GREAT BRITAIN 2.
a6, and the Zeiss Rectifier,
The 1/1250 survey is based on minor control provided
by ground survey, sufficient to control each overlap.
The analytical method of aerial triangulation, which was
developed successfully for surveys at 1/2500 (1, 11), has
not yet proved sufficiently accurate for the 1/1250. The
bulk of the plotting is still done graphically, but
increasing use is being made of precision plotters.
Contouring is done both graphically and by plotter, based
on map and ground control. Tne contours are added to
planimetry already furnished at the 1/10,560 scale.
It is intended to survey the Scottish Highlands by
plotter, based on an aerial triangulation tied to the
National triangulation network (average side length 7
miles), with additional ground height control.
Increasing use is being made of electronic computing
in reducing the stereocomparator observations.
The Directorate of Colonial Surveys has continued, as
its main task, to produce 1/50,000 scale maps of relatively
undeveloped territories. More developed areas have been
mapped at 1/25,000 and 1/10,000. In the ten years of the
Directorate's existence, over 1 million square miles have
been photographed (8) and some 400,000 square miles have
been mapped. About 2-g- million air photo prints have been
supplied to Colonial Survey Departments, where a steadily
increasing use is being made of photogrammetric methods.
Equipment includes Williamson-Ross Multiplex, Wild A5>
Zeiss C6, Hilger and Yatts Radial Line Plotter, and the
Grant Projector.
For 1/50,000 mapping, planimetry is provided by
slotted template followed by graphical plotting (the radial
line plotter has only recently been introduced), and
contours by multiplex bridging between controls at every
eighth overlap, graphical adjustment of bridges, and
contour plotting in 3-projector units. Larger scale
mapping is done in the A5 and c6, air triangulation
observations being adjusted by computation for planimetry
(16) and by graphical methods for elevation. Undeveloped
