any way to figure out the projection a .dwg is in?

[RaquelAguirre1]

I've obtained city data for one of my projects in DWG form that no one knows the projection for. I'd very much like to convert data to shapefiles and use it, but I'm hitting a dead end trying to find the projection.

Is there any way to figure out a projection by looking at the data?

thanks,

[PeterStratton]

It can be complicated, and to describe the process and all the various pitfalls would be akin to writing a book.  Luckily, such a book has already been written:

http://www.esri.com/news/releases/10_2qtr/lining-up-data.html

Best of luck though!

[RaquelAguirre1]

Good news and bad news, I guess...  Thanks very much.

[kellydoherty]

If the data is only available in .dwg form it is very possible that the data is not projected at all, or georeferenced in any way. If it is in a local coordinate system, you would need to tie that network to 2 or more stations that have lat/long coordinates, or tie 2 of the corners of the project directly to lat/long using GPS so that you have lat/long locations and can project it from there.

[MelitaKennedy]

You can use ArcMap to help identify a coordinate system. Leave the CAD file with an unknown (undefined) coordinate system. Add it and some reference data (that does have a known coordinate system) to ArcMap.

You can now set the data frame's coordinate system to possible candidates. In the US, that would include various State Plane zones (in meters, US survey feet, or int'l feet), UTM zones, state-wide grid, etc. It's less likely that city data uses one of these because sometimes there's a city coordinate system that we (Esri) just hasn't heard about.

So as you set the data frame's coordinate system, the unknown data will continue to be displayed where it was. That is, the software can't do anything to it. The reference data will be projected to the new coordinate system. If you can get the reference data to line up with the CAD data, then you've discovered the coordinate system.

Another trick is to look at the coordinate values. If the X or Y values have at least 6 digits, then the data may well be using a 'known' coordinate system. Five or less usually means a local system.

Melita