Question: plotting using plot3d

I have to plot a graph where an equation C is dependent to two variables, and the equation changes depending on value of d.

I know plot3d(equation, range, range) gives you graph, but is there a way to plot a graph in only places where data sets are given instead of one smooth graph?

Below is something I tried to do which is not giving me results i want.

with(plots); for v from 2 by .1 to 8 do

for d from -30 by 2 to 30 do

q := convert(d*degrees, radians);

if d < 0 then if v < 4*sin(q) then C := (.5*1000)*((.2*2)*cos(q)+1.5*(4*sin(q)-v))/(7.2*10^5*6)

elif v = 4*sin(q) then C := .8*cos(q)/((1.44*10^3*4)*sin(q))

elif v > 4*sin(q) then C := (.5*1000)*((.2*2)*cos(q)+1.5*(v-4*sin(q))-2*sin(q))/(7.2*10^5*6)

end if else C := (.5*1000)*((.2*2)*cos(q)+1.5*(4*sin(q)+v))/(7.2*10^5*6)

end if;

plot3d(C, v, q)                      <==where i'm having problem at.

end do

end do