Matching feature classes with the same coordinate system but different projections

[CorinnaVan_Cayzeele]

Dear Co-GIS-Users,

I have an ArcMap Project in the WGS_1984_UTM_Zone_40N coordinate system (for north-east Iran), using data and shapefiles from different sources. For the xy-data and the image I use as basemap, I defined the above coordinate system which the map-image is originally in as well as a DEM of the area. Other shapefiles (roads, rivers etc.) had no coordinate system defined. I defined it by creating a new feature dataset and adding the files into it. However, even though all of my data now have the same (geographic and projected) coordinate system, they are shown very far apart. If I try to align them, e.g. by clicking "set data source" in the properties window or by creating links in Editor-Spatial Adjustment, the shapes of the features change completely. I have this problem no matter the order in which I add the data into a new map project.  Does anyone have a hint on what the problem might be and how I could overlay the data?
Many thanks,
Corinna

[WesMiller]

You'll need to find out what coordinate system the data was originally drawn. Then Define Projection—Help | ArcGIS for Desktop (you could stop here and let arcmap project on the fly) and then use Project—Help | ArcGIS for Desktop that should get the data where it needs to be.

[AdrianWelsh]

Corinna,

I agree with what Wes said. How far off is your newly projected data from where it should be? Are we talking about a few feet/meters/miles? Or are we talking about on a different continent? Knowing this could help narrow down a few things, but it would be really helpful to know what coordinate system that the data was created in (like Wes said).

[NeilAyres]

Add to add...

What does the extents of the unknown stuff look like.

You may be able to guess what the coordinate system of the unknown data is, and then define it as that. It obviously wasn't originally the same as your base layers (not UTM40N). But far rather find out what the coord sys should be from the supplier of the data.