Methods, apparatuses, computer program products, and systems are provided for training a network to act as an overhanging structure detector using an unsupervised machine learning technique. An image is received that was captured by an image capturing device in communication with a probe apparatus on board a vehicle, wherein the image comprises at least a portion of a roadway. A sky projection is generated based on at least a portion of the image. It is determined whether the sky projection comprises a feature that defines a feature direction that is substantially non-vertical. Responsive to determining that the sky projection does comprise a feature that defines a feature direction that is substantially non-vertical, it is determined that the image comprises an overhanging structure.