Recently I have taken to adding Gantries, but I have a question about the guidance for these objects in regards to layer=*. The wiki instructs: “Optionally, add an appropriate layer=* tag.”
However, I find this statement ambiguous. A gantry is conventionally supported by some sort of post or lattice construction originating from the ground, this would imply layer=0. However, the gantry itself sits 1 conceptual layer above the road. Is the gantry layer=1 since it is over the road, or is it layer=0 since the supports sit at ground level?
layer=* is a z-order to tell rendering engines the relative order of things when you’re wanting to render them in the same order as they physically are.
I can see your line of thinking that, tagging a gantry at the same layer=* as the road says it’s a gantry for that road, but consider if you have one road on the “ground” at layer=0, then another road on a bridge going over it at layer=1 and then a gantry supported from the ground that covers both, if you set layer=0 it’s unclear what it covers over, so we should set layer=2 to indicate the gantry is over both roads.
A bridge is also sometimes supported by posts from the ground, but we still set the layer=* based on the order of the road surface, not where the supports start from.
On another topic, is there a good way to represent the supports of the gantry?
Some gantries will have a single support on one side and stick out over the road, with the other end of the gantry floating, and some gantries have supports on both sides. I want to know if there is a way to represent this
Although from what I can tell, most gantries at least in my region were tagged with bridge=yes (probably because they’re called a type of bridge in Germany[1]) which iD automatically adds layer=1 to it which may have scewed the tagging towards adding layer=1 to gantries.
if there are objects attached to the gantry, for instance a camera or a variable message sign, would that object also need to be tagged with the same layer=* as the gantry, or is the layer inferred by the object being connected to the gantry?
so perhaps each node should by default assume there is no support at that node, while each support could be represented by something like support=suspended or support=mast + tower:construction=* (lattice or freestanding or other values in edge cases)?
What exactly is the boundary between something being called a pole, and something being called a mast? The diameter of some gantry supports are very wide, which I would say is closer to a mast than a pole, which I think of as something with a small diameter, something like 1-4 inches.
Which is what I was suggesting when I said that bigger ones could be a mast. There’s also column, but that is specified as being stone.
I guess that, as with many things OSM, it all comes down to what you see when you look at it, & will I then see the same thing when I drive past later?