[Gdal-dev] NAD27 to WGS84

[Dadi, Upendra]

Hi,
  I am converting a dataset which has NAD27 as the datum to WGS84 using
gdalwarp. The dataset has the following coordinate system:

   PROJCS["Albers Conical Equal Area",
    GEOGCS["NAD27",
        DATUM["North_American_Datum_1927",
            SPHEROID["Clarke 1866",6378206.4,294.9786982139006,
                AUTHORITY["EPSG","7008"]],
            AUTHORITY["EPSG","6267"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4267"]],
    PROJECTION["Albers_Conic_Equal_Area"],
    PARAMETER["standard_parallel_1",29.5],
    PARAMETER["standard_parallel_2",45.5],
    PARAMETER["latitude_of_center",23],
    PARAMETER["longitude_of_center",-96],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]]]


 I issued the following command:

gdalwarp -s_srs <the above string> -t_srs EPSG:4326 conus.tif output.tif

And it worked fine.

My question is how does GDAL handle coordinate system transformation
from NAD27 to WGS84? You can see that there is no towgs84 parameter in
the projection above. I am guessing it handles NAD27 as a special case
and uses a more complex algorithm for transformation. What happens if I
add towgs84 parameter in the string? Will it overide the internal
algorithm used? Hope I am clear. 
Any insight on this is appreciated. 

Upendra