A two dimensional symbol is decoded by forming a sub-sampled image that is used to generate symbol margins for use in digitization. The sub-sampled image is based on local levels of contrast in the symbol. By using a sub-sampled image derived from local levels of contrast in the symbol to determine the symbol margins, margins can be located efficiently and in the presence of symbol contrast variation caused by irregularities in the marked surface angles. Digitization proceeds based on the determined margins.