I just had a guy come through my office in a big hurry, wanting to know how far apart along a river were some points in his GPS. I've done this a lot so I can't figure out why it is not working today. I downloaded the GPS points from his units, converted them into shape files in WGS 84, threw together a map in ArcMap 10.6 with a basemap from ESRI so we would be confident in the points position. Then I added a routed hydro layer, made a selection of just one route , the main river, and added the selected route as a layer. But when I run "Locate Features Along Routes" I don't get an output table. I just get an unhelpful ERROR 999999. Why isn't this working? Is it the coordinate system I have fallen into? Identify Route Location tool is working with my route layers.