Reliance on a chart. - The value of a chart must manifestly depend upon the accuracy of the survey on which it is based, and this
becomes more important the larger is the scale of the chart.
To estimate this, the date of the survey, which is always given in the title, is a good guide. Besides the changes that, in waters where
sand or mud prevails, may have taken place since the date of the survey, the earlier surveys were mostly made under circumstances that precluded
great accuracy of detail, and, until a plan founded on such a survey is tested, it should be regarded with caution. It may, indeed, be said that,
except in well frequented harbours and their approaches, no surveys yet made have been so minute in their examination of the bottom as to make
certain that all dangers have been found. The fullness or scantiness of the soundings is another method of estimating the completeness
of the chart. When the soundings are sparse or unevenly distributed, it may be taken for granted that the survey was not in great detail.
It appears to be insufficiently realised that the degree of reliance which may reasonably be placed on an Admiralty chart, even in surveys
of modern date, is mainly dependent on the scale on which the survey was made. The scale for publication is now generally that of the original
survey, except in the case of Coast sheets, which are sometimes reduced. It should not, therefore, be assumed that the original survey was made
on a larger scale than that published.
It must be borne in mind that the only way of ascertaining the inequality of the bottom of the sea is by the laborious process of
sounding, and that in sounding over any area, the boat or vessel obtaining the soundings is kept on given lines; that each time the lead
descends it only ascertains the depth of water over an area equall to the diameter of the lead, that is about two inches, and that consequently
each line of soundings, though miles in length, is only to be considered as representing a width of two inches.
Surveys are not made on uniform scales, but each survey is made on a scale commensurate with its apparent importance. For instance, a general
survey of a coast which vessels only pass in proceeding from one place to another is not usually made on a scale larger than one inch to the
nautical mile, while surveys of areas where vessels are likely to anchor, are made on a scale of three inches to the mile, and surveys of
frequented ports, or harbours likely to be used by fleets, on a scale of from six inches to ten inches to the nautical mile.
Close examination by sounding is the only method by which surveys on a large scale can be made, and in view of the vast mileage of surveys
yet requiring completion in the interests of navigation, it would be a waste of time to undertake large scale Coast surveys.
The scale on which a survey is to be conducted having been settled, it is manifestly superfluous to obtain more lines of soundings than can
be represented on the paper. 100 soundings, which is the maximum number that can be placed with clearness on every square inch of paper, means
that on a scale of one inch to a mile each sounding on the chart occupies an area representing eight acres of actual ground, whilst on a
scale of six inches to the mile each sounding represents an area of a little less than a quarter of an acre,i.e. of 100 square feet.
Little assistance in detecting excrescences on the bottom is afforded by the eye, when sounding in a boat, even in clear weather, on
account of the observer being within five feet of the surface; none in turbid seas. If, therefore, there is no inequality in the soundings to
cause suspicion, a shoal patch between two lines may occasionally escape detection.
Lines of soundings plotted as close as may be practicable on a scale
of six inches to the mile would be 100 feet apart, and each line would be only 2 inches in actual width.
thus, in a chart on a scale of one inch to the mile, an inequality of some acres in extent rising close to the surface, if it happened to
be situated between two lines, might escape the lead; whilst in a chart on a scale of 6 inches, inequalities as large as battleships, if lying
parallel to, and between the lines of soundings, might exist without detection if they rose abruptly from an otherwise even bottom.
General coast charts should not, therefore, be looked upon as infallible, and a rocky shore should on no account be approached within
the contour line of 10 fathoms, without taking every precaution to avoid a possible danger; and even with surveys of harbours on a scale of 6
inches to the mile, vessels shouls avoid, if possible, passing over charted inequalities in the ground, as some isolated rocks are so sharp
that the lead will not rest on them.
Blank spaces among soundings mean that no soundings have been obtained in these spots. When the surrounding soundings are deep it may
with fairness be assumed that in the blanks the water is also deep; but when they are shallow, or it can be seen from the rest of the charts
that reefs or banks are present, such blanks should be regarded with suspicion. This is especially the case in coral regions and off rocky
coasts, and it should be remembered that in waters where rocks abound it is always possible that a survey, however complete and detailed, may
have failed to find every small patch.
A wide berth should therefore be given to every rocky shore or patch, and this rule should be invariably followed, viz., that instead
of considering a coast to be clear, unless it is shown to be foul, the contrary should be assumed.