When was the Survey Carried

UK Hydrographic Office charts always have a source diagram on the chart. This shows both how and when the survey was carried out. Unfortunately not all charts have a source diagram and no vector charts I have seen have them either. Raster charts will if the original paper chart has one.

Vector charts will generally show the date of issue of the paper chart on which it was based, but this chart may well use cartography several hundred years old. Some charts are still based on Captain Cook's surveys and some Pacific Islands are reported to be up to 8 miles out of place.

Note how this area around the Channel Islands in the English Channel was surveyed mostly in the nineteenth century!

Surveying is very expensive and will not be carried out just to satisfy the leisure market'. Where commercial needs dictate, up-to-date surveys will be carried out, but this may be only in the channels and approaches used by commercial shipping, leaving the shallower areas unsurveyed by modern means.

In the United States of America, 40% of the shoreline has not been mapped since 1960. Around half of the soundings were carried out by lead line survey prior to 1940 (US Federal Advisory Committee Report 2007, which stated 'depending on the boater's location ... can render these charts slightly to grossly inaccurate'). Any electronic chart of the US will be based on these charts.

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