They can be measured by instruments on the ground, by satellite, by census takers, by GPS, or from maps. This is the process that you should be undergoing in your own individual projects. When we have determined what our data source and the level, how is this to be depicted in our GIS database. Every discipline has a certain way of looking at things. Geography's aspect is spatial.
Geographers have their own language to describe spatial relationships.
Everyone is a geographer. These are skills that every child develops. Geography has developed over its 2,500 year history, it has borrowed from other disciplines.