 This video will show you how to resolve a situation in which you have a vector data set in which the coordinate system is incorrectly defined. For this example, I'm going to be working with two data sets. One is the New Hampshire political districts. This is the data set I trust. The coordinate system is defined correctly. The other is a point data set of recreational points. Now someone has told me the coordinate system for this particular data set has been incorrectly defined. Because I trust the political district's data set, I'm going to add that to my map. As soon as I add it to my map, my map will adopt the coordinate system of this particular data set because it's the first data set added to the map. Going into the properties of the map, I can see that now the coordinate system of the map matches the coordinate system of the New Hampshire political districts layer. Now I'm going to add that point data set representing recreational areas to my map. Now of course the coordinate system is incorrectly defined and sure enough when I add it to the map, the points don't appear in New Hampshire. When I right click and choose zoom to layer, the points show up somewhere in eastern Canada. Definitely not ideal. Now if we go into the properties of the recreational points data set, we can explore the coordinate system. Under the source tab, we're going to scroll down to spatial reference and here we can see that the coordinate system is defined and that it's New Hampshire state plane, NAD 83 meters. Although the coordinate system is defined, the fact that it's not plotting out in the correct location is an indication that the coordinate system has been defined incorrectly. In the version of ArcGIS Pro I'm using for this demo, there's no good way to clear the coordinate system. You can see that leaving it blank or trying to type in unknown when running the define projection tool just results in an error. The workaround that I've found for this is to actually create a new feature class that has an undefined coordinate system and then use that feature class as the reference projection when I use the define projection tool. So my first step in this process, creating this new feature class, I'm going to call it unknown underscore CS and you're going to notice that I'm leaving the coordinate system information blank, meaning that it's going to be undefined. I'm not going to use this feature class for anything other than to define the projection of that recreational points data set so that it's unknown. If we go into the properties as this new feature class I've created, we can see that it has an unknown coordinate system. So now we can use this vector layer that has an unknown coordinate system in the define projection tool to clear the coordinate system from our recreational points data set. Going back into the define projection tool, I'm going to define the projection for the recreational points data set by setting it to match that of that new feature class I created that has an unknown coordinate system. When I run this tool, it will clear the coordinate system for the recreational points data set. We can confirm this by going into the properties of the recreational points data set, scrolling down to the spatial reference, and we see that the coordinate system is now unknown. Because the coordinate system is unknown, ArcGIS has no information to go on on projecting those points. It just assumes it in the same coordinate system as the map, which is New Hampshire State Plain NAD 83 feet. Because the points appear in the correct location, we can assume that the correct coordinate system of the recreational points data set is not New Hampshire State Plain NAD 83 meters as it was originally defined as, but of NAD 83 feet. So I can go back into the define projection tool and now set the coordinate system for the recreational points to match that of the map, which is New Hampshire State Plain NAD 83 feet. Going into the properties of the recreational points data set, we can confirm that the coordinate system information has been defined correctly. So when you have a data set in which the coordinate system has been defined incorrectly, the first thing you need to do is clear the coordinate system, then you can figure out what the coordinate system is. Once you've figured out what the coordinate system is, you can define the coordinate system and you're good to go.