Help for RPC2GRID

PURPOSE
     The RPC or RSM in a label give the transformation of a
     non-orthorectified image (line,sample) to rectified (lon,lat)
     coordinates.  Mathematically, the equations (RPF's) are reversed
     from lon-lat to line-sample.  This routine creates an ultrafine
     grid (should be finer than the underlying DTED used) that is
     equivalent to the transformations.

     The user can use tieconv to invert the ultrafine grid to apply to the
     raw image to get a mapped, orthocorrected image.  This process has been
     codified in the procedure RPCWARP.  Most users should use RPCWARP.
	   
CALL

There are two modes

     rpc2grid INP=(rpcim,dted) OUT=outgrid NAH=nah NAV=nav
  WHERE:
     rpcim          is the input data set (must have a GeoTIFF label with RPC's)
     dted           is a dted data set (must have a GeoTIFF label, lon-lat)
     outgrid        is the output ultrafine grid cols=(lon,lat,elv,line,samp)
     nah            is the number of areas horizontally in the grid (points-1)
     nav            is the number of areas vertically in the grid (points-1)

and

     rpc2grid INP=(rpcim,dted) OUT=outgrid NAH=nah NAV=nav LON1=lon1                 LAT1=lat1 LON2=lon2 ...etc
  WHERE:
     rpcim          is the input data set (must have a GeoTIFF label with RPC's)
     dted           is a dted data set (must have a GeoTIFF label, lon-lat)
     outgrid        is the output ultrafine grid cols=(lon,lat,elv,line,samp)
     nah            is the number of areas horizontally in the grid (points-1)
     nav            is the number of areas vertically in the grid (points-1)
     lon1,lat1      is the first corner in clockwise order
     lon2,lat2      is the second corner in clockwise order
     lon3,lat3      is the third corner in clockwise order
     lon4,lat4      is the fourth corner in clockwise order
  
OPERATION

In mode one, the corner (lon,lat) points are obtained from the label (these should
always be present for RPC labels).  The corners are enlarged outward by 4% relative
to the middle so the generated quadrangle will cover the image data better.

In mode two, the (lon,lat) points are given by the user.  The user should take care
to cover data.

Then the dted us used to look up elevation for each point using bilinear interp-
olation.  Then the RPCORRSMFWD routine is used to calculate (line,sample) for each
point.

The columns are always in that order.  The ncols parameter can add extra columns to
the file.

There is an unusual fix for bad data.   If the corner points are not in clockwise
order (points 3 and 4), then they are moved to a correct order so that the program will produce the desired grid output.

PERFORMANCE

The file is (nah+1)*(NAV+1) in length.  Almost all of the time will be spent in
rpcorrsmfwd.

Restrictions
------------

The ibis file must have at least 8 columns.

Original Programmer: A. L. Zobrist, 16 Mar, 2005
Current Cognizant Programmer: A. L. Zobrist

REVISIONS: 
  Fri Feb 1 2019 smyth switched to using rpcorrsmfwd to support RSM