The Draughtsman's Handbook of Plan and Map Drawing / Including instructions for the preparation of engineering, architectural, and mechanical drawings.

by George G. André

The sounded point determined by angles measured with the sextant may also be plotted by describing circles on the land-object lines as chords, to contain segmental angles equal to the measured angles. Such circles will intersect in the common land-object point and the sounded point. To plot the sounded point in this manner, requires the solution by construction of the problem, “to describe on a given line a segment of a circle that shall contain a given angle.” But this method is generally found too tedious in practice.

Errors and Error-sheets.

—There is a tendency, as we have previously remarked, for the measured lengths of lines to be a little too long, by reason of the irregularities of the surface. It is usual to allow for this source of error 1 in 1000 in fair open country, and 1¹⁄₂ in 1000 in close country. When the measurements differ by an amount exceeding these limits, the pencil trace should not be drawn between the reference points, but the line should be entered on an “office error-sheet.” The error-sheet should show the number of the plot-sheet, the triangle, the book and page in which the measurements are entered, and the scale and measured lengths of the line. To ascertain the source of the error, other lines referenced to the reference point or points of the line in error should be plotted, and the apparent source should be entered on the error-sheet. If the lines referred to the same point be found to plot to another point in the reference line, the scale measurement of this point should also be entered. And if the reference point in error be not directly surveyed in the survey of their respective lines, the measurements for reference and the arithmetical reductions will have to be examined. Besides the office error-sheet, there should be a field error-sheet for each book and triangle, upon which should be entered the book, the page, and the line in error, and some indication of the source of the error. This sheet will be forwarded with the field-book to the surveyor for correction. The following are examples of a common and very good form of error-sheet, but it may be varied in many ways if thought desirable:—

Office Error-sheet.

Plotter’s Name				Date
Book.	Lines.	Scale Measurement.	Reference Points of Lines.		Apparent Corrections.	Triangles.	Observations.
Page.
200	1953	1907	1118	1551	1651		Examine reference point and line.
23			2094	4020	4020
200	3000	3056	2814	1308	3056		Examine line.
28			4020	3082
200	1314	1323	1551	85	1651		Examine reference point and line.
42			4020	3028	4020

Field Error-sheet.

To A. B., Surveyor.			Date
Book.	Lines.	Triangles.	Observations.
Page.
			Examine reference points and line.
200	1953		1651	Reference point (August 10th, 1874).
23			4020
200	3000		Examine line. Line 3056 (August 10th, 1874).
28
			Examine reference points and line.
200	1314		1651	(August 12th, 1874).
42			4020
			Corrected in the field, A. B.

To Plot Vertical Sections.

—In plotting a vertical section, a fine and firm horizontal line is first drawn to represent the datum line. The reference points are then plotted upon this line from the level-book by means of a linear scale, in the manner already described for plotting such points. The reference points to be plotted upon the datum line are the chain lengths entered in the field-book in the column headed Distances. These distances are the points at which the levels were taken, and between them, unless otherwise stated in the field-book, the ground is supposed to slope uniformly. Moreover, these distances are assumed to be measured horizontally, and therefore care must be taken to ascertain whether or not they were so measured in the field; if not, they must be reduced before plotting, or the section will be too long. Having plotted the reference points on the datum line, a perpendicular must be erected from each of them, and a length laid off upon this perpendicular equal to the vertical height above the datum line indicated by the entry in the column of the level-book headed Reduced Levels against the distance to which the perpendicular corresponds. To render the differences of altitude more apparent, these vertical distances are plotted to a much larger scale than the horizontal, as explained in a preceding Section. To erect the perpendiculars, the T and set square furnish the most convenient means. The detail points thus determined upon the perpendiculars represent the points in the surface of the ground at which the levels were taken, and by joining these points we obtain the surface line. An example will clearly show the method pursued. Let it be required to lay down the section from the following level-book:—

Back Sights.	Fore Sights.	Rise.	Fall.	Re- duced Levels.	Dis- tances.	Remarks.
feet.	feet.	feet.	feet.	feet.	chains.
				100·00	..	B.M. north-west corner of church tower.
16·41	9·37	7·04	..	107·04	230
19·36	10·43	8·93	..	115·97	465
16·42	19·36	..	2·94	113·03	640
8·36	14·36	..	6·00	107·03	794
9·37	12·49	..	3·12	103·91	1030
11·64	19·76	..	8·12	95·79	1200
19·46	9·32	10·14	..	105·93	1564
17·64	13·62	4·02	..	109·95	..	Centre of road at 123 links.
18·76	12·64	6·12	..	116·07	1823
19·84	16·92	2·92	..	118·99	1964
19·76	11·64	8·12	..	127·11	2100	Forward station ☉ at corner of wood.
17·64	19·72	..	2·08	125·03	2250
9·73	18·64	..	8·91	116·12	2376
8·64	17·64	..	9·00	107·12	2590
18·76	16·24	2·52	..	109·64	2700
231·79	222·15	49·81	40·17
222·15		40·17		100·00
9·64		9·64		9·64	Difference between datum and last reduced level, or height of B above A.