Sperry Marine VISIONMASTER FT ECDIS-E, User Manual

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Charts ECDIS-E User Guide
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Chart Datum
The chart datum is the mathematical model used by a chart maker to map
the earth’s surface. The system VisionMaster FT uses the datum known as
World Geodetic System 1984 (WGS-84) Ellipsoid Earth model. Charts and
all chart objects are displayed using the WGS-84 datum whenever
possible.
While WGS-84 is the present standard for accurate mapping and display of
position information, there are hundreds of data in use, and each makes
different assumptions about the exact size and shape of the earth. This
means that a given latitude and longitude can correspond to different
locations when plotted on charts that are derived from different data. It is
therefore important to be aware of the datum of the displayed chart.
If a chart is based on another datum, the system offsets the chart to WGS-
84, if the offset to WGS-84 is known and provided in the chart file. Offset
correction values used for the chart are computed as average values for
the entire chart. These correction values may not be perfectly accurate
throughout the geographical area covered by the chart. Datum
discrepancies become more important when using larger-scale charts.
When displaying a chart that was created using a datum other than WGS-
84, a warning displays information about the chart’s datum. The Chart
Legend window also shows the datum of the displayed chart, see Legend
Detail.
Codes used to identify various known chart data are listed in Appendix B -
Chart Datum Codes .
Compilation Scale
Chart data is compiled at a specific scale, based upon the hydrographic
data from which the chart was created. This scale is called the chart’s
compilation scale, indicating the level of detail that is available on the
chart. When charts are first selected for display, they are displayed at their
compilation scale.
A chart's compilation scale is shown in the drop down list of the Charts
button (see Chart Database & Sub Menu Selection in Chapter 3), the Detail
tab folder of the Legend menu, under
Chart Usage/Scale , and Chart
Index, which shows the scale and other details of all charts in a selected
chart database.
When the scale ratio is selected at a larger scale than the chart's
compilation scale a permanent prompt appears with the message `
Chart
Overscale ', see Chart Overscale below.