REPORT FOR 1897--PART II. APPENDIX NO. 10.

677

The distances Sa and SA, fig. 52, are measured on the platting sheet and the ordinate aA, fig. 49, of the pictured point a, on the négative. Perpendiculars are then erected to SA in A and a and the latter is made ec^ual to the ordinate of a taken from the picture — Aa = a (a), fig. 52. If we now draw the line S (a) (to its intersection with the perpendicular in A), then the triangles Sa (a) and SA (A) will be similar and the angle AS (A) will represent the vertical angle (of élévation) of the visual ray from S to A revolved about SA into the plane of the horizon or into the platting plan. From the similar triangles Sa (a) and SA (A) we dérivé the proportional équation:

whence

A (A) : SA = a (a) : Sa

A(A) =

a (a). SA sa

The value found for A (A) measured on the platting scale will give the différence in élévation between caméra station horizon and the point A.

In practical work the élévations of the caméra stations are known, and by adding the heiglit of the instrument including the value A {A) to the élévation of S, fig. 49, the absolute height of A will be found, which, however, is still to be corrected for curvature and refraction.

A second value for the élévation of A may be found in the same manner for another négative •containing the image a (taken from another station), and the mean of several such déterminations is adopted for the final value for the height of A.

(c) Drawing the plan, including horizontal contours.—After some little practice points, pictured on different négatives but representing identical points in nature, will readily be identified by the observer and he will soon be able to pick out the characteristic points to reproduce the water courses, watersheds, roads, canals, etc., on the platting sheet. After these principal guide lines hâve been well located on the chart, buildings, outlines of woods, marshes, etc., are platted, including everything that is to be shown on the finished map.

Enough points should be platted iconometrically to give a good control for a correct délinéation of the relief. When the number of points determined on the plan is sufficient, or if they are favorably located to give an adéquate control only for the délinéation in the horizontal sense, additional points should be platted in order to obtain an equally good control of the terrene in the vertical sense.

The planimetric work completed, élévations of as many of the platted points as seem neces-sary (or additional ones) are determined and inscribed on the chart. Horizontal and équidistant contours may now be drawn, by interpolation, to harmonize with the élévations suffixed on the chart to the points of control, conforming their courses (between the located points) to the configuration of the terrene, as it is shown on the photographs.

It can not be denied that a certain amount of study and practical application are required to enable the draftsman to correctly interpret forms of the terrene, shown in perspective. Yet, it should also be admitted that such translation or conversion of the relief of the terrene into the horizontal map projections may be far more accurately accomplished (at one’s leisure) by means of geometrically correct perspectives, than could be accomplished by sketching in the field. When topographie features are sketched, as seen from one direction, they will frequently be found to hâve been misconceived when they are seen again from another (not anticipated) point of view. Of course, the platted forms may then be corrected in a measure, at least, still, many details are sketched which will not be seen again from other stations, and, even those that are seen again under other conditions may not be modified to conform to their true shapes, unless the original station, whence they were first seen and sketched, could be reoccupied to verify the suggested changes and corrections. Generally speaking, topographers regard a second occupation of a station with little favor, it being considered too great a waste of time, retarding progress, and considerably increasing the cost of the work.

In iconometric platting, however, it is always an easy matter to refer back again to panoramic views obtained from some other station, and the platting of topographie details should not be attempted without having first made a careful study of and a close comparison between the various pictures representing the same features but seen from different points of view.

(2) Method of Dr. A. Meydenbaur.—The pantoscopic lens (made by E. Bush, Rathenow,

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