G. R. Putnam

Methods of publication. An ideal process of publication for nautical charts would include the following features; rapidity in getting out new charts, facility in reprinting and correcting existing charts, clearness and sharpness of print, durability of paper and print, and correctness of scale. It is difficult to fulfill all these requirements by any method as yet developed. In the Coast and Geodetic Survey several different processes are in use at present; charts are engraved on copper and printed directly from the copper plate, or they are transferred from the copper plate to stone and printed from the stone, or a finished drawing is made and transferred to stone by photolithography and printed from the stone, or an etching is made on copper from a finished drawing and printed from a transfer to stone. Charts in other countries are in large part printed from engraved plates, excepting some preliminary charts by lithography.

FIG. 27. ENGRAVING A CHART ON A COPPER PLATE.

FIG. 28. ENGRAVING SOUNDINGS ON A COPPER PLATE WITH A MACHINE.

Copper plate engraving and printing have long been used in chart preparation. A drawing is prepared as a guide for the engraver; this must be correct as to all information to be shown but need not be a finished drawing. A true projection is ruled upon a copper plate. By photography a matrix is made from the drawing and a wax impression taken from this matrix. This is then laid down on the copper to fit the projection, and the impression is chemically fixed on to the copper. The work thus marked out is engraved by hand or by machine. A high degree of skill is required in the accuracy and finish necessary for chart engraving. Machines have been invented in recent years which can be used for portions of the work on copper plates, as for instance for cutting the sounding figures, the bottom characteristics, the border and projection lines, border divisions, compasses, line ruling, and stipple ruling. Stamps and dies have been successfully used for some symbols and notes, and roulettes for shading. By means of these various machines, many of which are American inventions, the process of chart publication from plates has been materially facilitated.

FIG. 29. ELECTROTYPING PLANT FOR ELECTROTYPING CHART PLATES.

When the plate is completed an alto, or raised copy, is made by depositing copper on to it in an electrotype vat, and from this alto another basso or sunken copy is made by the same process. This latter basso is used in printing. A copper plate may be used for about 3000 impressions, after which it may become too much worn for satisfactory chart printing. By printing from a duplicate basso the original plate is preserved and additional copies can be made when needed. The use of the alto also greatly facilitates matters when a considerable correction to the chart is required. All the portions of the chart to be changed can be scraped off the alto, and when a new basso is electrotyped from this scraped alto all such areas will of course appear as smooth copper, on which the new work can be engraved. Numerous small corrections are called for on charts, and on copper plates where these are to replace old work the latter is removed either by hammering up the back of the plate or by scraping its face.

Printing directly from plates is a laborious process. After the press bed has been carefully padded to take up inequalities in the plate, the surface of the latter is covered with ink and then carefully wiped off by hand, leaving the ink only in the engraved lines. The paper, first dampened, is laid on the plate, and passes with it beneath the cylinder of the press under considerable pressure. The prints are calendered by being placed in a hydraulic press under 600 tons pressure. The charts are beautifully clear and sharp, not equalled by other methods of printing. Owing to the wetting and drying of the paper, the finished print is, however, quite appreciably smaller in scale than the plate, and the shrinkage is greater in one direction than in the other. The average day's work for one press and two men is 75 prints. This is small compared with the output practicable with lithographic presses. On the other hand a plate can be prepared for printing more readily than a lithographic stone. For small editions the plate printing compares well in economy with lithographic printing, and the plate can also be printed on short notice. Because of changes in aids to navigation and other corrections, it is usually desirable to print at one time only a sufficient number of copies of a chart to meet current demands, and not to carry a large stock on hand.

FIG. 30. PRINTING CHARTS FROM COPPER PLATES; FINAL CLEANING OF THE PLATE BY HAND; PLATE PRESS ON THE LEFT.

The copper plates, bassos, and altos make a very convenient and enduring means of preserving the chart ready for printing or for further correction. A large number of plates can be placed in a small space, and if properly cared for they may be stored indefinitely without deterioration.

With plate printing it is not practicable to print more than one impression on the chart or to use more than one color, and plate-printed charts are therefore in black only.

Engraving on stone. On the United States Lake Survey the charts are first engraved on stone, and by a special process the work is then transferred to small copper plates, which are preserved. The final publication is by lithography, transferring again from the plates to stone.

Photolithography is a quick method of publishing a chart. It would be practicable by this means to reproduce the original survey sheets, but ordinarily these are not suitable as to scale and legibility, and it is necessary to make a new drawing, usually on tracing vellum. This is photographed on to glass plates, on the scale of the proposed chart. From these glass negatives positive prints are made on sensitized lithographic paper. These prints are fitted together and then inked, taking the ink only where the lines appear. This transfer print is then laid face down on the lithographic stone and run through a press under pressure, the stone absorbing the ink from the paper. The stone is then treated so that the inked portion remains slightly raised, and from this stone an indefinite number of charts can be printed in a lithographic press at the rate of 1000 an hour. The paper is not moistened, and consequently there is little distortion or change of scale in prints from stone. If desired to shade the land or use another color for any other purpose, additional impressions can be made on the same charts from other stones. Because of the bulk of the stones, work cannot ordinarily be retained on them, but the chart is cleaned off and the stones repeatedly used until worn thin. The original drawing as well as the negatives is preserved, from which the chart can again be published. For republication, the process is, however, not entirely satisfactory; the negatives are not always permanent, the work must again be assembled and transferred to the stone, changes or corrections are not very conveniently made on either drawing or negative, and after repeated changes the drawing becomes difficult to use in photolithography. Whether the charts are actually printed from copper or stone, there are decided advantages therefore in the matter of correction work and future editions in having the charts engraved on copper. On the other hand, the advantages of the photolithographic process are the ability to publish new drawings promptly, to use more than one shade on a chart, to obtain prints with little change of scale or distortion, and to print large editions rapidly.

Lithographic printing by transfer from engraved plates. An impression on transfer paper may be taken from an engraved plate and this laid down on the stone in a manner similar to that used in laying down the prints from the glass negatives in photolithography. Prints are then made from the stone the same as in photolithography, but with superior results as to clearness. This general process is extensively used in both map and chart publishing in this country, as it combines the advantages of the plate in preservation of the chart record and facility of correction, and the advantages of the lithographic printing in less distortion of the printed chart, ability to print more than one shade, and facility for large editions. As the transfer from the plate can be readily made it is also better applicable to small editions than is photolithography. It is, however, not as convenient in the latter respect as plate printing, and it does not give a resulting impression equal in clearness or durability to the impression directly from the plate.

Etching on copper for chart publication has been recently developed in the Coast and Geodetic Survey. A finished tracing is made, the surface of a smooth copper plate is sensitized, and by exposure to the sun a print is made on the sensitized surface. It is essential to use an air-exhausted printing frame so as to get good contact between the vellum and the plate. The work is then etched into the copper and the plate cleaned and touched up, after which it may be used the same as a hand-engraved plate, either for transfer to stone or direct plate printing. The expense and time required in the etching process are much less than for hand engraving. The process has been successfully used for a number of harbor charts. The etching of course will be of the same scale as the vellum at the time of the print, and vellum varies somewhat in scale with weather conditions and age. Unless overcome by the substitution of some more invariable material in place of vellum, this might be an obstacle to the use of the process for general charts where a true scale on the copper plate is desirable because of future work to be done on the plate. It must also be taken into account that the etching requires a finished tracing in ink, which is not essential for the hand engraver; if, however, the chart is first published by photolithography, as is the usual practice in the Coast and Geodetic Survey, the same tracing is used for both processes.

Distribution of charts. Charts published by the government are sold to the public at a small price, estimated to cover the cost of paper and printing. The charts may be obtained direct from the publishing office or from the chart agents who are to be found in all the principal seaports. Catalogues are published from time to time giving complete lists of the current charts and the main facts regarding them. Index maps show graphically the area covered by each chart. The notices to mariners contain announcement of new charts or new editions published and of charts or editions cancelled, as well as of all corrections.