 In this video, we'll take a look at how to deal with a vector dataset in which the coordinate system is unknown. This is extremely frustrating. An unknown coordinate system can be the result of the coordinate system being accidentally deleted, never defined in the first place, or perhaps lost during data conversion, such as moving from CAD to GIS. Let's begin in ArcGIS. I've got a base map loaded in, the World Dark Gray Base Map, and also the Vermont State Boundary. The Vermont State Boundary is in Vermont State Plain coordinates, and because it's the first non-base map layer I've loaded into ArcGIS, the map data frame has adopted the coordinate system from that layer. So the map data frame is set to Vermont State Plain, at 83 meters. This means that any layer that's added into this map document, if it has a defined coordinate system, will be automatically re-projected on the fly to Vermont State Plain. If it doesn't have a coordinate system, ArcGIS will have no idea how to plot it out. Now I'm going to go into ArcCatalog and add an open space layer. This open space layer does not have a defined coordinate system. As a result, when it's added to ArcGIS, all ArcGIS can do is assume it's in the same coordinate system as the data frame, which is Vermont State Plain. Going into the properties of this layer, we see that the coordinate system is in fact unknown. This open space layer is for the city of South Burlington, but zooming into that location shows no evidence of the layer. When we right-click and choose zoom to layer, it zooms us to the location of the open space layer. The good news is that we have polygon data of the open space locations. The bad news is that it's not showing up in the correct location. When we zoom out, we can see that it's appearing somewhere up at the Arctic Circle. This is clearly incorrect. This problem is caused by the fact that we have a map data frame with the coordinate system set to Vermont State Plain at 83 meters and an open space layer, which although it has polygons that have a location, that coordinate system is undefined and it's clearly not Vermont State Plain. Now we need to start the coordinate system guessing game. The way this works is we're going to continually change the coordinate system in the data frame until the open space layer lines up in the correct location. Guessing the correct coordinate system can be really difficult because there's thousands of possible options. Guessing the coordinate system can be really intimidating because there's thousands to choose from. Fortunately for me, someone told me that the open space layer is in the UTM coordinate system. That's not enough information by itself, but it certainly narrows things down because Vermont crosses only two UTM zones. I first tried UTM zone 19, but we can see when I set the coordinate system for the data frame, the open space layer is still not in the correct location. Given that 19 didn't work, I'm going to give zone 18 a try now. So I'm going to go back in and adjust the coordinate system of my map data frame. Once I set the coordinate system to UTM zone 18 and click OK, we see that the open space layer appears where it should in South Bowlington. Now the open space layer still doesn't have a defined coordinate system, but because the map data frame is set to UTM zone 18, it assumes that any layer without a defined coordinate system shares that UTM zone 18 coordinate system. I'm just switching base maps now to an image layer just to confirm that these open space polygons appear in the correct location, and they do. So this is great news. I've been able to determine that the coordinate system is UTM zone 18. Now my next step is to use the defined geoprocessing tool to actually write that coordinate system information into the file. I'm going to run the define tool on my open space layer, setting the coordinate system to match the current map, which as we know is set to UTM zone 18. Because my open space layer now has a defined coordinate system, I'm going to change the coordinate system of my map data frame back to Vermont State Plane, now at 83 meters, and my open space layer will still appear in the correct location. That's because ArcGIS is projecting it on the fly, which is to say it's temporarily transforming the coordinate system from UTM to State Plane for display purposes. Now I can use the project tool to actually transform the coordinate system from UTM zone 18 to State Plane to create a new open space layer that is projected to the Vermont State Plane coordinate system. Now let's review the two open space layers that I have in my ArcGIS Pro project. My original open space layer that originally has an undefined coordinate system now is correctly defined as UTM zone 18. After I define the coordinate system to UTM zone 18 for the original open space layer, I then use the project tool to create a new layer in which the open space polygons were transformed to the Vermont State Plane Natity III coordinate system. I did this final step because the Vermont State Plane coordinate system is the standard for mapping in Vermont, allowing me to do the most accurate measurements and easily integrate it with other geospatial data sets. In this video we showed you how to deal with an unknown coordinate system. The first thing we did was make sure that we have an ArcGIS Pro project with a reference layer with a known coordinate system. A base map works just fine. Then we continually changed the coordinate system of a map data frame until our unknown layer appeared in the correct location. After it showed up in the correct location, we used the define tool to define it to the coordinate system we determined it to be in. Finally, and as an optional step, we projected it to a new coordinate system to allow us to make more accurate measurements and integrate it with other data.