1. It's good to align with the base whenever possible, but not absolutely necessary. Like you mentioned, for non-highrise buildings where there may not be a clearly identifiable base, the outline of the roof would be good enough.

2. `covered` corresponds to whether the road is covered by something in real life or not. Basically, think of it as the road having shelter (people wait here when it rains). In most cases this may not be identifiable just based on satellite imagery, but would need local knowledge/survey.