Hi Ed:
In SBuilderX one has to option of setting elevation for individual polygon vertex points or to use 1 fixed (aka "continuous") elevation for all polygon vertex points, provided that use id using the proper type of CVX vector polygon type.
Generally, this is a sub-type of Airport Background polygon (although water polys can also have Alt assigned in a similar way).
[
EDITED]
It sounds like you successfully made a terrain mesh clinging land class texture polygon in SBuilderX (which IIRC can either be a FSX "Park" type CVX vector, or a FS2Kx legacy VTP object enabled only by use of the "Modified Terrain Cfg file").
If you open the BGL you made in ADE within SDK TMFViewer, right click the polygon and display the vector ID, you will know what type of GUID to use for your vector object if you try this again sometime in SBuilderX.
UPDATE: I see that this is a FSX "Park" type CVX vector GUID: {5CC8F288-39C9-4DC0-9C3F-16CE1B6FB446}
SBuilderX - Polygon Properties > [Vector Polys] tab > pick-list Type: 19 - 'LC_Cement'
LandClassPoly_Cement {5CC8F288-39C9-4DC0-9C3F-16CE1B6FB446}
https://docs.microsoft.com/en-us/previous-versions/microsoft-esp/cc526968(v=msdn.10)
This polygon does not currently have individually-assigned vertex point Altitudes in "
30F_ADEX_EJW_CVX.bgl" linked at:
https://www.fsdeveloper.com/forum/threads/sbx-will-not-compile.443681/post-806836
NOTE: In SBuilderX, by using a "
Constant" Altitude value of 0 Meters AGL for all vertices, this textured CVX vector polygon "Park" sub-type will be terrain mesh-clinging.
FYI: By using a "
Variable" Altitude value in Meters AMSL for each vertex, a CVX vector polygon (whether textured or un-textured) of the Airport Background type will become a sloped terrain flatten.
[
END_EDIT]
In either case, the default Land Class texture type for this CVX vector object will not show on top of your custom photo-real imagery Land Class textures compiled by SDK Resample (although Altitude attributes impacting terrain could still be implemented).
One can also do this in Sketchup, import the 3D model to MCX, then export it as a "sloped flatten" CVX vector BGL.
Happy scenery building !
GaryGB