Units There should be no ambiguity about the units in use at a particular time. The units listed below should be indicated in the display legend (1) Position latitude and longitude in degrees, minutes and decimal minutes. (2) Depth metres and decimetres. (3) Height metres. (4) Distance nautical miles and decimal miles, or metres. (5) Speed knots and decimal knots. g Legend A standard legend of general information relating to the area displayed, applicable to the ships position, should be shown on a graphic or text display. This legend should contain at minimum (1) units for depth (2) units for height (3) scale of display in addition overscale indication where appropriate (4) data quality indicator (5) sounding/vertical datum (6) horizontal datum (7) the value of the safety depth if used (8) the value of the safety contour selected by the mariner, as well as the value of the safety contour displayed (which maybe different from that
22 S, Edition 6.0 March 2010 selected by the mariner) (9) magnetic variation (10) date and number of last update affecting the chart cells currently in use (11) edition number and date of issue of the ENC (12) chart projection 2.3.2 Priority of information a Priority layers The IMO PS divides SENC information into three categories that determine what data is to be on the display Display Base (always present on the display Standard Display (the default display and Other Information (displayed on demand. (IMO PS section 3 and Appendix 2). (See section 2.3.3a). There are 10 priority layers for the drawing sequence of the data on the display (1) ECDIS visual alarms/indications (e.g. caution, overscale) (2) HO-data: points/lines and areas + official updates (3) NtMs, manual input and Radio Navigational Warnings (4) HO-caution (ENC cautions) (5) HO-colour-fill area data (6) HO's on demand data (7) Radar information (8) Mariners data points/lines and areas (9) Manufacturers data points/lines and areas (10) Mariners colour-fill area data This list is not intended to indicate a drawing sequence, but to specify that the information content of category n must not obscure the information content of category nor any higher category (i.e. n etc. Category (7) should have a radar off switch to facilitate its removal. The lookup tables and conditional procedures of the Presentation Library assign a category, and a display priority (drawing sequence, to every object (object class- attribute combination) in the ENC. b Radar priority The priority of HO chart data over radar is carried out by the single action "remove radar" control (IMO PS 7.2). When present, the radar data is always written over the
23 S, Edition 6.0 March 2010 eight opaque area fills (see 3.1.2). Chart line and point features should normally be written over the radar image, with some exceptions, as described in the "over-radar" field of the Presentation Library lookup table. But in order to meet the requirements of IMO PS 11.4.14 to adjust the ship's position, the ECDIS may incorporate the capability of changing the radar priority of the Presentation Library. Operation of this feature should be clearly indicated. 2.3.3 Display categories a IMO categories The IMO "Standard Display" provides a starting point. It is a list of objects that the mariner may either add further objects to, or remove objects (except Display Base) from, in deciding what is to be displayed. Note that as soon as any object on this list is removed from the display, or any object not on this list is added to the display, the display no longer shows the IMO "Standard Display.