I am expecting the output of my visibility analysis to be 0 or a positive number, but I am also getting negative values. Do negative values represent not visible?
In visibility did you create the AGL raster? I don't see any reference to negative values, so it is really possible or something else, at which point I would check to see what type of file you are using (Grids best), that the input data are projected and the X,Y and Z have the same units (ie meters) and a variety of other things