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I have 8 shapefiles which have a common coordinate system but none of them have it defined i.e. "unknown" is written when I look in the properties dialog box. However, I can see the extent which I am sharing here:

Top: 42.349145 Bottom: 41.710962 Right: -82.430463 Left: -83.126193

I need to define the projection of these shapefiles so that I could do further analysis. I know that the features these shapefiles represent are located in Windsor, Ontario i.e.they should have a projection of WGS 84 UTM Zone 17N. Can I directly define this projection without knowing about the geographic coordinate system?

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The values are in decimal degrees. That makes it harder to identify the actual coordinate reference system, because the values are so close. It's a lot easier to differentiate between a UTM and a MTM zone. If the data was collected recently, they may be in NAD 1983 CSRS.

On a related note, the Define Projection tool and the data's property page in ArcCatalog allow you to assign the current coordinate reference system. They update the metadata, not the actual values of the dataset. The data has to be georeferenced / in that coordinate reference system already. To create a new dataset in another coordinate reference system, you use the Project Tool (but the input data must have its correct coordinate ref system!).

  • I just realized that UTM system is a projected coordinate system and because my layers are in decimal degrees (as you said) and there is no .prj file, this means that the GCS must be NAD27 or NAD83 or WGS84. Now, beacuse I already know that features are located in Windsor, ON so they must be projected in UTM Zone 17N. I think there is no problem if I define the projection in WGS84 UTM Zone 17N now, beacuse if it is not WGS84, Arcmap would say that the shapefile's GCS is different. – beginagain Jul 8 '13 at 19:43
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    If I understand you correctly, you can't define the data directly as UTM. You must first define the current coordinate system, and then project the data to the new coordinate system. – mkennedy Jul 8 '13 at 19:49
  • I have defined the GCS of data frame as WGS 84. Now, if I define the projection of undefined shapefiles, using define projection tool, isn't it OK? If not, how should I go about? – beginagain Jul 8 '13 at 20:49
  • Yes, that's okay. Use the Define Projection tool to set the coordinate system of the shapefiles. I'm obviously not being clear today--you can set ArcMap's coordinate system to anything. It will project on-the-fly (in memory) any data layers in a different coordinate system to the data frame's. – mkennedy Jul 8 '13 at 21:30
  • I used define projection tool to define the projection of all layers to WGS84 UTM Zone 17 N but now when I click full extent I can't see all of them together. Does this mean that the projection is not appropriate? – beginagain Jul 8 '13 at 21:41

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