* They represent a specular interface between two media of different material compositions for calculating the reflection, transmission and possibly diffraction coefficients.
[[EM.Terrano]] has generalized the concept of '''Block''' as any object that obstructs and affects radio wave propagation. Rays hit The following table summarized the facets of a block and bounce off the surface of those facets or penetrate them and continue their propagation. Rays also get diffracted off the edges of these blocks. Â In [[EM.Cube]]'s [[Propagation Module]], blocks are grouped together by the type of their interaction with rays. [[EM.Cube]] currently offers three Block Types|block types of blocks for use in a propagation scene:Â # '''Impenetrable Surfaces:''' Rays hit the facets of this type of blocks and bounce back, but they do not penetrate the object. It is assumed that the interior of such blocks or buildings are highly absorptive.# '''Penetrable Surfaces:''' These blocks represent thin surfaces that are used to model the exterior and interior walls of buildings based on the "Thin Wall Approximation". Rays reflect off the surface of penetrable surfaces and diffract off their edges. They also penetrate such thin surfaces and continue their paths on the other side of the wall.# '''Terrain Surfaces:''' These blocks are used to provide one or more impenetrable, ground surfaces for the propagation scene. Rays simply bounce off terrain objects. The global ground acts as a flat super-terrain that covers the bottom of the entire computational domain. Â [[EM.Cube]]'s [[Propagation Module]] allows you to define block groups of each of the above three types. Each block group has the same color or texture and its members share the same material properties: permittivity ε<sub>r</sub> and conductivity σ. Also, all the penetrable surfaces belonging to the same block group have the same wall thickness. You can define many different block groups with certain properties and underneath each introduce many member objects with different geometrical shapes and dimensions. The table below summarizes the characteristics of each block type:
{| class="wikitable"
! scope="col"| Block Type
! scope="col"|Physical Effects
! scope="col"|Admissible Legitimate Object Types
|-
| Impenetrable Surface
| Reflection, Diffraction
| All Solid solid & Surface surface CAD Objectsobjects|-| Penetrable Volume| Reflection, Diffraction, Material Medium Transmission| All solid CAD objects
|-
| Penetrable Surface
| Reflection, Diffraction, Thin-Wall Transmission| All Solid solid & Surface surface CAD Objectsobjects
|-
| Terrain Surface
| Reflection
| Tessellated Objects Onlytessellated objects either imported externally or created [[using Terrain Generator]]
|}
[[Image:prop_manual-12_tn.png|thumb|600px|An imported external terrain model.]]
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=== Importing & Exporting Terrain Models ===