Just figured I would throw these links out that come in handy for listening to scanners.

For St. Mary's:
St. Mary's County, Maryland (MD) 10 Codes and Maps (http://www.radioreference.com/apps/db/?tab=files&ctid=1206)

Dont overlook the Maps and Related Images


For Calvert:
Calvert County, Maryland (MD) 10 Codes and Maps (http://www.radioreference.com/apps/db/?tab=files&ctid=1192)

Not many files there


And unfortunately there are no files listed for Chalres but I'll put out the link anyway in case it is updated later.
Charles County, Maryland (MD) 10 Codes and Maps (http://www.radioreference.com/apps/db/?tab=files&ctid=1196)