MAP REPRODUCTION The general heading of reproduction covers the various proc esses of drawing, photography and printing, which come between the survey on the ground and the completed map. There is a stage generally carried out by draughtsmen called compilation, which, however, antedates fair drawing and is equivalent to sur vey. Small scale maps of the less well known parts of the world are made up from material of various sorts and generally of unequal value.
The next stage is fair drawing. The draughtsman's first task is to plot the graticule and margin of his sheet on the chosen projection. He then plots the control points in proper position and begins to incorporate his material. Whether the latter is compilation, or field sheet, it is probable that the photographer will be called upon to provide bromides at the required scale. The next point to decide is the number of colours to employ. If the whole map is drawn on one sheet of paper the photog rapher will have to divide colour from colour during his prepara tion of the zinc plates. If each colour is drawn separately then subsequent photography and zincography is made easier, but the fit of one colour upon another, commonly known as the register, will suffer; for, once embarked upon, the various different draw ings may expand or contract unequally. The procedure chosen is often a mixture of these two principles.
Sometimes the field sheets are assembled and photographed to be printed in blue and to act as a direct key to the drawing of various colours. Blue is chosen because, in the subsequent photography of the completely drawn colour sheet or plate, blue will not appear on the negative, which will record nothing which has not been inked in. This system is handy, and guards against bad register. The use of a complete key of this sort implies that the whole of the material can be collected and photographed into position at one and the same time. In other cases each main
colour is drawn on its own tracing paper, the various plates being compared and examined, over each other, at frequent inter vals, to prevent the clash or overprinting of the different colours. In all such drawing the colours used must be photographically opaque. Where main colours are treated separately each is drawn in black. Where all the colours are drawn together a difference must be made as a guide to subsequent separation but the colours employed need not be those of the final map.
The main photographic process of preparing the printed record from the finished drawing is heliozincography. In this process the original drawing is photographed, and the glass negative is then laid over a sensitized zinc plate. The negative and zinc are held in close contact in a frame from which the air can be pumped. Light penetrates the lines and names left clear on the negative and hardens the surface below. The remainder of the sensitized sur face, protected by the negative, remains soft and can be washed away.
In the heliozincographic process each plate is photographed to the proper dimensions. The great advantage of heliozinc ography lies, however, in the possibilities of touching up or add ing fresh detail actually on the negative. A special staff is employed in large map establishments for this purpose. If all the colours of the final map have been drawn together it is at this stage that separation occurs. As many negatives are made as there are to be main colours. On each negative everything irrele vant to the particular colour in view is duffed out. The glass negatives then become the final records, from which any number of printing plates may be made.
The zinc plates are now given over to the printers. (See LITHOG RAPHY.)