The center of the galaxy, in the direction of Sagittarius, is in the center of the image. The north celestial pole, where the North Star would be, is the black spot on the upper right, and the south pole is in the lower left. Each pixel in the image is the sum of

   pow( 0.3981, magnitude )

for each star occupying the pixel. The resulting linear brightness map contains values on the order of 1E-4 to 1E-7. These are scaled linearly into the conventional 0 to 255 range. The version shown here has been reduced in size, and a gamma of 1.4 has been applied to compensate for the tendency of web browsers to darken JPEG images.

As the image makes clear, the catalog was built from computer analysis of thousands of separate photographic plates. The tile effect is the result of systematic errors in the computer analysis (or imperfections in the plates, depending on your point of view). Astrometry errors near the plate edges caused stars to be counted more than once in the overlap areas. The brightness and sharpness wasn't uniform on different parts of each plate or from one plate to another.

The first purpose of the catalog is to provide star positions for pointing the Hubble Space Telescope, and I don't think these errors have much effect on how well the catalog serves this purpose. But the GSC is also used in consumer star charting software, and there it may mislead users who aren't aware of its limitations.

This image is based on version 1.1 of GSC, which is still the version used by Hubble and in widest public circulation. GSC 1.2 has reportedly corrected many of the defects in 1.1, but its availability is limited, and it's not intended for use by astronomers using Hubble. GSC II is a newer effort using somewhat different methods and derived from a database of about a billion objects. See the Space Telescope Science Institute's GSC page for more information.