To get rid of buildings, roads, autogen of different kinds you have to do an exclude switch. There is an xml file if I remember correctly, there you can enter the coordinates for the area and what you want to exclude.
IIRC the way to get the coordinates is to use a square where you take the point of the upper left corner and the bottom right. I'll have to research that a bit more, I am at work right now, but you can search some by yourself too using the above info
Airliners.net Crew-Head Support 5D Mark III, 7D, 17-40 F4 L, 70-200 F2.8 L IS, EF 1.4x II, EF 2x III, SPEEDLITE 600EX-RT
Quoting JohnKrist (Reply 8): some nice person has made an app to aid in making exclude switches, search for ExcBuilder Version 2.0
Excellent. I had a go at making a version of Sek Kong airstrip in Hong Kong as it was in the 1970s but there were trees all over the apron and in the hangars. I lost interest after trying to learn about exclude files. I might revisit that now.