Page 212 - MATLAB Recipes for Earth Sciences

8.5 Georeferencing Satellite Images 207

imwrite(naivasha_rgb,'naivasha.tif','tif')
This command saves the RGB composite as a TIFF-fi le naivasha.tif (ca.
50 MB large) in the working directory that can be processed with other
software such as Adobe Photoshop.

8.5 Georeferencing Satellite Images

The processed ASTER image does not yet have a coordinate system. Hence,
the image needs to be tied to a geographical reference frame ( georeferenc-
ing). The raw data can be loaded and transformed into a RGB composite
by typing

I1 = hdfread('naivasha.hdf','VNIR_Band3N','Fields','ImageData');
I2 = hdfread('naivasha.hdf','VNIR_Band2','Fields','ImageData');
I3 = hdfread('naivasha.hdf','VNIR_Band1','Fields','ImageData');

naivasha_rgb = cat(3,I1,I2,I3);
The HDF browser can be used

hdftool('naivasha.hdf')

to extract the geodetic coordinates of the four corners of the image. This
information is contained in the header of the HDF file. Having launched the

HDF tool, we activate File as HDF and select on the uppermost directory
naivasha.hdf. This produces a long list of file attributes including product-

metadata.0, which includes the attribute scenefourcorners that contains the
following information:

upperleft = [-0.319922, 36.214332];
upperright = [-0.400443, 36.770406];
lowerleft = [-0.878267, 36.096003];
lowerright = [-0.958743, 36.652213];

These two-element vectors can be collected into one array inputpoints.

Subsequently, the left and right columns can be flipped in order to have
x=longitudes and y=latitudes.
inputpoints(1,:) = upperleft;
inputpoints(2,:) = lowerleft;
inputpoints(3,:) = upperright;
inputpoints(4,:) = lowerright;
inputpoints = fliplr(inputpoints);