Maps – Advanced Search

So here's an interesting one. The product is: imagine google maps, we map all places, but places change throughout time. As place names and borders change, we are collecting data on historical places.

Most users will use a 'simple search' to find their place. this a single text box where they type place name.

The mockup below represents an "advanced search". Place name is self-explanatory.

Places within will do a search but only look for places that are inside a larger place (for example if I choose United States, the search results will bring back the 50 states as the results).

Place type filter results for a specific type (cemeteries, cities, provinces, etc)

Years are to specify a historic time period of a place

Location will return all the places within a specified radius of a point the user selects (latitude/ longitude)

This area has not user test well. Users either try to fill in all the fields (not realizing they are optional) or they just have a general difficulty grasping the model of how the system works. Any thoughts to help me with this I'd appreciate.