Under similar conditions, but where more than one "marker" is used, the transit stadia is a dependable and fast method that may be employed. The use of this method enables the geologist to follow his key-horizons through all kinds of conditions; and he may easily change from the stadia to the spirit level method. It is of great value in a wooded country where numer ous "set-ups" are required.
Attention is called to mistakes that may be caused by the changing from the stadia method to the spirit level. The stadia measurements are generally figured from the ground, and if the method is changed, the proper allowance is to be made for the height of the instrument, of the spirit level. For example: If a change is made from the level to the stadia, and if the last "shot" was a back sight, it will be necessary to subtract the height of the instrument above the ground, from the height of the instrument above sea-level, which is the last computation in the notebook; and vice versa, when changing from stadia to the spirit level method, and if the next sight is to be a fore sight, it will be necessary to add to the ground elevation the height of the instrument above the ground to obtain the H.I. above sea-level before such a fore-sight may be taken. (Fig. 24.) The plane table is used to best advantage when a topographic map is not obtainable, as by use of it, both topographic and geologic maps may be made simultaneously. The plane table is at a disadvantage in a wooded country, where numerous set ups are necessary. It is advisable that a note-book should also be used in connection with the plane table.
The plane table is by far the most important instrument for the geologist and it is most frequently used in all kinds of geo logical work; both the results obtained and the time and expense connected with it are its main features. The main points in its favor are that the outcrops and their elevations as well as all topographic details may be obtained and mapped right in the field so that the actual construction of the map in the field may show errors, which may be eliminated, such errors if obtained only in the office when the field notes are worked up would necessitate an extra trip back in the field. No cumbersome notes are kept and the calculations are eliminated. Horizontal angles are not read and recorded but plotted on the map. Any
necessary sketching is done at once and not made from memory, so the details themselves may serve as models which in office work would not be obtainable. The limit within which the survey closes both as to horizontal and vertical measurements will be evident whenever a check reading is taken on various known points, thus a check on the work is had at all times. By means of it maps may be made to any convenient scale directly and economically and it is by far the most suitable instrument for the geologist and if properly handled it is a guarantee against inferior results.
It is customary for the geologist to carry the rod so that he may pick the proper outcrops and continually examine them, while the surveyor at the table may give all his attention to the mapping; otherwise, if the geologist were at the instrument and a rod man used, the geologist would not only have to attend to the instrumental work, but would quite often be compelled to leave it to attend to the geological features which may need his attention.
Regardless of what system of geological surveying is employed, it is always necessary that all surveys should be closed or checked. The vertical distances are the most important in this connection and therefore in all cases either the original bench mark should be used or the survey closed at another bench mark, so that a check on the correctness of the work may be had.
During the progress of the field work care must be taken and many columnar sections made throughout, so that the several key-horizons may be reduced to a common one; also lateral variations be noted and its general direction and extent determined. Do not hesitate to go back to any point where certain possibilities may offer themselves; if you are in doubt as to the correlation of a formation, stay with it until you solve it. The work may progress along smoothly for days then some thing may occur that will hold up the work, but it is better that you be sure of your formations and the general geology, than to make a hasty guess which might involve serious errors. Remem ber there is no need for any guessing to be done, the geological work in the field must be absolutely correct in accordance with the highest standards.