subroutine grdshp  r.b. goldstein and s. brody  feb 78
planet altitude grid for topography - local shape model
grdshp called from radpl if nshp=2
altitude at subradar point is determined by a 4 point int-
erpolation scheme using the grid of altitudes stored in
/shphar/. planetary flattening at the observation latitude
is also taken into account. the radar time delay (tmdly)
is then increased or decreased accordingly and returned. (see
bodred for shape model explanation).

the flag "zero" is set to true only if both grid and lgrid
are all zero. even if grid is all zero, the partials are
non zero and values computed in grdshp are needed when
grdpar is called

obtain lat. indices for nearest grid points and quantities for
interpolation. this differs from the long. section because
the maximum latitude strip corresponds to ipb=1 (i.e. the
top row is maximum latitude as on a map).
also there is no rollover permitted.

make sure there are 4 partials on tape so that same no. of
partials for every point in series as required by analyze
link.
do not insert if there are no grid partials at all