ASTER User's Guide - Aster - NASA

ASTER Users Handbook
In the case of an ASTER L1A image, the location of the four corners correspond to the area of
the actual image, but in the case of an ASTER L1B image, these locations correspond to the
corners of the file (in other words they include the fill or no-data area).
In general, what are the differences between geodetic & geocentric coordinates?
Geodetic coordinates specify a location on the Earth's oblate (non-spherical) surface. Geodetic
latitude is defined as the angle between the equatorial plane and a line normal to the surface at
that location. Geodetic longitude is the angular distance between the location's meridian and the
Greenwich meridian.
Geocentric coordinates relate to a reference system where the origin is the center of the Earth.
Geocentric latitude is defined by the angle between the equatorial plane and a line from the local
position to the intersection of the axis of rotation with the equatorial plane.
Geodetic longitude and geocentric longitude are the same because they share the same reference
meridian and axis.
Do ASTER Level-1 data conform to geodetic or geocentric coordinates?
The coordinates within an interior ASTER swath are geocentric while all the metadata are
geodetic. So if you use the interior swath-based coordinate information (from the embedded
metadata in the hdf file), you will need to convert it to geodetic coordinates. All the higher-level
products use geodetic coordinates, both in the metadata and in the swaths.
How do I convert my geocentric coordinates in the swath to geodetic ones?
The equation to convert the swath-based geocentric coordinates to geodetic coordinates is:
Geodetic = Arctan [(tan (Latitude)) / 0.99330562]
This equation is for the WGS 84 datum only. Geocentric longitudes do not need to be converted
to geodetic since they both are the same and they also share the same reference meridian and
axis.
What are the 11 x 12 (VNIR), 104 x 107 (SWIR), 11 x 12 (TIR) latitude and longitude files
for a Level-1A, and 11 x 11 (VNIR), 11 x 11 (SWIR), 11 x 11 (TIR) latitude and longitude
files that come bundled with a Level-1B granule?
These files represent the Geometric Correction Table (GCT) and are internal to the swath data
structures. Also referred to as the swath metadata, these geolocation files provide the values for
the lattice points for each of the sensors. As part of the geometric correction, each scene is
divided into block units, and the processing of the scene is done block by block in both the
along-track and cross-track directions. The values for the lattice points constitute coordinates for
each lattice block located by their center pixel and their corresponding latitudes and longitudes in
geocentric coordinates. The lattice blocks and their intervals for ASTER L1A are:
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