A user has posed a question about projection and datum/spheroid info for both SeaWiFS and MODIS data at different processing levels.

The answer, supplied by Norman Kuring, is pasted below for the benefit of others:

The SeaWiFS and MODIS level-2 data have not been mapped to any
projection (part of the definition of level 2).  Pixels are stored
in the level-2 files in a raster in the order in which they were
scanned in by the sensor.

For MODIS files, latitudes and longitudes at certain control
points along each scan are stored with the pixel data.  A simple
spline interpolation is sufficient to get coordinates for the
pixels between control points.

For SeaWiFS files, you need to do some extra computation starting
with the orbit position vector (orb_vec), sensor transformation
matrix (sen_mat), and scan ellipse coefficients (scan_ell) which
are provided for each scan line in the file.  If you have SeaDAS,
there is a FORTRAN routine called geonav that will show you how
this is done.  A paper describing the technique is:

   Exact closed-form geolocation algorithm for Earth survey sensors
   F.S. Patt and W.W. Gregg
   Int. J. Remote Sensing, 1994, Vol. 15, No. 18, 3719-3734.

Don't worry too much about choice of a datum.  At one-kilometer
resolution, it doesn't make much difference.

You can find level-2 file format descriptions under the
"Documents" link at the top level Ocean Color web page
http://oceancolor.gsfc.nasa.gov/