Each nautical chart is based on surveys and other information that have been compiled over many years. Some depths on a particular nautical chart may be based on leadline and sextant hydrographic surveys conducted before 1940. On the same nautical chart, other depth information may be based on multibeam surveys conducted with modern sonars and GPS positioning. Therefore, the nautical chart represents a mosaic of information patched together over a period of time. Some of the information found on a particular chart may be more accurate than other information because it is based on more recent survey information.
Many people believe that the printing of a new chart edition implies that completely new survey information was acquired and applied to the new edition. This is generally not the case. Information such as aids to navigation, wrecks, obstructions and other individual features might have changed between chart editions. In most cases, the depths and shoreline topography are carried through from the prior chart edition. The exception to this rule is when a modern hydrographic or photogrammetric survey is conducted. In that case, the new survey information will supersede that information which is currently charted.