Select Processing outputs precisely

Temporary outputs can retain user-selected layer names. Batch Processing also accepts temporary outputs. For raster output, set creation options and NoData values where exposed; for vector packaging, remember that an extent filter still creates an empty packaged layer when nothing intersects.

Control QGIS Server recovery and caching

Set QGIS_SERVER_PROJECT_CACHE_SIZE to configure the server project cache. Set QGIS_SERVER_RETRY_BAD_LAYERS=true when each request should retest layers previously accepted as invalid and restore them after dependencies recover.

Common correctness checks

Labels and symbols

• Budget symbol extent buffers: larger buffers fix off-canvas generated-symbol clipping but increase rendering work.
• HTML label backgrounds do not work on curved text; negative margins are limited to the bottom margin.
• Cross-layer duplicate suppression compares text case-sensitively.
• Whitespace-ignoring collisions apply only to curved placement and default off.
• Multipart split labeling drops surplus lines when parts are insufficient; vertex-based curved placement similarly drops excess characters.

Rasters and geometry

• Raster rank excludes NoData by default; its alternate mode propagates any input NoData.
• Extract Min/Max Pixel returns one point when multiple pixels tie.
• Approximate Medial Axis uses a 2D projection and ignores Z.
• native:forcecw and native:forceccw use opposite exterior/interior ring conventions; select the intended convention explicitly.
• Enabling Z transformation in Reproject Layer changes vertical coordinates as well as horizontal ones.

Layouts and projects

• Project scale method affects displayed/API scale, visibility, Processing, layouts, and server renders, but not map-unit symbol sizes.
• Geospatial PDF layer-tree preservation exports visible and invisible project layers and enables attributes for either all layers or none.
• Project-embedded Python is governed separately for macros, expressions, actions, and form initialization; do not assume a single trust switch.

3D and point clouds

• STL export does not preserve textures; use OBJ when textures matter.
• Environmental cube-map lighting applies to physically based materials but not fixed-gradient backgrounds.
• Some instanced 3D Tiles encodings are unsupported: quantized positions, oct-encoded rotations, and feature IDs.
• Remote VPC display does not imply remote editing support.

Version and dependency discipline

Treat provider-advertised capabilities and compiled dependency versions as runtime facts. In particular, check GDAL before COG, GeoPackage-domain, and dataset-identification work; PDAL and wrench before point-cloud algorithms; SFCGAL before medial-axis extensions; and GeoPandas before dataframe export. When preserving an older project's behavior, set formerly implicit options explicitly and verify results visually as well as through API or algorithm outputs.