Commercial and open source GIS packages enable users to generate their own spatial information. One way to do this is through a process known as digitizing. In the example below, aerial imagery is used as a reference to identify potential sites suitable for solar panels on public properties.
I also edited OpenStreetMaps to include some of the buildings that I identified above on their website. Here is a screenshot of some contributions: notice the two red buildings below, which I have identified on Open Street Maps as public, borough property.