Assuming this is true about Sketchup. Not holding you to it because I know you use 3DSMax for your work.
I've attempted this in Sketchup but was unsuccessful. This was awhile ago, and I easily just moved on to other things.
If anybody knows how to do this in Sketchup, it would be greatly appreciated if you could share your workflow with the Aerofly Community.
Flattening Ground Mesh is a HUGE desire for alot of user in this SIM.
Antoines post states that the ground poly's vertices must be painted black to appear transparent in Aerofly, which means that the 3D software you use needs to have a vertex coloring or weight painting mode. I highly doubt that the free sketchup version has this advanced feature, maybe the Payware version does. Blender is free and has many advanced features almost comparable to 3DS max but IPACS still doesn't support it with an export tool. My suggestion would be to create a tool that can read polygons from Google Earth kml files and assign an elevation to the polygon. In the freeware tool SBuilderX for FSX and P3D IIRC you could even assign an elevation to each edge of a Flatten polygon to create a sloped mesh. The next priority should be the possibility to create an exclude polygon that removes every default object in a given area so you can put your own objects on it. I'm tired of putting hours of work into a small scenery only to see default trees sticking out of the roof of my buildings and the only way to amend this is to delete a vegetation file with 100 sqkm worth of trees and completely rebuild the vegetation from scratch. It's a shame that the simulator with the most modern, best performing engine doesn't provide tools to do the most basic things in scenery development, tools which FSX had 13 years ago. Maybe the top freeware developers here on this forum could start a petition to IPACS to focus on providing these tools so that more people can create content for this platform, and I'm convinced that more content will spark more user interest in Aerofly and help improve sales. IMHO this should get priority over the next ten pages of the A320 FMC. ORBX effectively canceling their development of True Earth GB South for Aerofly should ring the alarm bells at IPACS. For me it's the beginning of the demise of a great platform and if they don't do something about it soon it won't go well.
End of rant!