Why there are no depth contours on any of our sheets
A contour is a line drawn between two soundings by somebody who was not there. On a bar, that line is the difference between a passage and a grounding.
Somebody asked last week why our sheets look unfinished. It is a fair question. Open any chart on the shelf and the water is banded — a five-metre line, a ten, a twenty, tints between them, the shape of the bottom laid out at a glance. Ours have figures and one line, and the one line is the coast.
That is deliberate, and it is the only strong opinion this company holds about drawing.
What a survey actually produces
A survey boat runs lines. It measures a depth, at a position, at a moment, and it reduces that depth to chart datum using the tide it observed at the time. What comes back is a scatter of points: a few hundred of them on a small sheet, a few million on a modern one.
That is the measurement. Everything else on a chart is drawing.
A depth contour is produced afterwards, on land, by somebody deciding where the five-metre line runs between the soundings. On even ground that decision is nearly free. Over a bank that shifted last winter, on run lines fifty-eight metres apart, it is a guess — and it is a guess that comes out looking exactly as authoritative as the soundings it was interpolated from. Same ink, same weight, same sheet.
The soundings are data. The contour is an opinion about the ground between them, drawn in the same ink as the data. That is the whole of the objection.
Skellow Bar is why we care
Run the line spacing over Skellow Bar and the problem is not abstract. The bar is a shoal that runs across the approach, and its crest is narrow. A contour drawn between two soundings either side of the crest will pass straight over the top of it, and the sheet will then show clear water where the least depth is. That is not a defect in the cartographer. It is what interpolation does to a feature narrower than the sample interval.
The figures cannot lie in that particular way. If there is no sounding on the crest, the sheet shows no sounding on the crest, and the gap is visible as a gap. You can see where nobody went. That is worth more than a smooth picture.
What we draw instead
One stroke, and every depth as a figure
Each plate carries exactly one stroked path — the coastline, which is observed rather than interpolated — and puts every measured depth on the sheet as a numeral. Plain figures are metres below chart datum. Underlined figures are drying heights: ground that stands above chart datum and uncovers. There is no tint, no fill, and no contour on any sheet in the set.
Across the seven sheets that is 1,531 soundings and zero contours, and both of those numbers are counted out of the shipped files rather than claimed. You can count them yourself; the figures are text in the SVG.
The honest caveat
Our seven sheets are not survey records. They come out of a modelled seabed written for this purpose, and the survey page says so at the top rather than in a footnote. The argument above is about how a sheet should be drawn, not a claim that we have been out there with a boat.
If you are working real ground, work it off the official survey. If you are looking at ours, look at the gaps.
Every sheet, at full size, offline
All seven plates ship inside the app at sheet scale, with the constituent tables beside them.
Wrackline is an aid to planning and is not a substitute for an official tide table. Do not navigate on it.