Working with coordinate systems in ArcGIS

All geographic datasets have a geographic coordinate system (GCS). Some datasets also have a projected coordinate system (PCS). When you add a dataset to ArcMap™, ArcMap detects the geographic coordinate system and the projected coordinate system if there is one.

If all the data you want to display on a map is stored in the same geographic coordinate system, you can just add it to the map—the layers will overlay properly. If some of the datasets also have projected coordinate systems, even if they are different, you can also just add them to the map without data alignment worries—ArcMap will automatically make the layers overlay using a process called "on-the fly projection." The geographic coordinate system is the common language. ArcMap can convert the geographic coordinate system to any projected coordinate system and it can convert any projected coordinate system back to the geographic coordinate system.

An issue arises when you want to display datasets that have different geographic coordinate systems on the same map. The first layer you add to an empty data frame determines the coordinate system for the data frame. If that layer has a projected coordinate system, the data frame will have that same projected coordinate system. If you add a layer that has the same geographic coordinate system but a different projected coordinate system (or no projected coordinate system at all), ArcMap will perform an on-the-fly projection and convert the data to the data frame's projected coordinate system. The layers will overlay properly.

If, however, you try to add a layer that has a different geographic coordinate system, ArcMap will display a warning message telling you that it may not be able to properly align the data. ArcMap can still project the data on the fly, but it can no longer guarantee perfect alignment. (For perfect data alignment, you need to apply a transformation to make the geographic coordinate systems match—transformations are beyond the scope of this course.)

How do you know what coordinate system your data is stored in? You can view the coordinate system information for a dataset in ArcCatalog™, in its metadata. If a dataset has no coordinate system information in its metadata (it's missing), you may not be able to display the data in ArcMap. You may need to do some research to find out the coordinate system, then define the coordinate system using the ArcGIS tools provided. You will do this in the exercise coming up.

 

More information What happens when coordinate system information is missing?

When you add a dataset to ArcMap that is missing coordinate system information, ArcMap will try to read the coordinates of the data and determine whether they have been projected. If the coordinates are in the range of longitude-latitude values (x = ±180, y = ±90), ArcMap will add the data to the map and project it on the fly, although there may be inaccuracies because ArcMap cannot determine the geographic coordinate system for the data.

If the coordinates are not in the range of longitude-latitude values, ArcMap will display a warning. It will still add the data to the map, but it cannot project it on the fly. The result is usually that the data doesn't "fit" in the same coordinate space as the rest of the data, and either doesn't display or has serious alignment problems. In this case, you'll have to enter the necessary coordinate system information yourself in order to display the data properly on a map.