The SHOWAREA instruction was designed to symbolize area objects. It performs a variety of fill operations. The prime requirement is that the area symbolization shall always be clearly visible in the part of the area that lies within the viewing window of the ECDIS. If the area covers a large part of the viewing window, more than one symbol may be required. On the other hand, a secondary requirement is not to show more symbols than necessary, as this will cause distracting clutter. One solution is to centre a symbol in the part of the area exposed by the viewing window.
8.4.1 Fill Operations
An area can be identified in several ways:
- with an opaque colour fill (e.g. depth areas);
- with a transparent colour fill (e.g. traffic separation zone);
- with a pattern of symbols (e.g. traffic arrows) or texture (e.g. pack ice)
- with a symbol or text located on a position inside the area (e.g. traffic arrow)
A transparent colour fill may overlap an opaque fill and a patterned fill may overlap any other fill, including another patterned fill. For overlapping fills the respective area has to be filled more than once in a sequence of several area‑fill operations.
The presentation library supports two methods of transparent fill.
with only a percentage of the pixels having the fill colour (stippled fill, pseudo transparency);
2.) by mixing the fill and underlying colour at each pixel, according to the fill percentage.
When method 1 and a 4 pixel group is used to achieve transparency then only the percentages 25%, 50% and 75% can be used for the transparency. For compatibility with both transparency methods only percentage values 25%, 50% and 75% are used within the presentation library. The following explains the pseudo-transparency that can be achieved by method 1.
If an area of 4 by 4 pixels has to be filled with a transparent colour only 3, 2 or 1 pixel(s) of this area are tinted with the opaque fill colour while the remaining pixel(s) are tinted using the colour 'TRNSP' (= 100% transparent, see 4.2.1), which means the colour fill is not performed for these pixels. Thus the colour of the underlying pixels still can be seen through. On a high resolution screen the result will be very close to a real transparent fill.
Centred symbols are used to reduce clutter in areas of heavy traffic. Since such areas may be large we use large symbols and since many restrictions may apply to a given area (e.g. traffic lane; precautionary area; no anchoring or fishing) the symbols have built-in offsets to prevent overwriting.
A pivot point for centred symbols and text will be at the centre of the area, or close enough to the centre that it is evident which area the symbol applies to. The offsets for symbols and text are given with respect to the pivot point.
Multiple centred symbols are often used. For example, a traffic lane with restrictions on entry and on fishing will have a centred traffic arrow and an offset “entry restricted” symbol with a subscript “!” to indicate that other restrictions apply.
If the centre of the symbol bounding box falls outside of the area then it shall not be drawn.
There are situations the chart display will split objects into multiple parts appearing in the ECDIS display as separate objects. In this situation the system shall calculate the c of g of each part of the object and display centred symbols for each part.
IHO S-64 contains examples of cases which an ECDIS shall be capable of handling. Text is allowed to extend beyond the boundary of an area. The result will be that the Mariner can clearly identify the area.