Sperry Marine VISIONMASTER FT ECDIS-E, User Manual

Page: 97 / 425

ECDIS-E User Guide Charts
65934012 4-5
Navigational Purpose
In addition to the compilation scale, the level of detail and expected
accuracy available on a chart is also related to its navigational purpose. For
example, Harbour, Approach, Coastal or General. A vector chart's
navigational purpose is shown in the Detail tab of the Legend sub menu,
under
Chart Usage/Scale. The navigational purpose of all charts is shown
in Chart Index.
Chart Overscale
There are three ways in which the system indicates
that chart data is being displayed at a larger scale than
its compilation scale: the Display Overscale Prompt
(detailed in Prompts), the Display Overscale Pattern (detailed below) and a
permanent indication in orange lettering at the top right of the chart display,
below the Watch mode button.
Display Overscale Prompt
The display overscale prompt appears as Primary Display Overscale
when any of the following conditions are met:

The largest scale chart at the centre of the display is overscaled, in
most cases the display is covered by several charts.

An overscaled non-vector (e.g. raster) chart is displayed.

An overscaled vector chart from the most detailed usage level is
displayed.
A chart is considered to be overscaled if the scale ratio of the display is
larger than the compilation scale of the chart. For example, the overscale
prompt will be shown if the scale ratio is set to 1:20,000 and the displayed
chart at screen centre has a compilation scale of 1:25,000. The scale of
each chart can be viewed in the Chart Index window.