if we applied the same criteria to plastic contamination on land, would we find any square km of land area NOT counting as a Garbage Patch?

http://en.wikipedia.org/wiki/Great_Pacific_Garbage_Patch

The patch is not easily visible because it consists of very small pieces, almost invisible to the naked eye [3], most of its contents are suspended beneath the surface of the ocean[4], and the relatively low density of the plastic debris at, in one scientific study, 5.1 kilograms of plastic per square kilometer of ocean area.[5]

...

The patch's size is unknown, as large items readily visible from a boat deck are uncommon. Most debris consists of small plastic particles suspended at or just below the surface, making it impossible to detect by aircraft or satellite.[12] Estimates on size range from 700,000 square kilometres (270,000 sq mi) to more than 15,000,000 square kilometres (5,800,000 sq mi) (0.41% to 8.1% of the size of the Pacific Ocean), or up to "twice the size of the continental United States".[13] The area may contain over 100 million tons of debris.[13] (However, this figure is not supported by the densities quoted below.) It has also been suggested that the patch may represent two linked areas.[14]

In August 2009, the Scripps Institute / Project Kaisei SEAPLEX survey mission of the Gyre, found that plastic debris was present in 100 consecutive samples taken at varying depths and net sizes along a 1,700 miles (2,700 km) path through the patch.[15] The survey also confirmed that while the debris field does contain large pieces, it is on the whole made up of smaller items which increase in concentration towards the Gyre's centre, and these 'confetti-like' pieces are clearly visible just beneath the surface.[16]

...