- OpenCPN User Manual
These buttons allow you to zoom in and out on the chart currently being displayed.
Will zoom the chart in for more detail, larger scale.
Will zoom the chart view out for more area, smaller scale.
Alternatively, the + and - keys on your keyboard will zoom in and out. If you have a mouse with a scroll wheel, it can also be used to quickly zoom in and out.For zooming in smaller steps try
- Ctrl + for fine scale zooming in.
- Ctrl - for fine scale zooming out.
- Ctrl + scroll wheel, zooming in/out in small steps.
If you zoom in enough there will appear a warning "OverZoom" in the upper left part of the chart area.
This means that you have zoomed in way to far, and is using the chart at a scale that was never intended, and that is not supported by the original survey; no new information will be seen, and the situation is potentially dangerous as it could give the impression of increased distance between dangers.
On a raster-chart pixelation will be seen, but on a vector chart it is not so obvious when you have over zoomed. This is where the warning is useful.
Your first action when the warning appears should be to zoom out at least one snap.
Charts are generally based on surveys in twice the scale of the released chart, so when zooming in beyond a factor of 2, there is no support, increased details etc, in the underlying survey.
Nigel Calder "How to read a Nautical Chart, Second ed. 2012", is recommended to all users who want to know more about charts, the surveys they are based on, and their horizontal and vertical accuracy.
OpenCPN allows quite a bit of over-zooming by default, but it is also possible to change this by editing a line in the opecpn.ini (config) file. Find this line:
and change it to
This will still allow over-zooming up to a factor of about 2.5 for larger scale raster charts and about a factor 4 for similar vector charts.
A stricter recommendation based on how charts are compiled, and their de facto accuracy, would limit this to a factor 2.
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