I think everyone in the GIS world has run into this problem. You get some new data, add it to your map, and...it doesn't overlay with your other datasets.
Here are the first steps you should take to solve the problem or to ask for help here or elsewhere.
- Identify the current coordinate reference systems for all datasets and the map/data frame/etc. It's fine if one or more dataset has an 'undefined' coordinate reference system.
- Identify the extents for all datasets. If you know that the problematic dataset has certain latitude-longitude extents, that's great! If not, just identify a general location for the data: city, county or other administrative district, etc. For example, "The data should be near Wapakoneta, Ohio."
At this point, you could certainly post the information and ask for help.
One thing you shouldn't do.
Change the coordinate reference system of the dataset to match your other, reference data with the Define Projection Tool or the data's property page in ArcCatalog. These processes update the metadata only. The data extents and coordinate values are not changed.
Part Two will discuss how to determine the correct coordinate reference system for the dataset. If you want to see a related and previously published top, check out Identifying an Unknown Coordinate System.