Practical marine surveying by Phelps Harry 1861-

Author:Phelps, Harry, 1861-
Language: eng
Format: epub
Tags: Hydrographic surveying
Publisher: New York, J. Wiley and Sons
Published: 1889-03-25T05:00:00+00:00

on the plate at the required angle and a fine line drawn along its edge, the right angle being laid off to the left of the centre line and vice versa. The rule is then removed and the glass placed face down on the projection, and moved until the three lines cover the required signals, when the observer's position is pricked through the centre h6le. The black lines on the glass show very plainly against the sheet, and there is no parallax on account of thickness of glass, etc. When one position is plotted the lines may be washed off the glass, and it is ready at any time for use and not affected by water.

157. As each position is plotted it is marked with a needle point, a circle drawn around it, the number of the position placed abreast, and a linQ drawn from the preceding position. These lines should terminate just outside the circles of the positions, not at the points themselves. When all the positions are plotted, or before stopping for the day, the positions are inked in by making a circle about -^ inch diameter around each one, and writing the number and ,day mark abreast every fifth position, also the first and last of the day. Different colors are used to distinguish between the boats and ship, as each one has the same series of day marks; red being generally used for the ship's work, green and blue for the boats; with a further distinction of capitals and small letters should there be more than three vessels. These auxiliary marks are never placed on the sheet in black, as they are not transferred to the finished chart.

158. The next step is to plot the soundings. If the same number of casts were taken between every two positions, the spaces might be laid off with proportional dividers, but as this is never the case for many intervals another device is used.

A frame about 6'^ square inside is made of strips J" X li'' as

in the sketch. On the edges AB^ CD draw a series of parallel

o lines about J'' apart and make a shallow cut along each mark with a penknife, and drive a fine tack or pin in the outer edges abreast each mark. String this up as a harp, using

g fine silk thread and drawing it taut across the frame in each cut. Turn the frame over and number each thread consecutively, calling the first one o. If the o thread is placed B ^ over any point on the chart and any other

thread over a second point, the intermediate threads will divide the line' joining the two points into a number of equal parts equal to the number abreast the thread which crosses the second point. The soundings are then written in black ink over each point of division and at each position, and the character of the bottom is put in at intervals.

159, Hitherto in discussing in-shore work it has been considered that every position was fixed by angles and the soundings plotted on the lines connecting them.

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