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in section 4. The last section draws conclusions                                   x - x1n         =  y     -     y  n    =     z - z1n    =  kng  ,k  n  ∈ [0,1] .
and puts forward expectations for further study.                                   x2n - x1n          y2n   -        1                                 g

                                                                                                                  y1n        z  n  -  z1n
                                                                                                                                2

2. Intervisibility Algorithm                                                    For the terrain data storing in RSG model

                                                                            whose interval ( precision ) is Δx = Δy = a,

     The intervisibility is acquired by computing                           intervisibility computing in one GCS tile is broken

the intersections between all of the segments in                            up into that in several RSGs. The intersection

different GCS tiles and the terrain cured surfaces                          point between the LoS segment and the current

orderly. Assuming the intersection point                                    RSG ( px,py) is M( x,y) and the line slope is kn

coordinates of the nth segment and the nth GCS tile                         =      (  y2n  -  y1n  )  /  (  x  n  -  x1n  )  .  Then       the  next      intersection
                                                                                                               2

are  P  n  ( x1n ,  y1n ,  z1n )  and  P2n   ( x2n ,  y  n  ,     z2n )     point N ( x∗, y∗ ) of the segment and the next
        1                                                2

respectively,       the  equation  of  this  segment        k  n  can       RSG            (  p∗x  ,     p  ∗  )     can        be    showed       graphically  in
                                                               g                                            y

be expressed as                                                             Figure 1.

                                  Fig.1.All situations of the next intersection point and the next RSG.

    The four vertexes of one RSG are ( px, py,                              condition through bilinear interpolation. The
pz1 ) ,( px +a,py ,pz2 ) ,( px +a,py +a,pz3 ) and ( px ,                    equation of the terrain surface is expressed as
py + a, pz4 ) respectively. Based on these
coordinates, the terrain surface is constructed to                                      Axy + Bx + Cy + D - z = 0,
                                                                            Where

restore the relatively actual terrain elevation

                                       A  =  pz1  -   pz2 + pz3          -  pz4 ,
                                                         a2

                                       B  = (py   +   a) ( pz2           -  pz1 )  +  px( pz4         -     pz3 ) ,
                                                                            a2

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