A method of sketching in shore-lines when making running surveys or reconnaissance which avoids the necessity of estimating distances, or depending on the eye to get the configuration of the shore-line, or taking bearings with the compass.
The instrument used is a modification of the ordinary plane table, and consists of a plane table or drawing board, mounted on a tripod, and two rulers. One ruler is attached to the edge of the board so as to move freely in the direction of its length across the board, with room underneath it for a sheet of sketching paper. This ruler is pointed at one end, and graduated on one edge from the pointed end as zero. The other is an ordinary straight-edge ruler with sight vanes at the ends, and is not attached to the board in any way. (See Figures 1 and 2, Plate I.)
To sketch in a shore-line, the ship or boat runs along the shore at as uniform a speed as possible, and with as few courses as is consistent with keeping at a convenient distance from the shore; the distance from the shore being determined by the minuteness of the details which it is desired to see and record. The board is mounted on deck so as to command as much of the horizon as possible, and is so placed that the pointed ruler is parallel to the ship's track; ordinarily this will be parallel to the ship's keel. When the ship is making drift or steering a range the ruler is to be parallel to the course actually made; how it can be so placed will be explained hereafter. A line drawn along the edge of this ruler will be the ship's track on the paper. The graduations on this ruler represent time, so that if at any instant its point is at the ship's position on the paper, then at any other time, say three minutes later, the point may be moved forward to the ship's new position by moving it forward three minutes or three divisions of its graduation. Thus at any instant when the ship is moving along the shore the point of this ruler may be put at her proper position on the paper at that instant; the draughtsman having a watch before him for that purpose. Now to put in the shore-line, bearings are taken of prominent points of the shore with the sighting ruler, having its edge against the point of the time ruler,—the latter being at the proper time or place in the ship's track,—and these bearings are drawn on the paper. As soon as more than one bearing is taken of the same point on shore from different places or times in the ship's track, that point is fixed on the paper by the intersection. It is, in fact, fixed by a triangulation on the point with the ship's track as a base line. (See Plate II.) It is not necessary, of course, to wait for even minutes or divisions of the ruler graduations, but at any instant when a bearing is wanted, the draughtsman, noting the time by the watch, can move the time ruler forward to the proper position, by estimating the fractions of minutes between the divisions. By selecting points as they come in sight on the bow, taking bearings then, again when nearly abeam, and again when on the quarter, good angles can always be made. The whole shore-line is thus put in by cuts on the prominent or necessary points, the draughtsman filling in the shore-line between these points as the ship passes along it. Any number of intersections may, of course, be drawn through the same point to verify the intersections; and many points may be kept in hand at the same time by slight pencil notes on the different lines drawn to keep them from becoming confused. Any prominent peaks or landmarks beyond the shore-line, or in fact anything in sight from the ship in any direction, may be plotted very accurately on the paper by taking bearings of them from time to time as the ship moves along. (See Plate II.)
When the ship's course is changed, turn the paper, underneath the time ruler, through the angle of change about the point of the time ruler occupying the ship's position on the paper at the time of turning, turning the paper to the right when the course is changed to the left, and vice versa; the edge of the time ruler will then be along the ship's new track. This angle of change may be laid off with a protractor; or if the ship is to head for a visible point, the new track may be drawn with the sighting ruler, the same as any other bearing, just before turning, and the paper then turned until the edge of the time ruler is along the new track.
The sketch is connected with known points and plotted in the following manner. The ship is carefully logged,—preferably by frequent readings of the patent log or a careful record of the revolutions of the engine,—and a record kept of the courses and distances, times of changing courses, &c. A time record is also kept on the sketch itself by putting the time down frequently along the ship's track; the separate record of the officer of the deck or other person than the draughtsman being used when finally plotting the work. When the sketch is from one known point to another, these points may be cut in the same as the other points of the shore-line; or the ship's starting and finishing points may be determined by sextant angles on known points or signals. To plot the work, the ship's track is laid down from one known point towards the other, and the second point is plotted from the sketch the same as the other points of the shoreline. Then, all known errors having been eliminated, the difference between the plotted position of this known point and its true position is an error which must be distributed throughout the ship's track. This may be done as shown in the figure. (See Figure 3, Plate III.) The full line represents the plotted track of the ship and the broken line the corrected track. The dotted lines are drawn through the points where the course was changed and parallel to the line through the plotted and true positions of the second known point. And the lines Be, cc, &c. are of such lengths that Bb is to AB as Dd is to ABCD, and that Cc is to ABC as Bd is to AB CD, &c. And Dd is equal to Be. The error is thus distributed proportionately throughout the whole track.
When there is only one known point, a run may be made along the shore-line as far as required and back to the same point, then plotting the whole course; the error in the final plotted position of the known point can be distributed in the same manner.
An examination of this method of making a running survey will render it evident that when running between known points, every error except unrecorded variations in the speed, unknown variations of currents, and unknown irregular errors of the compass, are eliminated; and these are errors necessarily occurring in every running survey not a triangulating one. As before suggested, it avoids all necessity of estimating or guessing distances with the eye, or of depending on the eye for the configuration of the shore-line. It has been found in practice that many irregularities, such as bights and bays in the shoreline which are unperceived by the eye, will be recorded by this method. Such sketching has been done heretofore, when making running surveys, by the clumsy and inaccurate methods of bow and beam bearings and compass bearings and off-hand sketching. The far greater number and more accurate bearings taken by this method add much to the accuracy of the shore-line obtained. Its rapid and easy use, its self-plotting, and its plotting while the shore-line itself is before the draughtsman, make it especially adapted to places where the shore-line is intricate, as where there are many islands and rocks and many breaks in the shore-line,—a few tangents to an island, from different points in the ship's track, completely defining it.
Various ways of adding to the efficiency of the method will suggest themselves to any one using it. For instance, if the steering is bad or difficult, the draughtsman can, by noting the bearing of some object ahead, know when the ship is exactly on her course, and by only taking bearings then, make the method nearly independent of bad steering. When this precaution is taken, if the bearings of points do not intersect well, then the time ruler is not parallel to the ship's actual course. This should be corrected by turning the board until the time ruler is parallel to the actual course. When the change of drift is equivalent to a change of course, then the paper should also be turned a corresponding amount in the opposite direction, the same as for any other change of course. It will be found that the instrument itself will render drift apparent which is hardly discernible in any other way, for when the time ruler is parallel to the true course the intersections will be remarkably close (see Figure 4, Plate III), and any deviation, especially if it repeat itself in successive points plotted, is good evidence of drift, and the amount by which a quarter-bearing fails to pass through the intersection of the bow and beam bearings shows how much the drift is and how much the board is to be turned to correct for it. Figure 4 is an exact copy of the intersections of a number of bearings taken of a point on shore from a ship going about six knots an hour and about two miles off the point; no precautions being taken to favor the method,—the quartermaster at the time was steering an ordinary course, not knowing that anything depended on his steering, and the bearings were taken haphazard as the ship went by the point, as a test of the method.
This method of making a running survey was devised and used on board the U.S.C.S. Hassler in the summer of 1883. Although the instrument there used was made on board ship, it was found to work exceedingly well; many miles of surveying were done with it, both in steam launches and in the ship itself. It was used for putting in the shore-lines between signals when they were far apart, and also for reconnaissance work independent of known points. An interesting mathematical problem will be found in determining the curve of the intersections of many bearings taken of the same point when the time ruler is not parallel to the true course. A modification of the same instrument can be used in navigating known waters, entering harbors or running along the shore, determining at any instant the ship's exact position, or tracing on the chart her exact course along a shore, through a channel or into a harbor.
U. S. Hydrographic Office,
February 9, 1885.