For the Line Density tool in ArcGIS 10.1, the ArcGIS Resources page states that when specifying the population_field, one can "use Shape if input features contain z". In my case, I am calculating the density of aircraft flight polylines that have a z value along the line.
When I use "Shape.z" as the population field, my raster output changes, so something is happening. However, I am not sure exactly what is happening. Is the line density weighted so that the length of each line intersection is multiplied by the z value of the line? And if so, what z value per line is used for calculating density, given that the z value keeps changing along the line?