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A .prj file contains the coordinate system information for a dataset and is required for ‘on the fly’ projection by GIS software. The file itself is a text file containing information in Well Known Text (WKT) format. The code snippet here shows you how to generate a .prj file, using Python, for your data if the .prj file is missing. See the post on CSV to Shapefile with pyshp for an example of using the function in a workflow. You will need to know what coordinate system the data is in and the EPSG code. For example, to find the EPSG code for WGS84 use a search engine and search for EPSG WGS84. The first site returned is usually http://spatialreference.org/. The EPSG code for WGS84 is 4326.

# function to generate .prj file information using spatialreference.org
def getWKT_PRJ (epsg_code):
     import urllib
     # access projection information
     wkt = urllib.urlopen("http://spatialreference.org/ref/epsg/{0}/prettywkt/".format(epsg_code))
     # remove spaces between charachters
     remove_spaces = wkt.read().replace(" ","")
     # place all the text on one line
     output = remove_spaces.replace("\n", "")
     return output

# create the .prj file
prj = open("filename.prj", "w")
# call the function and supply the epsg code
epsg = getWKT_PRJ("4326")
prj.write(epsg)
prj.close()

The filename should match the name of your data files. For examples, the .prj file for cities.shp or cities.tab should be cities.prj.

If you open the .prj file in a text editor you will see the text below.

GEOGCS["WGS84",DATUM["WGS_1984",SPHEROID["WGS84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]]

GEOGCS at the beginning of the WKT indicates that EPSG: 4326 is a geographic coordinate reference system (it would be PROJCS if it was a projected coordinate system). The information within the square brackets after DATUM provide the information for the parameters of the datum. We see the name of the datum, ‘WGS_1984’, and the SPHEROID information with a semimajor axis of 6,378,137 m with an inverse-flattening ratio of 298.257223563. The PRIMEM data tells us that it uses Greenwich as the prime meridian (where longitude is zero). UNIT specifies the measurement unit of the coordinate system, here it is ‘degree’, and the 0.0174532925199433 value is required to convert from radians into the stated units.

Open your data in a GIS and make sure that it is displaying correctly in relation to other features.