Sperry Marine Visionmaster FT ECDIS, User Manual

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ECDIS User Guide Charts
65900012 4-5
Factors affecting Chart Accuracy
A position plotted on a navigational chart is only as good as the accuracy of
the chart. Therefore it is important to understand the factors which may limit
the accuracy of both raster and vector charts. These factors are detailed
below.
Survey Data
Vector charts are generally based on recent hydrographic survey data.
However, the level of detail provided by different types of chart data (both
raster and vector charts) may vary greatly depending upon the geographic
location, these charts may sometimes use old survey data where more
recent surveys do not exist.
Chart Datum
The chart datum is the mathematical model used by a chart maker to map
the earth’s surface. 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 A –
Chart Datum Codes.