Each area also must contain the symbol for only one kind of map unit. Errors in symbols take various forms. A delineation on a field sheet might be closed without a symbol in it, or a symbol not listed in the legend might be used. Symbols might be illegibly drawn on the field sheet. Practice and care in lettering, good judgement in placing symbols, and care in erasing and reinking mistakes ensure legibility.
Various methods are available for checking field sheets.
A good method is to color each delineation by hand on photographic copies of the filed sheets. A color check reviews each delineation and inspects boundaries throughout their length. In another method, each field sheet is divided into sections of perhaps 50 to 100 square centimeters. The delineations within each section are checked one at a time, special care being taken at the edges of the section. If the map checkers are not familiar with the legend, they must be especially diligent in checking the symbols against the legend.