There is little more frustrating than receiving a GIS layer that doesn’t have a defined coordinate system. This becomes a headache when trying to view, let alone analyze, it with a layer in a different coordinate system. They just won’t line up until both have a defined coordinate system so one can be projected on-the-fly. Here are a couple approaches to figuring out what the missing coordinate system is.

**Are the coordinates lat/long?** If so, first try the WGS 1984 projection. Define the projections in the layer properties in ArcCatalog. Also remember that latitude is the y coordinate while longitude is x. If the data is flipped the latitude and longitude might be reversed, or missing negative values if the the longitude values are westings.
**Look at the coordinates of other data in the area of interest** – Keep a table of coordinate systems and the range of x and y in the area of interest. Here is part of the table I keep for New Brunswick and update it whenever a receive a layer in a new coordinate system. It doesn’t take long to maintain this table, but it can save a lot of time:
**Trial and error** – Create a new session with a layer with a defined coordinate system in the area of interest and the layer that is missing the coordinate system. Zoom to the layer with the defined coordinate system and change the coordinate system of the data frame until the other layer comes into view.
**Georectify the data** – A last resort can be to georectify the layer using a layer in the coordinate system you want. Use the Spatial Adjustment toolbar for vector data or the Georeferencing toolbar for raster data.

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