A valid frame, with its valid parts: without an invalid area, and without the parts that have their default value (north at the top, looking straight down). Undefined if nothing is left, which is the frame of a map without one.
A valid frame, with its valid parts: without an invalid area, and without the parts that have their default value (north at the top, looking straight down). Undefined if nothing is left, which is the frame of a map without one.