I have a problem determining the length of the routes (boat) I've surveyed in order to calculate the survey effort. I added the GPS points of the boat and then used the points to line tool and added a field in the attribute table with the length of each route. Then I used the steps described by PolyGeo♦ here: Calculating density map of cycleways using fishnet in ArcGIS Desktop? but for some reason it doesn't work for me, I only get zeros and not the length per grid cell. Any ideas why that might be?