Using OpenCPN with Google overlays does not help a lot. Such maps are usually georeferenced pictures of Google Earth. So they have no depth information and are raster charts only (bitmaps, called *.kap), no vector charts with additional informations. However there are also professional charts available for OpenCPN and most circumnavigators uses the old CM93 v2 vector charts even when the latest available version is from May 2015 for free.

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Gerhard Mueller
Amel Sharki #60
Currently Kalamata, Greece