Define Projection for a Batch of Shapefiles

I was working on a project where I had downloaded 51 shapefiles (state-based census tract files) from the Census Generalized Cartographic Boundary Files. Each file lacked a projection .prj file, so I had to define each one as NAD83. Not wanting to do this one at a time, I used the GDAL / OGR tools and a bash script to process them all in a batch. I wrote a little script in a text file and then pasted it in the command line:

#!/bin/bash
for i in $(ls *.shp); do
ogr2ogr -f “ESRI Shapefile” -a_srs “EPSG:4269″ ./nad83 $i
done

It iterates through a list of all the shapefiles in a directory, uses OGR to define them as NAD83, then writes them to a new subdirectory called NAD83.

After searching through the web for some guidance on this, I later realized that there was a nice, succinct example of this in a book that I had (yeah – remember books? They’re still great!)

#!/bin/bash
# from Sherman (2008) Desktop GIS Mapping the Planet With Open Source Tools pp 243-44

for shp in *.shp
do
echo “Processing $shp”
ogr2ogr -f “ESRI Shapefile” -t_srs EPSG:4326 geo/$shp $shp
done

This does the same thing, difference here is that it prints a message to the command line for each file that’s processed and uses the -t_srs switch (transform projection) rather than the -a_srs (assign an output projection), which in this case seems to do the same thing. Of course you could tweak this a little to transform projections from one system to another as well.

This is fine and good if you’re using Linux and can use bash (go here for more info about bash). If you’re using Windows, you can do this if you’re using a Linux / UNIX terminal emulator like MSYS; otherwise you can use the DOS Command Prompt and write a batch (.bat) file to do this instead – the post on this forum is the first thing I found in my quest to figure all of this out.

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