CHAPTER IY.

PHOTOGRAMMETERS.

The praetical value of a photogrammeter (photographie surveying instrument) dépends greatly upon the quality and general uniformity of its lens or lenses, upon the rigidity of the eomponent parts of the apparatus, its easy transportability, and on the rapidity with which it may be put into adjustment.

A good phototopographic lens should be free from spherical aberration (or diffusion of the light rays) ; it should possess no chromatic aberration, nor should the image show distortion of any kind, and the field of view (the range of lens) should be large, rapidity of the lens being désirable, but less important than the other requirements just mentioned.

The principal lenses in use for phototopographic purposes are : Dallmeyer’s rapid rectilinear, SteinheiVs aplanat, Bush1 s pantoscopic, Gorz’s double anastigmat, and, more reeently, Zeissh anas-tigmat lens.

The nodal points, the focal length, arc of visibility, and the arc which is perfectly free from distortion of every kind should be known for every lens used for phototopographic purposes, and the manufacturers of ail good lenses are best fltted to détermine those values with great précision for every lens.

I. REQUIREMENTS TO BE FULFILLED BY A TOPOGRAPHIC SURVEYING CAMERA.

A good surveying caméra or photogrammeter for topographie work should produce négatives which are geometrically true perspectives the éléments of which should be known, and the follow-ing desiderata should be fulfilled :

First. The plates to be exposed should be adjustable into vertical plane.

Second. The distance between image point and sensitive plate should be maintained unchanged for ail plates.

Third. This distance—the constant focal length—should be known or will hâve to be determined for every instrument.

Fourth. Means should be provided to trace or locate the horizon line upon every négative or print.

Fifth. Means should be provided for locating the principal point upon every négative.

Sixth. A ready orientation of the photographs (the picture traces) for iconometric platting should be provided for; and we may add as

Seventh. Enough characteristic stations (besides the triangulation points needed for the instrumental control) are to be occupied with the surveying caméra to give a full development of the terrene, which is to be mapped.

Until reeently photographie surveying instruments were not procurable in open market. Nearly every observer who made praetical application of the photographie methods for topographie surveys had an apparatus constructed for his particular need and according to his individual îdeas.

In the following we will describe such photogrammeters as may be regarded as spécial types, constructed to fulfill different requirements.

II. ORD1NARY CAMERAS ADAPTED FOR SURVEYING PURPOSES.

These caméras are generally supported by three leveling screws, and they are provided with a cireular level, or with two cross levels, for adjusting the sensitive plate into vertical plane. The distance between lens and sensitive plate (focal distance) may be made invariable by means of

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