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overzoom

Also called overscale, in ENC display, this means the user has zoomed the display scale to a value larger than the compilation scale of the chart. The overscale factor is the ratio of the scales, ie a 1:25000 chart displayed at 1:11,000 would be overzoomed by a factor of 25000/11000 = x2.3.

(Recall the way chart scale sizes are defined. 1:10,000 expressed as a ratio 1/10,000 is a larger number, and hence larger scale, than 1:40,000 expressed as 1/40,000. Thus large scales are smaller numbers and small scales are larger numbers.)

IHO and IMO standards call for a user notification on overzoom and especially for overzoom greater than a factor of x2.0. Different ECS present this different ways.

qtVlm has an elegant and effective solution for this. It shows a popup at the top of the screen announcing "Overzoom" when any one of the charts in view is zoomed past x2.0. A cursor on this sign brings up a tooltip telling us which charts in view are overzoomed and by how much.

Note that on all programs, the alert does not show up when you are just at a display scale larger than the compilation scale, but it must be at least 2 times larger. Thus a 1:40,000 chart is not overzoomed at 1:30,000 or 1:35,000 it will not trigger an alert until 1:20,000. Note too that qtVlm has an option to start blurring the chart once overzoomed, but we recommend turning that off. There are many good and perfectly safe reasons to look at an overzoomed chart and we do not need that extra warning.

See chart scale.


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