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10.1.7 Limit of ENC Coverage: Non-HO Data on the Display

Since the HO will not issue a data coverage diagram, the ECDIS shall compile a graphical index of the HO ENC data available, classified by navigational purpose, and make it available to the mariner.

The end of HO data need not be identified on the display. The appearance of the “No data” colour (NODTA) and fill pattern AP(NODATA03) will indicate the end of data.
If non-HO data is shown on the ECDIS display, its boundary shall be identified by the linestyle LC(NONHODAT). The display priority is 3; over radar; display base; viewing group 11060. Note that the LC(NONHODAT) is a “one-sided” line and the boundary of the non-HO data must be drawn according to S-57 rules to ensure that the diagonal stroke of the line is on the non-HO data side of the line.

10.1.8 No Data Areas

To make sure that the mariner is aware of areas where no data exist, the entire screen shall always be filled with grey NODTA colour fill together with the fill pattern AP(NODATA03) at the start, before any other information is drawn. The display priority is 0 (allowing non-ENC data to be overdrawn); radar flag suppressed by radar; the category is DISPLAYBASE and the viewing group is 11050.

10.1.9 Scale Boundaries

"Navigational purpose" is used in S 57 to express the concept of scale (ENC Product Specification 2.1).

10.1.9.1 Chart Scale Boundaries

The "chart scale boundaries", where the navigational purpose of the data changes, shall be symbolised on the ECDIS display by a simple linestyle LS(SOLD,1,CHGRD). Alternatively linestyle LC(SCLBDYnn) may be used, with the double line (indicating better resolution) on the side of the larger scale data. The display priority is 3; over-radar; standard display; viewing group 21030.

Only the significant changes from one navigational purpose to another shall appear as chart scale boundaries; boundaries marking minor changes in compilation scale that lie within the range of a navigational purpose shall not be drawn.

10.1.9.2 Graphical Index of Navigational Purpose

The chart catalogue for official charts may be used as a model for the graphical index of navigational purpose, except that minor changes in compilation scale that lie within the range of a navigational purpose shall not be drawn on this index. Only the significant changes from one navigational purpose to another shall appear.

10.1.10 Overscale

Scale is expressed as a fraction, representing: [length of an object on the chart] / [actual length of that object on the ground]. Thus 1/10,000 is twice as large a scale as 1/20,000, because the length of a given object on the chart is twice as long at 1/10,000 scale as at 1/20,000.

The "Compilation Scale" is the reference for overscale calculation. This is coded in the CSCL subfield of the DSPM field. The object M_CSCL, CSCALE is only used if the compilation scale is different for some part of the data set, and when encoded must also be taken into account. (S 57 App. B1, Ann. A - "Use of the Object Catalogue").

10.1.10.1 Overscale Indication

The overscale indication is intended to remind the mariner that the size of chart errors is magnified when he increases the display scale. A 1 mm error at compilation scale of 1/20,000 becomes a 1.3 mm error at a display scale of 1/15,000 and a 2 mm error at 1/10,000.

The overscale factor shall be calculated as [denominator of the compilation scale] / [denominator of the display scale], expressed as, for example "X1.3", or "X2" (using the figures in the example above.)

This shall be indicated on the same screen as the chart display, and treated as display base. Use colour SCLBR.

This overscale indication is required by IMO PS [3] whenever the display scale exceeds the compilation scale.
Note: If the display is compiled from more than one ENC of the same compilation scale, and if the mariner deliberately chooses to zoom in so that the display scale exceeds the compilation scale, then only the "overscale indication" shall be shown. The "overscale pattern" AP(OVERSC01) shall not be shown.

10.1.10.2 Overscale Area at a Chart Scale Boundary

Section 10.1.10.1 above dealt with overscale deliberately selected by the mariner. A different overscale situation arises when the ship approaches a scale boundary from a larger to a smaller scale ENC, typically when leaving harbour, as illustrated in the data coverage diagram below. In combining data from the large scale and the small scale ENCs to generate a display at the larger scale, the ECDIS will have "grossly enlarged" the small scale data to the left of the scale boundary on this figure.

In addition to drawing the scale boundaries, the "grossly overscale" part of the display shall be identified with pattern AP(OVERSC01), as illustrated. Its display priority is 3; over-radar; standard display; viewing group 21030.
In this context, " grossly enlarged" and " grossly overscale" shall be taken to mean that the display scale is enlarged/overscale by X2 or more with respect to the compilation scale. For example, at the left edge of Fig 5 the display scale of 1/12,500 is X4 the compilation scale of 1/50,000, and so the overscale pattern is required.
Note that in this situation the pattern AP(OVERSC01) shall only be shown on the area compiled from the smaller scale ENC. If the area from the larger scale ENC is also overscale, this shall be indicated by the "overscale indication". The pattern AP(OVERSC01) shall not be shown on the part of the display taken from the larger scale ENC. For example if the display scale of the situation in the data coverage diagram was 1/3,500 the area of compilation scale 1/12,500 would have an overscale indication of X 3.6 but would have no pattern AP(OVERSC01).

10.1.10.3 Larger Scale Data Available

Visualise the opposite situation from that in the data coverage diagram below. This time, the ship is approaching harbour from the left edge of the figure, displaying an ENC at a compilation scale of 1/50,000. As the display window begins to cover an ENC at a compilation scale of 1/12,500, the ECDIS shall indicate that larger scale data will shortly become available, as required by IMO PS [3].

Fig 5. Data Coverage Diagram
(The right hand side of the diagram shows the ENC layout with the screen window overlaid, and the left hand side is enlarged to show the ECDIS display on that screen.)

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