Are you indicating that you are making textures for the location using 5 cm per pixel resolution imagery ?
If so, is it made via SDK Resample as custom photo-real land class, or is it instead made as a textured ground polygon (aka "G-Poly") using ModelConverterX ( aka "MCX") Ground Polygon Wizard, or ADE GP Editor ?
When a flat 3D model textured as Aprons / Taxiways is placed onto the terrain in FS, regardless of its thickness, its top surface will be displayed as a 'Ground Polygon' (aka "G-Poly").
In FSX, one assigns the 'Altitude' a MDL-based G-Poly is placed at relative to terrain mesh / airport flatten Altitude ...in order to be visible.
And in FSX, if a FS2002 (aka "FS8") legacy format non-MDL G-Poly is used, it is placed at the exact altitude of the local terrain mesh / airport flatten, and its VTP 'Layer' must be assigned as a 'Positive' number in order to be visible.
Both of these MDL-based and non-MDL-based methods work in FSX.
But in P3D with MDL-based G-Polys, instead of assigning a 'Positive' VTP layer, one uses a "Negative" layer position number to represent the Z-sorting distance from the user aircraft AGL to the G-Poly surface ('negative' means less than the distance from the user aircraft camera to the underlying terrain surface) ...effectively causing the G-Poly to be displayed AGL.
These increments of distance, when interpreted as relative "layers", will determine whether shadows from user aircraft and IIUC, from 3D objects as 'Ground Shadows' ...can be displayed in FS at run time.
PS: Please help us... to help you... by answering the other questions in these posts above: