THE FILE FIELD GUIDE

.prj

Projection or Project Information

Maps & GPS

PRJ may describe a GIS coordinate system or another application’s project. For Shapefiles it is a sidecar; assigning a CRS differs from transforming coordinates.

Main use: Describe a dataset’s coordinate reference system
Data type: Multiple uses; identify the producer

All sections, questions and sources

Content reviewed：2026-10-19 · Editor：Kai

Opening steps

Choose your environment or purpose to see its steps.

QGIS/GDAL for GIS-origin files

Preserve the original and inspect a working copy in a compatible application.

Identify the producer and matching SHP companions.
Inspect WKT or other coordinate-system text.
Load the dataset and compare known locations.

Inspect the received file and its requirements

Establish dependencies and settings before proceeding.

For non-GIS projects, identify the original application.
Check datum, units and zone.
Do not assign a guessed CRS to unknown coordinates.

Menu labels and support vary by app, OS and version. Preserve the original before making changes.

Troubleshooting by symptom

No map: locate geometry data.
Large offset: inspect datum/units/zone.
Editing PRJ fails: distinguish assignment from reprojection.
Wrong application: identify other PRJ uses.

Dataset with PRJ → reprojected data

Establish the source CRS, then export a reprojected copy.
Reopen the output and check it against the preservation points in this guide before handoff.

Changing PRJ alone does not transform coordinates.

Assignment versus transformation

Assignment defines interpretation; reprojection recalculates coordinates. A wrong source CRS gives wrong output.

Source for these steps：GDAL · ESRI Shapefile / DBF / QGIS · Working with Projections

Worked example: checks before handoff

Example: compare boundaries at several known locations before exporting a transformed copy.

Source for these steps：GDAL · ESRI Shapefile / DBF / QGIS · Working with Projections

Related-format differences

Related-format differences
Format / method	Purpose and preserved information
.prj	Describe a dataset’s coordinate reference system
SHP / CPG	Geometry / attribute-encoding sidecar.
Dataset with PRJ → reprojected data	Changing PRJ alone does not transform coordinates.

Handling and security

Review the disclosure scope and precision of location data.

Questions and answers

Restore a map from PRJ?

Geometry and attribute data are needed.

Is an EPSG number always enough?

Confirm the exact system and units.

Open every PRJ in GIS?

Identify other producer-specific uses.

Format facts: name, steward and MIME type

Formal name: Projection or Project Information
Steward / specification: GDAL
MIME type: Consult the producer or delivery system. The extension alone does not establish a MIME type.

Primary and official sources

Project documentationESRI Shapefile / DBF
CPG encoding and PRJ coordinate systems.
GDAL · Link checked：2026-10-16
Software documentationWorking with Projections
Assigning coordinate systems versus transforming coordinates.
QGIS · Link checked：2026-10-16