13.6: C.6- Limitations
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Although hydrographic charts provide a very valuable contribution to our knowledge, the purpose of these charts is to assist navigators, rather than engineers. Soil data mentioned on the chart are generally only an indication of the surface of the seabed, they cannot be used in designing a foundation. The charts, and certainly the portions close to the shore, are meant to warn the sailors against running aground. Relatively more attention is therefore paid to shoals and low water conditions, rather than to gullies and extremely high water levels. Moreover, the scale of the charts is generally unsuitable for construction work. For the specification in tenders and project drawings, more detailed maps are required.
When comparing old and recent charts, one must be aware that the locations of buoys and lighthouses may have changed, since survey or the drawing of the original chart. Therefore, one should remain vigilant when using older maps for comparison. The most recent maps will probably be based on positioning with DGPS, an electronic positioning system using satellites as beacons. This eliminates most errors.