I have been mapping local creeks and canals recently (there is no shortage of them in Sierra Nevada foothills and California central valley). I have run into some challenges tagging aqueducts. What I mean by “aqueducts” are sections of irrigation canals built on man-made structures above ground level; they can pass over valleys, roads and other waterways. Here are a some examples:
I have tagged them with waterway=stream, bridge=aqueduct and layer=1. Mapnik does not seem to render them at all. Is this a wrong combination of tags, or is the bug in Mapnik?
I have a related question about siphons (technically, “inverted siphons” http://en.wikipedia.org/wiki/Siphon#Inverted_siphon)). These are tunnels or pipelines that dip to a lower elevation but then return to (almost) the original elevation. The pressure of the incoming water pushes the outgoing water up the opposite slope; hence, the conduit has to be a closed pipeline. A siphon can lead a canal across the bottom of a valley or under a natural stream. A siphon may run below ground, on the ground, or above ground (my third example has a siphon running below the original canal level but above a natural creek in a small canyon). Is there a way to differentiate between a closed, pressurised siphon and a regular culvert?