Assigning Vs Reprojecting: Understanding The Key Difference In Qgis
Understanding Coordinate Reference Systems
A coordinate reference system (CRS) is a method of associating numerical coordinates with positions on the surface of the Earth. It provides a common framework for defining locations so that spatial data layers can be overlaid and integrated accurately in a geographic information system (GIS).
There are two main types of CRSs used in GIS:
- Geographic CRS – Based on latitude and longitude coordinates defined on a spheroid model of the Earth
- Projected CRS – Based on planar xy coordinates that have been mathematically transformed from geographic coordinates onto a flat surface
Understanding CRSs is critical because data layers compiled using different CRSs will not align properly. All data in a GIS project must share a common CRS to produce accurate maps, measurements, and analysis.
When to Assign vs Reproject Layers
When bringing spatial data into QGIS from different sources, you have two options to reconcile CRS differences:
- Assign CRS – Define the correct CRS for a layer that does not have one defined or has an incorrect one defined
- Reproject – Transform layer coordinates from one CRS to another
Assigning a CRS
Assigning a CRS is necessary when:
- A layer such as CSV table is imported without spatial reference information
- A layer has an incorrect CRS defined that doesn’t match its actual coordinates
Assigning forces the layer to use the designated CRS without changing its underlying coordinate values.
Reprojecting Layer Data
Reprojection transforms coordinates from one CRS to another to match the desired project CRS. Reasons to reproject a layer include:
- Standardize all data to a common CRS for analysis
- Match basemap layer such as OpenStreetMap
- Reduce distortion effects from projection transformations
Reprojection resamples and calculates new coordinate values for a layer to place it accurately in the target CRS.
Key Differences
| Assign CRS | Reproject Layer |
|---|---|
| Defines how QGIS interprets coordinate values | Transforms coordinate values to a different CRS |
| No change to actual coordinate values | Resamples coordinates and calculates new values |
| Typically used when data lacks a defined CRS | Used to match CRS of other layers in analysis |
Step-by-Step Guide
Assigning CRS with Code Examples
To assign a CRS in QGIS Python console:
layer = iface.activeLayer() crs = QgsCoordinateReferenceSystem(4326) layer.setCrs(crs)
To assign CRS definition when loading layer:
uri = "point.csv?delimiter={}&crs=epsg:27700"
layer = QgsVectorLayer(uri.format(';'), "points", "delimitedtext")
Reprojecting with Code Examples
To reproject single layer to specific CRS:
project_crs = QgsProject.instance().crs()
layer = iface.activeLayer()
params = {'INPUT': layer,
'TARGET_CRS': project_crs,
'OUTPUT': 'memory:ReprojectedLayer'}
reproj = processing.run('native:reprojectlayer', params)
To reproject all layers to match project CRS:
project = QgsProject.instance()
projectCRS = project.crs()
mapLayers = project.mapLayers()
reprojected = []
for layer in mapLayers.values():
params = {'INPUT': layer,
'TARGET_CRS': projectCRS,
'OUTPUT': 'memory:ReprojectedLayer'}
reproj = processing.run('native:reprojectlayer', params)
reprojected.append(reproj['OUTPUT'])
project.removeAllMapLayers()
project.addMapLayers(reprojected)
Common Issues and Solutions
Troubleshooting Tips
- Layers not aligned – Check CRS definitions of each layer
- Measurements wrong – Data may be in different units (m vs ft) based on assigned CRS
- Features relocated or missing – Reprojection can shift locations and drop features near poles
- Performance issues – Try only reprojecting what’s necessary and clip to project area
FAQs
Q: Do I have to reproject to match my basemap?
A: No, QGIS can transform layers “on-the-fly” for display without altering data.
Q: How do I check coordinates or assign a CRS programmatically?
A: Use QgsSpatialReferenceSystem class and methods like createFromProj4 or createFromString.
Q: What happens if I save a project with mixed CRS layers?
A: Project CRS gets set to match first layer loaded. Other layers reproject dynamically.
Key Takeaways
- Consistent CRS vital for accurate GIS analysis
- Assign CRS to define coordinates, reproject to transform them
- Reprojection changes data coordinate values
- Check CRS issues first to troubleshoot alignment problems
- Automate CRS checks and reprojection for processing workflows
Additional QGIS CRS resources:
