Qgis Core Coordinate Systems

qgis-core-coordinate-systems

Quick Reference

Core CRS Classes

| Class | Purpose | Key Methods | |-------|---------|-------------| | QgsCoordinateReferenceSystem | Represents a spatial reference system | isValid(), authid(), isGeographic(), mapUnits() | | QgsCoordinateTransform | Transforms coordinates between two CRS | transform(), transformBoundingBox() | | QgsCoordinateTransformContext | Project-level datum transformation settings | Used as parameter for transforms | | QgsDistanceArea | Measures distances and areas in any CRS | measureLine(), measureArea(), setEllipsoid() |

CRS Creation Methods

| Input Format | Constructor Syntax | Example | |-------------|-------------------|---------| | EPSG code | QgsCoordinateReferenceSystem("EPSG:") | "EPSG:4326" | | PostGIS SRID | QgsCoordinateReferenceSystem("POSTGIS:") | "POSTGIS:4326" | | Internal ID | QgsCoordinateReferenceSystem("INTERNAL:") | "INTERNAL:3452" | | Proj string | QgsCoordinateReferenceSystem("PROJ:") | "PROJ:+proj=longlat +datum=WGS84" | | WKT string | QgsCoordinateReferenceSystem("WKT:") | "WKT:GEOGCS[...]" |

CRS Property Methods

| Method | Returns | Description | |--------|---------|-------------| | authid() | str | Authority identifier, e.g., "EPSG:4326" | | description() | str | Human-readable name, e.g., "WGS 84" | | isValid() | bool | Whether the CRS was successfully created | | isGeographic() | bool | True for geographic CRS (degrees), False for projected (meters) | | mapUnits() | Qgis.DistanceUnit | Unit enumeration for the CRS | | toProj() | str | Full Proj string representation | | toWkt() | str | Full WKT string representation | | postgisSrid() | int | PostGIS SRID value | | srsid() | int | QGIS internal ID | | projectionAcronym() | str | Projection type, e.g., "longlat" | | ellipsoidAcronym() | str | Ellipsoid identifier, e.g., "EPSG:7030" |

Common EPSG Codes

| Code | Name | Type | Units | Use Case | |------|------|------|-------|----------| | 4326 | WGS 84 | Geographic | Degrees | GPS coordinates, global data exchange | | 3857 | Web Mercator | Projected | Meters | Web maps (OpenStreetMap, Google Maps) | | 32631-32660 | UTM Zones 31N-60N | Projected | Meters | Accurate local measurements (Northern Hemisphere) | | 32701-32760 | UTM Zones 1S-60S | Projected | Meters | Accurate local measurements (Southern Hemisphere) | | 28992 | Amersfoort / RD New | Projected | Meters | Netherlands national grid | | 27700 | OSGB 1936 | Projected | Meters | UK Ordnance Survey | | 2154 | RGF93 / Lambert-93 | Projected | Meters | France national grid |

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Critical Warnings

NEVER create a QgsCoordinateTransform without a QgsCoordinateTransformContext. The bare constructor ignores datum transformation preferences and produces silently inaccurate results.

NEVER assign a CRS to a layer as a substitute for reprojection. layer.setCrs() changes the interpretation of existing coordinates -- it does NOT transform the data. Use QgsCoordinateTransform to actually reproject.

NEVER assume QgsPointXY takes latitude first for EPSG:4326. QgsPointXY ALWAYS takes (x, y) = (longitude, latitude), regardless of the EPSG axis order specification.

NEVER use deprecated creation methods (createFromSrid(), createFromEpsg()). ALWAYS use the string-prefix constructor: QgsCoordinateReferenceSystem("EPSG:4326").

NEVER skip isValid() checks after CRS creation. An invalid CRS silently produces wrong results in ALL downstream operations (transforms, measurements, spatial queries).

NEVER run standalone PyQGIS scripts without QgsApplication.setPrefixPath() set correctly. Without it, QGIS cannot find the srs.db database and ALL CRS operations fail silently.

ALWAYS use QgsProject.instance().transformContext() instead of a bare QgsCoordinateTransformContext(). The project context includes user-configured datum transformation preferences.

ALWAYS check datum transformation grid availability before transforming between datums that require grid files. Missing grids cause silent loss of precision.

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Decision Tree

Which CRS to Use?

Need to handle coordinates?
├── Receiving GPS/WGS84 data?
│   └── Use EPSG:4326 (geographic, degrees)
├── Displaying on a web map?
│   └── Use EPSG:3857 (Web Mercator, meters)
├── Measuring distances/areas accurately?
│   ├── Local area ( destination)
pt_utm = xform.transform(QgsPointXY(18.0, 5.0))

# Reverse transform (destination -> source)
pt_wgs = xform.transform(pt_utm, QgsCoordinateTransform.ReverseTransform)

Pattern 3: Transform a Geometry

from qgis.core import (
    QgsCoordinateReferenceSystem,
    QgsCoordinateTransform,
    QgsProject,
    QgsGeometry,
    QgsPointXY
)

geom = QgsGeometry.fromPointXY(QgsPointXY(5.0, 52.0))
xform = QgsCoordinateTransform(
    QgsCoordinateReferenceSystem("EPSG:4326"),
    QgsCoordinateReferenceSystem("EPSG:28992"),
    QgsProject.instance().transformContext()
)
geom.transform(xform)  # In-place transformation

Pattern 4: Measure Distance with QgsDistanceArea

from qgis.core import (
    QgsDistanceArea,
    QgsCoordinateReferenceSystem,
    QgsPointXY,
    QgsProject,
    Qgis
)

d = QgsDistanceArea()
d.setSourceCrs(
    QgsCoordinateReferenceSystem("EPSG:4326"),
    QgsProject.instance().transformContext()
)
d.setEllipsoid("WGS84")

point1 = QgsPointXY(5.0, 52.0)
point2 = QgsPointXY(5.1, 52.1)
distance_m = d.measureLine(point1, point2)

# Convert to kilometers
distance_km = d.convertLengthMeasurement(distance_m, Qgis.DistanceUnit.Kilometers)

Pattern 5: Set Project CRS

from qgis.core import QgsProject, QgsCoordinateReferenceSystem

# Set project CRS: all layers render in this CRS via on-the-fly reprojection
QgsProject.instance().setCrs(QgsCoordinateReferenceSystem("EPSG:3857"))

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Common Operations

Get CRS from an Existing Layer

layer = QgsProject.instance().mapLayersByName("my_layer")[0]
crs = layer.crs()
print(crs.authid())  # e.g., "EPSG:4326"

Transform a Bounding Box

from qgis.core import QgsRectangle

bbox = QgsRectangle(4.0, 51.0, 6.0, 53.0)  # xmin, ymin, xmax, ymax
xform = QgsCoordinateTransform(
    QgsCoordinateReferenceSystem("EPSG:4326"),
    QgsCoordinateReferenceSystem("EPSG:3857"),
    QgsProject.instance().transformContext()
)
bbox_transformed = xform.transformBoundingBox(bbox)

Measure Area of a Polygon

from qgis.core import QgsDistanceArea, QgsCoordinateReferenceSystem, QgsProject, Qgis

d = QgsDistanceArea()
d.setSourceCrs(layer.crs(), QgsProject.instance().transformContext())
d.setEllipsoid("WGS84")

for feature in layer.getFeatures():
    area_sqm = d.measureArea(feature.geometry())
    area_ha = d.convertAreaMeasurement(area_sqm, Qgis.AreaUnit.Hectares)

Reproject a Layer via Processing

import processing

result = processing.run("native:reprojectlayer", {
    "INPUT": layer,
    "TARGET_CRS": QgsCoordinateReferenceSystem("EPSG:32632"),
    "OUTPUT": "memory:"
})
reprojected_layer = result["OUTPUT"]

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Reference Links

• [references/methods.md](references/methods.md) -- API signatures for QgsCoordinateReferenceSystem, QgsCoordinateTransform, QgsDistanceArea
• [references/examples.md](references/examples.md) -- Working code examples for CRS operations
• [references/anti-patterns.md](references/anti-patterns.md) -- CRS pitfalls and what NOT to do

Official Sources

• https://qgis.org/pyqgis/master/core/QgsCoordinateReferenceSystem.html
• https://qgis.org/pyqgis/master/core/QgsCoordinateTransform.html
• https://qgis.org/pyqgis/master/core/QgsDistanceArea.html
• https://docs.qgis.org/latest/en/docs/pyqgisdevelopercookbook/crs.html

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: Impertio-Studio
• Source: Impertio-Studio/QGIS-Claude-Skill-Package
• License: MIT

Install and usage instructions live in the source repository linked above.