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be laid in a direct line from A, towards G, where let a peg be left, as at c ; and again, the like distance from A in a direct line towards B, where another peg is also to be left, as at do let the distance from d to c be measured, and placed in the field-book, in the second column under the denomination of angles, in a line with station No. 1 ; and in the same line, under the title of distances, in the third column, let the measure of the line AB in chains and links be inserted. Being now arrived at B, let one chain be laid in a direct line from B towards A, where let a peg be left, as at f, and again, the like distance from B in a direct line towards C, where let also another pegbe left, as at e : the distance from e to f is to be inserted in the field-book in the second column, under angles, in a line with station No. 2; and in the same line, under the title of distances in the third column, let the measure of the line BC, in chains and links, be inserted : after the same manner we may proceed from C to D, and thence to E, but because the angle at E, viz. FED, is an external angle, after having laid one chain from E to h, and to g, the distance from g to his measured, and inserted in the column of angles, in a line with station No. 5. and on the side of the field-book against that station, we make an asterisk, thus”, or any other mark, to signify that to be an external angle, or one measured out of the ground. Proceed we then as before, from E to F, to G, and thence to A, measuring the angles and distances, and placing them as before, in the fieldbook, opposite to their respective stations; so will the field-book be completed in manner following. * N. B. After this manner the angles for inaccessible distances may be taken, and the method of constructing or laying them down, as well as the construction of the map, from the following field-notes, must be obvious from the method of taking them.
The form of the field-book, with the title.
A field-book of part of the land of Grange, in the parish of Portmarnock, barony of Coolock, and county of Dublin; being part of the es
tate of L. P. Esq. let to C. D. farmer. Surveyed January 30, 1782.
Taken by a four-pole chain.
- No. Angles Distan. Remarks. Sta!Ch. L. Ch. L. Mr. J. D’s part of Grange | 1 |1.80 17.65
2 | 1.79 18.50 Mr. L. P's part of Portmar- 3 1.76 28.00 nock strand 4 |1.41; 20,00 * | 5 |1.87; 14.83 Widow J. G.'s part of Grangel 6 1.14 19.41
7 | 1.89 24.53 TClose at the first station.T
Explanation of the remarks.
Mr. J. D’s part of Grange bounds or is adjacent to the surveyed land from the first to the third station; Mr. L. P's part of Portmarnock bounds it from the third to the fourth station; the strand then is the boundary from thence to the sixth, and from the sixth to the first station, the widow J. G.'s part of Grange is the boundary.
It is absolutely necessary to insert the persons' names, and town-lands, strands, rivers, bogs, rivulets, &c. which bound or circumscribe the land. which is surveyed, for these must be expressed in the map.
In a survey of a town-land, or estate, it is sufficient to mention only the circumjacent townlands, without the occupiers’ names: but when a part only of a town-land is surveyed, then it is necessary to insert the person or persons’ names, who hold any particular parcel or parcels, of such town-land, as bound the parts surveyed.
When an angle is very obtuse, as most in our present figure are, viz. the angles at A, B, C, E, and G: it will be best to lay a chain from the angular point, as at A, on each of the containing sides to c and to d, and any where nearly in the middle of the angle, as at e : measuring the distances ce and ed; and these may be placed for the angle in the field-book. Thus,
For when an angle is very obtuse, the chord line, as cd, will be nearly equal to the radii Ac and Ad; so if the arc ced be swept, and the chord line cal be laid on it, it will be difficult to determine exactly that point in the arc where cal cuts it: but if the angle be taken in two parts, as ce and ed, the arc, and the angle thence, may be truly determined and constructed.
After the same manner any piece of ground may be surveyed by a two-pole chain.
To take a survey of a hiece of ground from any floint within it, from whence all the angles can be seen ; by the chain only.
PL. 6. fig. 6.
Let a mark be fixed at any point in the ground, as at H, from whence all the angles can be seen; let the measures of the lines HA, HB, HC, & c. be taken to every angle of the field from the point H.; and let those be placed opposite to No. 1, 2, 3, 4, &c. in the second column of the radii: the measures of the respective lines of the mearing, viz. AB, BC, CD, DE, &c. being placed in the third column of distances, will complete the field-book. Thus,
If any line of the field be inaccessible, as suppose CD to be, then by way of proof that the distance CD is true, let the measure of the angle CHD be taken by the line oo, with the chain: if this angle corresponds with its containing sides, the length of the line DC is truly obtained, and
the whole work is truly taken. a
Note, That in setting off an angle, it is necessary to use the largest scale of equal parts, viz. that of the inch, which is diagonally divided into 100 parts, in order that the angle should be accurately laid down; or if two inches were thus divided for angles, it would be the more exact; for it is by no means necessary that the angles should be laid from the said scale with the stationary distances.
7% take a survey by the chain only, when all the angles cannot be seen from one fioint within.
Let the ground to be surveyed be represented by 1, 2, 3, 4, &c. Since all the angles cannot be seen from one point, let us assume 3 points, as A, B, C, from whence they may be seen; at each of which let a mark be put, and the respective sides of the triangle be measured and set down in the field-book; let the distance from A to 1, and . from B to 1, be measured, and these will determine the point 1; let the other lines which flow from A, B, C, as well as the circuit of the ground, be then measured as the figure directs; and thence the map may be easily constructed.
There are other methods which may be used; as dividing the ground into triangles, and measuring the 3 sides of each; or by measuring the base and perpendicular of each triangle. But this we shall speak of hereafter.