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I got a Shapefile but it's coordinate system is unknown and there is no .prj file.

How I can identify it now? Are there any tools that could help?

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What software do you have ArcView, ArcGIS - QGIS? – Mapperz♦ Mar 28 '11 at 17:56
In fact all of them. Why? – com Mar 28 '11 at 17:58
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load the shapefile in arcmap and then project it to an known projection (change the data frame projection) export as data frame then the shapefile is in a known projection - reproject that known shape to your own projection to fit existing data. – Mapperz♦ Mar 28 '11 at 18:03
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You can also tell a lot by looking at the coordinates in the dataset. What are the range of x and y values within the dataset? How much of an area do you expect it to cover? – scw Mar 29 '11 at 2:54
Not sure what you mean. Why do the shapefiles not have a defined projection? Can you clarify your operating environment. – Dan Patterson Dec 4 '11 at 3:39

8 Answers

up vote 16 down vote accepted

There's always the "brute force" method:

  1. Take a layer with a known coordinate system that is supposed to overlay with your unknown layer.

  2. Now make some educated guesses on what projection the unknown layer could be. (UTM, Plate Carree, etc). Project your known coordinate system layer into each projection until you find one that matches the unknown layer as much as possible.

Good Luck

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this is my method also. I utilize two different software to "prove" it. You might also peruse this question for some basic direction on projections. gis.stackexchange.com/questions/2769/… – Brad Nesom Mar 28 '11 at 20:16
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You could simplify step 2 a little by just changing the dataframe (ArcMap) projection, and letting the known layer reproject on the fly. – Brandon Copeland Mar 28 '11 at 20:50
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Note that you shouldn't be projecting the data. You should be defining the projection. Projecting the data changes the coordinates. Defining the projection just assigns a projection to that data and tells that software what projection the data is already in. These are two different tools in ArcMap, "Define Projection" is the one you want to use. – Sean Aug 31 '11 at 12:36
up vote 5 down vote

The best answer I've found to this question is non-technical: find out where your data came from. Agencies and organizations tend to be consistent with their use of projections. Know it came from your state DOT? Look at the rest of their data and see what it tells you. Don't know where it came from? An educated guess is just as likely to send you down the right road.

At least it makes tackling the problem with brute force a little more do-able!

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up vote 6 down vote

Seriously outdated at this point, but Werner Flacke and Birgit Klaus posted Find Projection on ArcScripts in 2007. I don't think the source code is there, unfortunately. It's VBA-based so only usable in ArcGIS Desktop v9.2 and possibly 9.3. It does include two shapefiles with the areas of interest from the EPSG Geodetic Parameter Dataset which could be used to narrow the possibilities.

Blue Marble Geographic Calculator and Geographic Transformer have coordinate system recovery tools.

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Thanks for the tip Melita. The code is there (*.frm). The license is public domain. PrjFinder is a companion to the book "Working with Projections and Datum Transformations in ArcGIS" (2006). – matt wilkie♦ Aug 30 '11 at 22:27
up vote 7 down vote

Not a tool (I don't know of one in existence that would let you do that), but check out @mkennedy's reply to a similar question. She explains how she arrived at the correct spatial reference. SpatialReference.org and patience will be your friends.

Additionally, ESRI provides a guide on how to guess a coordinate system (though I prefer mkennedy's method if you know a bit more about the data).

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up vote 7 down vote

Believe it or not people, you might be surprised what a simple google search can sometimes reveal!!

At my last job, I had a layer of Geology data (polygon) named "FSU_Geol.shp". My boss gave it to me and asked me to find out a number of things. First off, he was handed this shapefile by the client, and there was no .prj file, so he wanted me to figure that out. He also wanted me to figure out what the categories of Geology were. I could go on-and-on, but lets cut to the chase .... I googled it, and I ended up HERE.

I was beside myself!! The acronym in the naming convention stood for "surface Geology of the Former Soviet Union", and the Google search led me "directly" to the source (USGS). Everything, and I mean "EVERYTHING" I could ever need to know about this shapefile was at the top link I hit. I'm not saying that Google can find anything & everything, but let me tell you, I was fresh out of university, and just taking a "shot in the dark", and look at the feedback I got!! My boss was so impressed, yet I just got lucky!!
In another instance, someone in the office had downloaded a number of shapefiles using a batch function. I don't remember the name of the software package at the moment, but the .prj files were missing. I simply went in to the metadata in ArcCatalog, and I actually found my answer in there. Again, don't quote me, but I think there was a URL to the source in there.

The above answers are all excellent, and having said that, I'll be bookmarking this discussion for future reference. Ever since that day however ..... I "start" with a google search if there's no clues in the metadata!!

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up vote 11 down vote

There are two great links from Esri that go into detail on this:

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downloads2.esri.com/MappingCenter2007/arcGISResources/more/… is a recent post in the resource centre (mapping section) that goes over the process, similar to brenth's suggestion – SaultDon Mar 29 '11 at 16:04
up vote 10 down vote

I'm promoting comments by Mapperz and Brandon Copeland by adding an answer that uses their technique.

  1. If you have ArcGIS Desktop, add some reference data that covers the same area to ArcMap. This reference data must have a valid coordinate system (projection) definition.
  2. Add the data in an unknown coordinate system.
  3. Research what possible coordinate systems are used in this area. One place to check is the EPSG Registry.
  4. Set the data frame's coordinate system to the possibilities. If the data-with-a-known-coordsys line up with the data-without-a-known-coordsys, you've probably identified the coordinate system.

This technique takes advantage of the fact that ArcMap can convert data to a new coordinate system in-memory. The data-without-a-known-coordsys cannot be converted so it's just displayed. By projecting the known data in-memory, we can see quickly what coordinates it has in the various possible coordinate systems.

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Side note: In QGIS reprojecting in-memory is called "on-the-fly". – underdark♦ Mar 29 '11 at 6:08
It's called 'on-the-fly' in ArcGIS as well! I keep finding a lot of people who don't understand that phrase though, so I've been trying to use 'in-memory' instead. – mkennedy Mar 29 '11 at 22:32
up vote 7 down vote

Ask the producer.

If you know common CRS for your geographic region, you can try some of them. But asking is better.

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The only problem is i cannot ask him...This was also the first thought which i had. – com Mar 28 '11 at 17:53
Then, you'll have to go brute force as brenth said. – underdark♦ Mar 28 '11 at 20:08

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