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This work of laying off a blank sheet and locating the position of signal points, is done in the office, and on its accuracy depends in great measure the accuracy of the finished chart.
The complete equipment of a planetable party is as follows:
One plane table; two rods; one alidade and
equipment; one or more blank sheets; one visions of the rod lying between the umbrella; one launch, for river and coast outside hairs in the eyepiece indicates work; one tender; from four to six men; and with quite surprising accuracy the dis
axes, shovels, tools, glasses, clocks, compasses,
etc. If any inland or traverse work is to be tance away the rod is from the instru
done, the equipment will include a team and ment. The alidade is also fitted with a vertical arc, by means of which elevations How the Field Work is Done are determined when necessary.
Let us suppose that the party is coniWhen a plane-table party goes into , fortably located at some resort, farm the field, it is supplied with a blank sheet house, or other place of living, as nearly —the most important and most carefully central to the work in hand as possible. guarded part of the equipment. This The first thing in the morning, after the sheet is made of heavy Whatman draw- weather prophets have assured the topoging paper, cut so that it will have the rapher in charge of the party that no imleast shrinkage and expansion from mediate rain is in prospect, is to load all dampness and dryness, and is backed the instruments into the launch. The with heavy cloth. Sometimes there are party then proceeds to the signal from two pieces of paper, with grains running which work is to commence. If the sigin opposite directions, and cloth between, nal is still standing, well and good. In where great extremes of temperature and the center of it will be found a large peg dampness are expected. On this blank driven into the ground; and in the peg, sheet are drawn lightly the meridians and a nail, which indicates the exact location parallels; and in these squares are indi- of that particular "point.” If the signal cated, by triangles inclosing dots, the has been destroyed by wind or water, or positions of the various triangulation sta- the malicious ignorance of the people of tions, and other signals by means of cir- the locality, it must be rebuilt. If the cles and dots, from which the party is to peg is gone, the party digs down three work in drawing the actual topography. feet or more until a bottle or stone jar comes to light, also with a mark upon it, are preferred, although two may be used indicating again the position of that in an emergency. "point." This mark, called a ground Let us suppose that the various signals mark, is of course not found beneath are known by number, as 1, 2, 3, etc. church steeples or tall chimneys, or Actually they have all sorts of namestemporary signals, but is a most impor- such as “Colton's Point," "Blue Sow No. tant part of the erection of a triangula- 2," "Higgins Creek,” etc.—names taken tion signal, inasmuch as if it were omit from the nomenclature of the country. ted, and the surface mark obliterated, re- The party has located over No. 1. The covery of the point would mean much sheet is on the table, so adjusted (for triangulation work for a party sent out convenience only) that the little dot and to do work of another kind.
triangle representing signal No. 1 is We shall suppose that the signal has somewhere near the center. The alidade been recovered and put in order. The is now placed upon the chart so that one plane-table man, who carries and has of its straight edges just touches No. I charge of that instrument, now removes and, further along, No. 2. The table is the protecting top of the movement, and now revolved until signal No. 2 can be erects the tripod over the ground peg, or seen on the central cross-hair in the telas close to it as he can get. The table it escope. The table is then clamped. The self has previously been removed from alidade is now lifted and placed so that its case, and attached by the proper its edge touches No. I and No. 3. If, screws to the movement. The sheet is without movement, No. 3 signal is exthen taken from its waterproof copper actly on the cross-hair of the telescope, cylinder, and stretched across the top of the table is considered "oriented ;" that the table, so that the signals to be used is, the sheet upon the table, with its dots are on the plane surface. The rest of the representing signals, bears an accurate chart is rolled up and secured under the and exact relation to the same signals edges of the table with metal clips. The alidade is now placed upon the table, and the table leveled by means of its leveling screws, with the aid of a level located upon the base of the alidade. Meanwhile, some other member of the party has been engaged with glasses in locating other signal points about the coast-line --or across the river, if river work is being done. Three are essential, and more
PLUMBING A TEMPORARY PLANE-TABLE SIGNAL PRIOR TO ITS FASTENING.
The plane-table man is covering the "movement" of the instrument with its case,
at hand the following data :— the distance away the rodman is, actually; the distance away his rod should be on the chart in the reduced ratio ; and, indicated by the edge of the alidade, the direction from base in which the rodman is. With a fine-pointed pencil, therefore, he draws along the edge of the alidade a light line, the length of the distance between his compass points. He then, by a certain prearranged signal — generally a movement of the arms over the head-signifies to the rodman to move on; and turns his attention and his instrument to the other rodman, with whom he repeats the process just described. When he gets again around to the first rodman, and takes the distance, measures it on the compass, etc., he does not draw a line along the alidade ruler, but makes a point on the sheet,
with his pencil, on the edge of the ruler, BUILDING A PERMANENT SIGNAL OVER A RECOVERED TRIANGULATION STATION,
and the compass distance. This point is
then connected to the end of the first line actually erected. The north and south of drawn. the chart are exactly with the north and It is obvious that this process, kept up, south of the corrected compass.
will produce a reduced facsimile of the
coast-line which is accurate as to disDrawing the Chart
tance, direction, and contour. This is The table having been accurateiy the elementary essential of plane-table oriented, actual charting is to commence. work. There comes a time when the rods The two men who act as rodmen unfold can no longer be read—when they appear their rods, and erect them, fastening so small in the telescope as to preclude them by means of screws provided for accuracy, or when a bend in the coastthe purpose. The rodmen separate, go- line, or undergrowth, prevents their being in opposite directions along the coastline. They stop when they reach the first bend, indentation, or promontory of any size, and turn, facing the instrument under the signal, holding their rods upright, with the scale as nearly square to the instrument as possible. The topographer now orients the alidade so that the image of one of the rods appears across the cross-hairs, being particular to see that one edge of the base of the instrument passes over the point on the chart which • is the location of signal No. 1, over which his instrument is erected. Ile reads in the telescope the number of meters distant the rodman is from him — thirtythree, let us say. He then takes a pair of dividers, and measures off with their points, 33 meters from a metal scale which is reduced twenty thousand times ; in other words, the chart is being made on a scale of 1 to 20,000. He now has
GETTING READY TO GO TO WORK U'NDER A SIGNAL. The topographer and his assistant are working at the instrument. The plane-table man is at his never-ending work of
putting needle-points on pencils. One rodman is waiting for instructions to start,
and the other is locating signals with the glasses,
around and sighting on the signal, with the alidade edge running over the rod station as plotted, and the signal point on the chart, the table is again oriented, and checked up, if necessary, by observations on other signal points. From this station the work goes on as before; only in this case both rodmen are forward rod
charts. The surroundings of the coast are of great importance to navigators. They want to know the location of roads, the location of villages, and the character of the shore. Accordingly, much work is done a short distance inland. Here it sometimes becomes necessary to run what is called a traverse line, in which