116 CONVERGENT VERSUS VERTICAL PHOTOGRAPHY, SCHERMERHORN
In addition to the advantages resulting from the geometrical position of convergent
photography, mentioned also by Brucklacher, Thompson, although not giving any figures,
mentions the greater accuracy of reading in models because of more favourable angles
of intersection of corresponding rays.
As a second point he mentions the versatility and efficiency of the ER-55 projector,
developed by the Geological Survey under the leadership of R.K. Dean. The low cost of
this plotting unit of about $ 5,000 complete, counts favourably if we consider that the
ER-55 Projector is easily adaptable for use with vertical, convergent, transverse or
high oblique photography. Thompson mentions “that the instrument is capable of standard
accuracy plotting under C-factor conditions closely approaching current practice with
the most expensive instruments”. He furthermore explains the economy of the system
by means of “the solution of the horizontal aerial triangulation problem by the stereo-
templet system developed by Mr. M. B. Scher of the Geological Survey”.
At the end of his note Mr. Thompson mentions also the use of the super wide-angle
camera. The Geological Survey purchased two of the super wide-angle cameras at the
first moment they became available. Thompson says:
“During the past interim of twelve years from 1948 to 1960 we have made an effort
to exploit the advantages of increased base height ratio by the only means available:
convergent photography. It may well be said that if the use of convergent photography
had accomplished nothing else, it would have been worthwhile for its role in spurring
the development of super wide-angle cameras. Immediately upon receipt of the new
super wide-angle cameras the G.S. initiated a programme to test their performance
with very satisfactory results. There remain the technical and economic problems of
securing competible instrumentation for plotting and related operations".
It is obvious that such disadvantages count in particular in great services which
have at present an instrumentation adapted to one particular type of camera, which res-
titution machines cannot take immediately super wide-angle photographs. It must be said,
however, that this problem may be solved at present along different and equally satis-
factory lines. Thompson mentions as a potential disadvantage of the super wide-angle
the small negative scale, offered by the 88 mm focal length. He will understand, however,
that this trouble can be eliminated by choosing a lower flying height than applied at
present with the 6 inch convergent cameras. One remark at the end of the memo, that
the use of convergent photography unquestionably demands better calibration of cameras
and instruments and more care in many phases of the mapping operations, must be con-
sidered seriously. He mentions, however, that in the U.S.G.S. these problems have es-
sentially been overcome and experience shows the value of the system.
The difficulty in comparing the systems can be illustrated in considering the cost of
investment. Thompson mentions the cheap plotting equipment for medium-scale topo-
graphie work, although he also has to add equipment for vertical triangulation and the
cost of a convergent camera, plus the cost for the equipment necessary for accurate
calibration of cameras and projectors, which are in this case more necessary than in that
of vertical photography, for the same type of map production.
Kasper considers this problem from another angle, viz. from investment of a photo-
grammetric office, in particular for map production in a scale between 1 : 500 and
1 :10,000 which has available $ 200,000. He distinguishes between two cases: a. for con-
vergent with standard angle camera and with a vertical wide-angle and b. with only
vertical photography. He comes then to the following proposition :
