USGS

Digital Mapping Techniques '99 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 99-386

Offset Printing of Raster Image Files

By Elizabeth V. M. Campbell1 and Robert B. Fraser2

1Virginia Division of Mineral Resources
P.O. Box 3667
Charlottesville, VA 22903
Telephone: (804) 951-6343
Fax: (804) 951-6366
e-mail: ecampbell@geology.state.va.us

2U.S. Geological Survey
National Center MS 903
Reston, VA 22092

The Virginia Division of Mineral Resources has an ongoing commitment to vectorizing both new and previously published geologic maps in Virginia. Our goal is to provide not only paper maps but also GIS layers. We have digitized over half of our published maps and several previously unpublished maps. In getting to this point, we have had to deal with many issues. This paper will discuss one of these issues, specifically the use of offset printing to produce the required initial quantity of a map.

OVERVIEW OF HISTORY AND METHODOLOGY

The Virginia Division of Mineral Resources (VDMR) became interested several years ago in creating digital geologic GIS layers and in reducing publication costs by using digital production methods. It was important to avoid redundancy of effort by digitizing a map only once while meeting both goals. When none of the existing software met our needs, the Virginia Division of Mineral Resources helped to develop customized software.

The software is MS-DOS-based and runs on MS Windows systems. Data entry involves "heads-up" tracing of lines, creation of polygons, attribution of lines and areas, and entry of point data (strike and dip, well locations, etc.) over scanned images. Undergraduate geology majors have done the bulk of this work. DMR staff does the subsequent editing. All digital maps undergo the same detailed scrutiny as used in final editing of any traditionally produced map. The time required to digitize existing geologic maps varies with complexity and scale of the original map ranging from 15 days for a 1:24,000 quadrangle to 60 days for a complete 1:100,000 map.

We create a raster image of the geologic map from vector files that replicate the original published (paper) map in all respects. Geology, in color, appears over a subdued topographic/culture base. The raster image can be distributed on a CD-ROM, via the internet, or plotted on our HP Design Jet 2500 CP. Publishing a geologic map image on a Web page or CD-ROM has the advantages of quick distribution and low storage costs. However, many clients still want a paper copy and either do not have the ability to print in color or want the entire map printed. Plotters allow printing of high-resolution large paper maps and allow an agency to avoid storage costs for low demand maps.

OFFSET PRINTING TO MEET INITIAL QUANTITY REQUIREMENTS

In Virginia, the Division of Mineral Resources is required to supply 45 copies of a newly published map to various library collections. After the initial sales of another 40 to 50 copies, the demand then drops to one copy per month or less. We have found it to be impractical to print the initial 80 to 100 copies using an inkjet printer. Our alternative is to establish conventional offset printing as a means of meeting the initial demand, since it is still the most economic way to produce larger quantities of a map.

In a cooperative venture with the USGS, a standard output raster image of a geologic map was sent to the USGS printing shop. Adobe PhotoShop was used to convert the raster file of the finished map from RGB (red, green, and blue) to CMYK (cyan, magenta, yellow, and black), for ink trapping enhancements and color separation. The CMYK plate-making negatives were written on a large-format film writer. Traditional four color printing methods were then used. Experience shows that the process from color separation through printing requires fewer than eight man-hours, including time on the press, and that the quality of the resultant print is as good as that produced by traditional manual cartographic methods.

Costs

In the past, we paid $6,000 - $25,000 for 700 copies of a 1:24,000 quadrangle map depending on the map complexity and paper. We printed 700 copies partly because we divided the production cost by the number of copies produced in order to derive a selling price. Printing a larger number did not increase the printing costs as much as it decreased the final selling price for the map.

However, eighty percent of the production cost of printing a color geologic map was the cost of scribing and making peel coats. By using a raster image, we only have to pay the actual cost of printing. Therefore, the number of copies needed to have a reasonable per copy selling price is lower. The minimum number of copies printed depends on the type of printing press, the cost of setting up a print run and the type of paper.

In addition to lower production costs, we save time because we do not have to make line weight and color guides, nor create mylars for structure and formation symbols. Another advantage is that we can proof the map prior to sending it to the press because we are able to see how the text, symbols, line weights, and patterns will look on the printed map.

Problems Creating a Raster Image for Offset Printing

PPI
Some issues associated with offset printing need to be addressed. Two of the most notable are resolution of the digital image (pixels per inch or PPI) and color shifts. When determining PPI for a raster image, the question usually comes down to resolution versus file size. The higher the resolution, the more clear and the less blocky the detail. If the image is to be published digitally or even printed on an inkjet plotter, the only limitation is the size of the image file, which can rapidly become unmanageable as the PPI increases. However, for an image going to an offset printing press, there is another consideration. At higher PPI's too much ink is put on the paper causing the map to be dark. The general rule of printing is the PPI should be two times the lines per inch (LPI). LPI changes depending on the paper and the press. LPI generally ranges between 72 and 150. We have gotten good results with image resolutions of 250 and 300 PPI. We want to create only one final image file for each map that can be published digitally or printed on inkjets or run through an offset printing press. For this reason, we settled on an image resolution of 300 PPI, which preserves sufficient detail, produces a file of manageable size and produces a good-quality print (though a little dark).

Color Shifts
We are in the process of resolving the issue of color shifts. The problem is that computer monitors create images by using red, green and blue (RGB) light. Inkjet plotters/presses generally use cyan, magenta, yellow and black (CMYK) ink. Some colors that can be created using RGB cannot be duplicated using CMYK. These colors are said to be "out-of-gamut." Out-of-gamut colors look muddy when printed because the position in color space has been shifted slightly to bring it within the CMYK gamut or range of colors. It is difficult to predict the appearance of a printed map that was created in RGB.

One solution is to use "Spot" colors in the printing process. Spot colors are specific inks created to produce a specific color not in the CMYK gamut. Since the color produced is not the result of a combination of four colors but rather one ink with a defined composition, the color is always the same. The classic use of spot colors is in the reproduction of logo colors like "3M blue" where it is important to the company that the color is the same every time it is used. On geologic maps, red is perhaps one of the more important colors that is difficult to reproduce in CMYK. Red spot ink is used sometimes as a fifth color in CMYK printing to obtain a true red. Using spot colors can be very expensive since an individual mask plate is necessary for every color.

Another solution is to only use colors that are in the "common gamut". The common gamut is that range of colors common to both RGB and CMYK; therefore, no shift is necessary. Some programs display a gamut warning and offer substitutes. It can be time consuming to go through every "out-of-gamut" color on a map and find a substitute particularly if the image does not use a color palette. Furthermore, RIPing programs that translate an image into plotter language can cause color shifts. This is most noticeable in the yellow, which frequently develops greenish tones. It necessary to adjust the program's gamma curve. Not all programs allow the user to do this.

The issue of color shifts is further complicated by the unreliability of using an inkjet plotter to proof colors for offset printing. Most inkjets use dye inks while offset printing presses use pigmented inks. Dye inks are water-soluble with the color in solution tinting the water. The ink is somewhat translucent and is susceptible to fading upon exposure to UV light. Pigmented inks, on the other hand, are oil-based and the color is produced by tiny particles of pigment suspended in the ink. Pigmented inks are more resistant to fading and are more opaque than dye inks. Pigmented ink is available for the HP DesignJet 2000 series printers. We have noticed that blues in particular are different when printed in dye ink and pigmented ink.

While it is possible to buy pigmented ink for inkjet printers and calibrate a monitor and a RIPing program to approximate the CMYK press colors, it is still only an approximation which must be constantly maintained because ambient light, warmth of the cathode tubes and a myriad of other things affect our perception of color. Because of the difficulty of proofing offset printing colors ahead of time, many people who are only interested in the printed result use one of several specific color systems, for example TruMatch® and Pantone®. Each of these systems has a swatch book printed on an offset printing press under specific conditions. These swatch books allow you to see how a color will look when printed. Once you select a color from the swatch book, you either enter the code for the color or the CYMK values for the color. The major image processing programs like Adobe Photoshop, CorelDraw, and Canvas support these two systems.

The Division of Mineral Resources has not reached a definitive solution to the color problem. We will probably use a combination of the solutions mentioned above to develop a chart of defined colors to be combined with patterns for various formations in Virginia. Our goal is to produce one final raster image for each map where the image will look the same regardless of how it is displayed or printed. In order to discern different formations within the same age, colors need to maintain their distinctiveness when they are printed. We are working with the USGS printing office on this problem. Together, we have created two TIF files: a RGB color chart and a CMYK color chart. The USGS is planning to print the CMYK color chart on their offset printing press. It is possible to send the color chart TIF files through various combinations of RIPing programs to various printers. Using these charts and perhaps the TruMatch swatch book and color system, we will create a standard set of colors for each unit following the international stratigraphic color scheme.

CONCLUSION

The Virginia Division of Mineral Resources has successfully created an acceptable offset-printed map from a 300 PPI raster image file. Sending digital image files to an offset printing press enables us to economically print the initial number of copies required to release a newly published map with significant savings in both time and money. We are planning to use this procedure for offset printing of digital files to publish all new maps.

ACKNOWLEDGEMENT

We would like to acknowledge Lawrence G. Matheson of the Map Application Center, National Mapping Division, U.S. Geological Survey for his efforts in processing the image files through the pre-press procedures and for his guidance in color management/separation.

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