MISR Regional INTEX-B Map Projection

The map projection used for the Regional Data Products is a Lambert Azimuthal Equal Area map projection. The parameters of the map projection are:

Projection	Lambert Azimuthal
Datum	none
Units	meters
Radius of reference sphere	6370997
Longitude of projection center	-92° 30' 00.00"
Latitude of projection center	24° 51' 10.00"
False easting (meters)	725531.687500
False northing	794894.625000

Regional data are generated at two resolutions (1.5 km and 24 km), depending on the resolution of the underlying Level 1 or Level 2 data. The Regional Data are at a slightly coarser resolution than the Level 1 or 2 data to reduce artifacts when reprojecting the data.

The images displayed on the View Data page have coordinates of (0, 0) at the lower left hand corner. Note, these are the coordinates of the bounding box; the center of the pixel is at (750, 750) for 1.5 km data and (12000, 12000) for 24 km data.

HDF-EOS Library

If you are using the HDF-EOS Library to read the MISR Regional data, nothing special needs to be done to set up the map projection. Information about the map projection is present as metadata in the Regional files, and HDF-EOS automatically reads this metadata (but see comments below about spherical vs. WGS-84 latitude).

General Cartographic Transformation Package (GCTP) Library

If you are using the GCTP package for map projections, the projection parameters are (6370997,0,0,0,-92970000,24051010.493317,721031.687500,800144.625000,0,0,0,0,0) (see comments below about spherical vs. WGS-84 latitude).

IDL

If you are using IDL, the following code snippet sets up mappings to go from the map projection to latitude/longitude (relative to sphere of radius 6370997 meter, not WGS-84):

      mp = map_proj_init(111,DATUM=19,SPHERE_RADIUS=6370997,CENTER_LONGITUDE=-92.5,$
                        CENTER_LATITUDE=24.852914810278,FALSE_EASTING=725531.687500,$
                        FALSE_NORTHING=794894.625000)

After this set up, the idl commands MAP_PROJ_FORWARD and MAP_PROJ_INVERSE can be used to convert between latitude/longitude and the Lambert Azimuthal projection (see the IDL documentation for details, and see comments below about spherical vs. WGS-84 latitude).

Spherical vs. WGS-84 latitude

A refinement is needed if you are doing high precision work. The packages mentioned about (HDF-EOS library, GCTP, and IDL) all convert to and from latitude/longitude for the same reference ellipsoid as the map projection, which for the map projection described here is a sphere of 6370997 meters. Often what you actually want is latitude relative to the WGS-84 ellipsoid.

The difference between using spherical latitude vs. WGS-84 is small, and can often be ignored. For the area of the INTEX-B campaign, the difference is an error of about 15 km.

If you need greater precision in your latitude, you need to do a second conversion step. The conversion in IDL is:

      wgs84_a = 6378137.0000
      wgs84_b = 6356752.31424517929
      wgs84_esq = (wgs84_a * wgs84_a - wgs84_b * wgs84_b) / (wgs84_a * wgs84_a)
      deg_to_rad = 3.14159265 / 180.0
      wgs_lat = atan(tan(Sphere_lat * deg_to_rad) / (1 - wgs84_esq)) / deg_to_rad

The conversion in other languages is very similiar.

There is no conversion of longitude needed, only the latitude depends on the reference ellipsoid.

Note that more sophisticated GIS tools (e.g., ERDAS Imagine) will do this conversion automatically for you. Consult the documentation for your tool to see if you need to do a second conversion or not.