A method of reading degraded symbols is described. A symbol has an initial shape, which is marked on an object. Marks, shapes, symbols, and object ID are managed separately. A symbol library with symbols and associated shapes is initially created. Shapes in the library are updated from shapes of marks as the marks degrade over time. A read mark is compared to all the shapes in the library to determine a most likely shape. A selection set is used to limit symbol selection, based on the comparison, to valid symbols. The symbol library and selection set may be customized to each usage of the method. Comparison methods use probability distributions. Confidence values are used to validate output, generate warnings, and to control updating of the library. Weighted averaging may be used at the level of shapes, comparison distributions, or selections. One application is reading tattooed marks on rodent tails in a vivarium.