PRJ File Documentation

Summary

A project file (.prj) is a generic extension used by many unrelated programs, so its meaning depends on what created it. The most concrete and widely searched type is the Esri projection file: a small plain-text sidecar that tells GIS software which coordinate system a Shapefile uses, written in Well-Known Text (WKT). It sits beside the .shp, .shx and .dbf and is read automatically by QGIS or ArcGIS; you can view it in any text editor. If your .prj is not GIS — it may be a Visual Studio, 3ds Max or audio-app project — open it with the program that made it.

Technical details

FeatureValue
Extension.prj
Common meaningGeneric “project file” (ambiguous)
Most concrete typeEsri projection (coordinate-system) file — Shapefile companion
Format type (GIS)Plain-text coordinate-reference-system definition
Encoding (GIS)Well-Known Text (WKT / WKT2)
MIME type (GIS)text/plain
Developer (GIS)Esri; WKT CRS standardised by the OGC
IntroducedWith the Esri Shapefile (early 1990s)
Open standard (GIS)Yes (OGC WKT for coordinate reference systems)
Magic numberNone — text starting PROJCS[, GEOGCS[, PROJCRS[ or GEOGCRS[
Belongs with.shp, .shx, .dbf, .cpg (a Shapefile is a file set)
Opens in (GIS)QGIS, ArcGIS Pro, GDAL/OGR, any text editor
Other meaningsVisual Studio, Autodesk 3ds Max, Tracktion/WaveLab audio, HEC-RAS, AIMMS
Related extensions.shp, .dbf, .wkt, .qpj
Structure at a glance (GIS projection file)

A GIS .prj is a short plain-text file with no binary signature. It holds one Well-Known Text coordinate-system definition, beginning with a keyword such as PROJCS[ (a projected system) or GEOGCS[ (a geographic one); newer WKT2 uses PROJCRS[ / GEOGCRS[. A typical file is a single line like PROJCS["WGS_1984_UTM_Zone_33N", GEOGCS["GCS_WGS_1984", DATUM[...], SPHEROID[...]], PROJECTION["Transverse_Mercator"], PARAMETER[...], UNIT["Metre",1]]. Identify it by these keywords and open it in any text editor. An application project .prj (Visual Studio, 3ds Max, audio tools) has a completely different, program-specific layout.

What is a PRJ file?

The .prj extension is genuinely generic — short for “project” — and dozens of unrelated programs use it for their own workspace or project files. Because there is no single format behind the extension, the honest answer to “what is a .prj” depends entirely on which program wrote it. This page leads with the most concrete and most searched meaning, the Esri projection file, and then covers the application-project meanings so you can tell which one you have.

An Esri .prj is a tiny plain-text file that accompanies a Shapefile and defines its coordinate reference system (CRS): the map projection, datum, ellipsoid and units. It is not a data file — it contains no features or geometry — only the description of how the Shapefile’s coordinates map onto the Earth.

A companion in the Shapefile set

A Shapefile is not one file but a set that must travel together. The geometry lives in the mandatory .shp, the index in .shx, and the attribute table in the dBASE .dbf; the .prj sits beside them and tells GIS software how to place that geometry on the map. Without a .prj, the coordinates in the .shp are just numbers with no known reference frame, so a layer may land in the wrong part of the world or fail to line up with other data. That “my Shapefile is in the wrong place” problem is very often a missing or incorrect .prj, and the fix is to supply or correct it.

Well-Known Text: what the file actually contains

The projection is written in Well-Known Text (WKT), an open Open Geospatial Consortium (OGC) standard for describing coordinate reference systems as readable text. A projected system starts with PROJCS[ and a geographic (lat/long) system with GEOGCS[; the newer WKT2 revision uses PROJCRS[ and GEOGCRS[. The definition nests the pieces of the CRS:

PROJCS["WGS_1984_UTM_Zone_33N",
    GEOGCS["GCS_WGS_1984",
        DATUM["D_WGS_1984",
            SPHEROID["WGS_1984",6378137.0,298.257223563]],
        PRIMEM["Greenwich",0.0],
        UNIT["Degree",0.0174532925199433]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["False_Easting",500000.0],
    PARAMETER["Central_Meridian",15.0],
    PARAMETER["Scale_Factor",0.9996],
    UNIT["Metre",1.0]]

The DATUM and SPHEROID fix the model of the Earth; the PROJECTION and its PARAMETER entries define how the curved surface is flattened onto a plane; and UNIT gives the linear or angular units. WKT2 can also carry an AUTHORITY reference to the EPSG code, the numeric catalogue ID (for example EPSG:32633 for the UTM zone above) that many tools prefer over a full .prj. Because WKT is an open standard, QGIS, ArcGIS, GDAL/OGR and other GIS software all read the same definition.

Opening, reading and fixing a GIS PRJ

You open a GIS .prj in two ways. Normally you never open it directly at all: loading the Shapefile into QGIS or ArcGIS Pro reads the .prj automatically and sets the layer’s CRS. To read or edit the projection string, open the file in any text editor — it is short, readable WKT. To fix the classic misalignment problem, you re-define the projection rather than moving the data:

# read the CRS a .prj describes, or get its EPSG code
gdalsrsinfo file.prj
gdalsrsinfo -o epsg file.prj

# write a new .prj by assigning the correct CRS to a Shapefile
ogr2ogr -a_srs EPSG:4326 out.shp in.shp

In QGIS you export the layer with the correct CRS; in ArcGIS the Define Projection tool writes a corrected .prj. Note the distinction between assigning a CRS (telling the software what the existing coordinates already are, which fixes a wrong .prj) and reprojecting (recomputing the coordinates into a different system) — using the wrong one is a common source of misplaced data.

The other PRJ: application project files

The competing meaning is the generic application project file, and these formats share nothing but the three letters. Each is opened only by the program that created it:

  • Microsoft Visual Studio (older versions) saved a project as .prj.
  • Autodesk 3ds Max and some CAD tools use .prj for a scene/project — this is the source of the .3ds/.obj/.dae conversion pairings you may see, which belong to 3ds Max, not to GIS.
  • Audio software such as Tracktion and WaveLab store a session as .prj.
  • HEC-RAS, the US Army Corps hydraulic-modelling package, confusingly uses .prj for its project — a different file from the GIS projection despite the identical extension.
  • Optimisation tool AIMMS, the Monarch data tool and various device programmers each save their own .prj.

There is no universal .prj opener for these. A GIS projection .prj is just a text coordinate string and cannot be turned into a .3ds or .obj model — that conversion only makes sense for a 3ds Max project, exported from 3ds Max. If your file is not GIS, identify the creating application and open it there.

References