Last week I got approached by different developers with suggestions for ModelConverterX to create flattens. So given multiple questions about the same subject I was trigged to look into this issue. Since I mainly create scenery for the Netherlands I have never had trouble with flattens myself. I think in the Netherlands you would even get away without any flattens, since the terrain is already flat.
But in more hilly or mountainous areas the problem is that FSX airports have to be flat, so a flatten is created for the airport. But if the terrain around the airport has quite some elevation differences, you are likely to end up with your airport on a plateau or other undesirable results.
By creating a sloped flatten you can usually make a more gradual transition from the airport to the surrounding terrain. Tools like SBuilderX and ADE allow you to create such flattens. But finding the correct elevation for the points can be tricky.
With this new functionality that will be available in the next development release another way to create sloped flattens is possible. You can now model the terrain shape in your favourite 3D modelling tool and convert it to a flatten with ModelConverterX. What this functionality does is create a sloped flatten for each polygon in the model.
Simply model the shape you want and export it to a MDL file. Then you import the MDL into ModelConverterX and enter the coordinates of the reference point. Next under export scenery you will find the new format “FSX flatten BGL file” and this will use shp2vec to create the flatten BGL from your object. Below is a picture where I turned one of my test objects into a flatten (I know, this is not a useful terrain shape, but it demonstrates the concept).
I am not sure yet how easy it will be to model a shape in your 3D modelling tool and still get a nice transition to the surrounding terrain. To do that well I guess you need to be aware of the terrain shape in your modelling tool. I would be happy to hear any feedback from developers that try this new approach on how that works and which improvements could be made there.