A.PPENDIX NO. 10—1897.

PHOTOTOPOGfBAPHIC MBTHODS AND INSTEUMENTS.

By J. A. Flemer.

INTRODUCTION.

Topography is that brandi of surveying which pictnres the shape of the outer visible surface of the earth, in reduced scale, as a horizontal projection, yet showing the relative positions of points of the terrene also in the vertical sense. It is, therefore, supplementary to geodesy in representing areas of the earth’s surface, inclnding ail the necessary details and changes in the terrene, by means of instrumental measurements made in the field.

The work of filling in the details—topographie surveying in the doser sense—may be accomplished by various methods, differing in the matter of costs, time, and attainable accuracy; one may be advantageously employed for one class of work, while another may be préférable for another class or locality, under different conditions, and the method best adapted for any partic-ular région should be employed to obtain the best resnlts. Minute and detailed methods, with ensuing accurate results, should be applied to cities and ail closely settled régions, to the Coastal belts, larger river valleys and lakes, particularly when navigable, and this work should be platted on a large scale.

Arid, barren, and mountainous régions, as well as prairies and swamp lands, when sparsely settled, should be more generalized in their cartographie représentation and platted on a small scale.

Topographie surveys may be accomplished in various ways, of which the following are the methods and instrumental outfits more frequently in use :

I. The direct platting to scale in the field of ail features to be represented on the finished chart:

(a) With a plane-table and steel tape measure.

(b) With a plane-table and telemeter or stadia rods,

(c) With a tachygraphometer and telemeter or stadia rods.

(d) With either outfit mentioned under a and 6, but with a leveling instrument in addition for a more précisé location of the horizontal contours.

(e) Using a barometer instead of a level for less accurate work.

II. The compilation of ail available data—cadastral surveys, public land and county surveys, raiïroad and canal surveys—giving principaliy the horizontal distances and rnaking a supplementary survey to supply the missing data, which in this case are principally élévations that may be supplied by leveling profiles, by trigonométrie leveling, by interpolation and sketching.

III. The records of the survey are in the shape of field notes and sketches (tachymetry), the map being produced by platting the recorded data in the office:

(a) With a surveyor7s compass and steel tape, locating the relative positions of characteristic points in the horizontal sense, while their relative élévations are ascertained by means of a level and minor details are sketched.

(b) By means of a transit and steel tape points are determined both geographically and hypsometrically (using vertical angles), and minor details by sketching.

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