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Displaying Soundings on a Chart

The last part of the article from Andy about soundings


There are a lot of numbers to display in a small space – even on a paper chart 40” wide.

Conventionally, soundings are displayed...


without thousands separators or units


with a single decimal digit displaced ½ a line down instead of with a decimal point (if less than about 30 feet)


with a bar under the number for drying heights, instead of using a minus sign

Units are implied by the chart – in the UK, non-metric charts were black and white and metric charts are colored. Everything is done to try and prevent someone misreading a dangerous depth for a safe one.

Sounding Units in S-57 (ENC)

ENC is one of several chart products based on the S-57 chart standard. Part of the ENC specification is that all depths and heights be specified in metric units (meters).

But the surveys in the US have been done in English units (feet). When a sounding in feet is converted to meters to be stored in the ENC format and is then converted back to feet for display, rounding errors creep in...

Original depth recorded in survey in feet

Stored in ENC in meters (to 1 decimal)

Displayed on chart in feet (to one decimal)













We could argue that 0.1’ here and there does not make the chart useless, but it does make the chart significantly more crowded with all those decimal digits. In Nuno Navigator, depths from US sourced chart data are rounded to whole feet, on the assumption that the survey was in feet in the first place.

Soundings can also be very crowded – clutter on the chart, particularly when the chart is displayed at a less detailed scale than it was intended for. The ECDIS display standard (ISO 61174) also specifies rather large text and symbols – larger than are needed on a modern LCD display. The standard S-57(ENC) presentation draws sounding values using turtle graphic symbols instead of text – symbols cannot benefit from clear-type etc.

Taking all this together the display of soundings changes from




Orignal S-57(ENC)


Nuno Navigator

At a smaller scale the difference is even more marked




Orignal S-57(ENC)


Nuno Navigator

The Nuno Navigators soundings might be a bit small to read, but the original is too crowded to read and the existence of small text is the clue you need to know that there is something there – all you have to do is zoom in a bit to read it. Everything else about the chart remains intelligible – you can still see the contours clearly for instance.

Highlighting Soundings

The “Properties at Point” display is an important element of an ENC chart. It shows all the detail of the last thing you clicked on and is one of the big advantages of a vector chart.

It is important to know what you actually clicked on.


We highlight this on the chart.

Here, I clicked on a buoy.


But it didn’t work so well for soundings, which by a quirk of the S-57 design are grouped together.

Here I clicked on the 11.5m sounding lop left.


But, by remembering the depth of the sounding that was actually clicked on, we can now make it work much better



By topsy.com | Mon, Jul 2010 01:53 | #

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