« PreviousContinue »
(233) The prismatic compass is generally held in the hand, the bearing being caught, as it were, in passing; but more accurate readings would of course be obtained if it rested on a support, such as a stake cut flat on its top.
In the former mode, the needle never comes completely to rest, particularly in the wind. In such cases, observe the extreme divisions between which the needle vibrates, and take their arithmetical mean.
(234) Defects of compass. The compass is deficient in both precision and correctness."
The former defect arises from the indefiniteness of its mode of indicating the part of the circle to which it points. The point of the needle has considerable thickness; it cannot quite touch the divided circle; and these divisions are made only to whole or half degrees, though a fraction of a division may be estimated, or guessed at. The Vernier does not much better this, as we shall see when explaining its use. Now an inaccuracy of one quarter of a degree in an angle, i. e. in the difference of the directions of two lines, causes them to separate from each other 5 inches at the end of 100 feet; at the end of 1000 feet nearly 4 feet; and at the end of a mile, 23 feet. A difference of only one-tenth of a degree, or six minutes, would produce a difference of 12 feet at the end of 1000 feet; and 9 feet at the distance of a mile. Such are the differences which may result from the want of precision in the indications of the compass.
But a more serious defect is the want of correctness in the compass. Its not pointing exactly to the true north does not indeed affect the correctness of the angles measured by it. But it does not point in the same or in a parallel direction, during even the same day, but changes its direction between sunrise and noon nearly a quarter of a degree, as will be fully explained in Chapter VIII. The effect of such a difference we have just seen. This direction
The student must not confound these two qualities. To say that the sun ap pears to rise in the eastern quarter of the heavens and to set in the western, is correct, but not precise. A watch with a second hand indicates the time of day precisely, but not always correctly. The statement that two and two make five, is precise, but is not usually regarded as correct.
may also be greatly altered in a moment, without the knowledge of the surveyor, by a piece of iron being brought near to the com pass, or by some other local attraction, as will be noticed hereafter. This is the weak point in the compass.
Notwithstanding these defects, the compass is a very valuable instrument, from its simplicity, rapidity and convenience in use; and though never precise, and seldom correct, it is generally not very wrong.
THE FIELD WORK.
(235) Taking Bearings. The "Bearing" of a line is the angle which it makes with the direction of the needle. Thus, in Fig. 147, page 124, the angle NAB is the Bearing of the line AB, and NAC is the Bearing of AC. The Bearing and length of a line are named collectively the Course.
To take the Bearing of any line, set the compass exactly over any point of it by a plumb-line suspended from beneath the centre of the compass, or, approximately, by dropping a stone. Level the compass by bringing the air bubbles to the middle of the level tubes. Direct the sights to a rod held truly vertical, or "plumb," at another point of the line, the more distant the better. The two ends are usually taken. Sight to the lowest visible point of the rod. When the needle comes to rest, note what division on the. circle it points to; taking the one indicated by the North end of the needle, if the North point on the circle is farthest from you, and vice versa.
In reading the division to which one end of the needle points, the eye should be placed over the other end, to avoid the error which might result from the "parallax," or apparent change of place, of the end read from, when looked at obliquely.
The bearing is read and recorded by noting between what letters the end of the needle comes, and to what number; naming, or writing down, firstly, that letter, N or S, which is at the 0° point nearest to that end of the needle from which you are reading; secondly, the number of degrees to which it points, and thirdly, the letter, E or W, of the 90° point which is nearest to the same end of the needle. Thus, in the figure, if when the sights were directed along a line, (the North
point of the compass being most distant from the observer), the North end of the needle was at the point A, the bearing of the line sighted on, would be North 45° East; if the end of the needle was at B, the bearing would be East; if at C, S. 30° E; if at D, South; if at E, S. 60° W; if at F, West; if at G, N. 60° W; if at H, North.
(236) We can now understand why W is cn. the right hand of the compass-box, and E on the left. Let the direction from the centre of the compass to the point B in the figure, be required, and suppose the sights in the first place to be pointing in the direction of the needle, S N, and the North sight to be ahead. When the sights (and the circle to which they are fastened) have been turned so as to point in the direction of B, the point of
the circle marked E, will have come round to the North end of the needle, (since the needle remains immovable,) and the reading will therefore be "East," as it should be. The effect on the reading is the same as if the needle had moved to the left the same quantity which the sights have moved to the right, and the left side is therefore properly marked "East," and vice versa. So, too, if the bearing of the line to C be desired, half-way between North and
East, i. e. N. 45° E.; when the sights and the circle have turned 45 degrees to the right, the needle, really standing still, has apparently arrived at the point half-way between N. and E., i. e. N. 45° E.
Some surveyors' compasses are marked the reverse of this, the E on the right and the W cn the left. These letters must then be reversed in the mind before the bearing is noted down.
(237) Reading with Vernier. When the needle does not point precisely to one of the division marks on the circle, the fractional part of the smallest space is usually estimated by the eye, as has been explained. But this fractional part may be measured by the Vernier, described in Art. (229), as follows. Suppose the needle to point between N. 310 E. and N. 3110 E. Turn the tangent screw, which moves the compass-box, till the smaller division (in this case 310) has come round to the needle. The Vernier will then indicate through what space the compass-box has moved, and therefore how much must be added to the reading of the needle. Suppose it indicates 10 minutes of a degree. Then the bearing is N. 31° 10' E. It is, however, so difficult to move the Vernier without disturbing the whole instrument, that this is seldom resorted to in practice. The chief use of the Vernier is to set the instru ment for running lines and making an allowance for the variation of the needle, as will be explained in the proper place. A VernierA Vernier arc is sometimes attached to one end of the needle and carried around by it.
(238) Practical Hints. Mark every station, or spot, at which the compass is set, by driving a stake, or digging up a sod, or piling up stones, or otherwise, so that it can be found if any error, or other cause, makes it necessary to repeat the survey.
Very often when the line of which the bearing is required, is a fence, &c., the compass cannot be set upon it. In such cases, set the compass so that its centre is a foot or two from the line, and set the flag-staff at precisely the same distance from the line at the other end of it. The bearing of the flag-staff from the compass will be the same as that of the fence, the two lines being parallel
The distances should be measured on the real line. If more convenient the compass may be set at some point on the line prolonged, or at some intermediate point of the line, "in line" between its extremities.
In setting the compass level, it is more important to have it level crossways of the sights than in their direction; since if it be not so, on looking up or down hill through the upper part of one sight and the lower part of the other, the line of sight will not be parallel to the N and S, or zero line, on the compass, and an incorrect bearing will therefore be obtained.
The compass should not be levelled by the needle, as some books recommend, i. e. so levelled that the ends of the needle shall be at equal distances below the glass. The needle should be brought so originally by the maker, but if so adjusted in the morning, it will not be so at noon, owing to the daily variation in the dip. If then the compass be levelled by it, the lines of sight will generally be more or less oblique, and therefore erroneous. If the needle touches the glass, when the compass is levelled, balance it by sliding the coil of wire along it.
The same end of the compass should always go ahead. The North end is preferable. The South end will then be nearest to the observer. Attention to this and to the caution in the next paragraph, will prevent any confusion in the bearings.
Always take the readings from the same end of the needle; from the North end, if the North end of the compass goes ahead; and vice versa. This is necessary, because the two ends will not always cut opposite degrees. With this precaution, however, the angle of two meeting lines can be obtained correctly from either end, provided the same one is used in taking the bearings of both the lines.
Guard against a very frequent source of error with beginners, in reading from the wrong number of the two between
which the needle points, such as reading
34° for 26°, in a case like that in the figure.