I notice that sometimes Shapefiles that I have received contain .prj files. Sometimes they don't. Wikipedia notes that the .prj file is not "mandatory" and that it contains the :

projection format; the coordinate system and projection information, a plain text file describing the projection using well-known text format

So my question is: why is it sometimes included, and sometimes not? Do newer versions of Esri tools include it, whereas older ones didn't?

  • This "Why" question is really directed at the individuals who don't choose to adequately define their data. It has nothing to do with Esri tools. – Vince Nov 5 '17 at 13:45
  • So whether a prj file is included is a conscious decision made by the creator of the dataset? – Steve Bennett Nov 6 '17 at 4:18
  • Well, not including it certainly is. – Vince Nov 6 '17 at 4:24
  • In my experience, inexperienced users/users from a different discipline are generally when I receive a shapefile lacking a *.prj. Often, I ask for the .prj and .dbf, .shx files explicitly, or that I expect ~7 files in a "shapefile". – bwp8nt Nov 6 '17 at 5:13

The original and only published version of the shapefile format https://www.esri.com/library/whitepapers/pdfs/shapefile.pdf defines only three mandatory files:

An ESRI shapefile consists of a main file, an index file, and a dBASE table.

Shapefiles were introduced with ArcVIEW 2.0 and it may be that it could not utilize .prj file. The .prj file itself was already used by ESRI in Arc/Info E00 coverage format https://www.e-education.psu.edu/geog488/sites/www.e-education.psu.edu.geog488/files/downloads/Arc_Info%20E00%20Format.pdf.

Possible reasons to not use .prj file

  • It is not compulsory
  • The strings to use in .prj are not documented by ESRI so other vendors have had to reverse engineer them.
  • .prj is not needed if user works all the time in one coordinate system
  • Data providers usually tell also by some other means what it the correct coordinate system

ESRI is using in .prj files Well Known Text strings which are close to, but not similar than the WKT defined by OGC. As an example here is the GDAL source code that tries to make those two definitions to match https://trac.osgeo.org/gdal/browser/trunk/gdal/ogr/ogr_srs_esri.cpp.

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