Back in the Day Part II: CMYK “flats” and Printing Maps

Four-Color Printing
Four-color printing, also known as “four color process” using CMYK, is a conventional color model for printing, similar to RGB in the digital universe. When cyan, magenta, yellow and black blend together, they create a wide range of tones and hues that you and I interpret as a full spectrum color image.

Welcome back! In my last entry, Back in the Day Part I: Making Paper Maps from Scratch, I barely scratched the surface about how printed maps come together. I talked about scribing roads by hand and creating a duplicate negative image from that artwork. Why a negative image? Well let’s take a step back from the actual content of the map and talk a bit about how the printing process actually works.

Most materials you see printed on paper come from a negative image ­— newspapers, magazines, baseball tickets, paper money, all of it. Printed on paper from some master source that happens to be upside-down and backwards, usually a plate that has been “burned” in a vacuum frame. Some images are black and white, some two-color (black and white and one color) and some four-color, also known as “four color process” using CMYK — a conventional color model for printing, similar to RGB in the digital universe.

So what is CMYK? Sounds like a European hockey team doesn’t it? CMYK stands for Cyan (Cyan or blue, it actually resembles more of a turquoise than anything), Magenta (“process red” that looks more like hot pink), Yellow (enough said), and Key (really, it’s black, but the old timers refer to it as “Key” because the other color plates were registered, or “keyed,” to the black plate during the printing process). When blended together, these four colors create a wide range of tones and hues that you and I interpret as a full spectrum color image.

Four-Color Map Printing
When printing in CMYK, four sets of negatives are required. CMYK stands for Cyan, Magenta, Yellow, and Key (or black). When blended together, these four colors create a wide range of tones and hues that you and I interpret as a full spectrum color image.

When printing a map in CMYK, four sets of negatives are required, organized by color. We call these negatives “flats.”  For example, a set of Cyan flats would contain features that appear blue on a map, such as open water and hydrology. Cyan flats will also contain tones that contribute to compound colors, such as greens and purples. The same principle applies to  magenta, yellow and black flats. We can think of these flats as being similar to layers in digital mapmaking. Each layer adds details to the map, in this case, the flats are adding color. Often times we will have five or six flats for one compound color.

In order to achieve the correct color tone, screens need to be applied to certain map features that we don’t want to print at 100% strength. When we print open water, for example, we use a 10% screen so that when the map gets printed from our open water negative, only 10% of the cyan will print on the paper, resulting in a light blue tone. These screens are measured by percentage and would be merged with other objects in composite form.[c4]

When all of the flats of each color have been composited (burned) on their respective plates (there should be four, right? Cyan, Magenta, Yellow and Black — see?  You’re catching on) The printers take these plates and register them on the press and start printing the “signatures.” A signature, or sig, is basically a printed sheet (both sides) that contains multiple pages. These sheets are then folded in a certain way so that the pages appear in sequence, like a book.  When I was a map technician, each atlas had a good number of signatures that were printed in order (1 through 12 for example), but keep in mind, the total signatures in the job reflected how big the atlas was. Alaska and Texas had over 30 signatures while Maine had only 12 sigs, for example.

After the signatures are printed by all four plates (CMYK), they are then sent along to the bindery where the sigs are trimmed to become one uniform size, then collated and bound into books that you and I recognize.

It’s fair to say that my bosses at the publishing company didn’t trust printers. Whenever we sent atlases for printing, we would order 30,000 or 50,000 books at a time, which, as you can imagine, was an expensive investment. We as publishers, also had to purchase our own paper. so there was no going back if a job got botched. Too many times books would come back with inconsistent blues, reds, greens, you name it.

In order to combat this problem, the map technicians would go on “press checks”, meaning we would QA/QC each signature after the plates were hung and the printing started. If the book had 36 signatures, that meant we did 36 checks. If we were printing 30,000 books, it would take 3-4 hours to print a signature. Every three hours we would be taken into the pressman’s area, shown a printed signature, and sign-off on it before they were given the OK to continue printing. This is what we did every three hours, non-stop, until the job was done. Overnight checks were brutal, and yes sometimes this would go on for days. Plenty of Mountain Dew and Diet Coke, let me tell you.

The golden rule for QA/QC was “CRC”.  COLOR, REGISTRATION, CONTENT.

So after our map  is printed, the books hit the shelves and they start selling like hotcakes. All according to plan, life is good. Then the phone rings in the Revisions Dept., and there’s someone who’s not too happy that their private driveway ended up on page 34.


Kris Berglund

Kris Berglund is currently the Vice-President (Sales) at Blue Marble Geographics and has been with the company for over fifteen years. Kris has been involved with digital mapping technology for over twenty years, and demonstrates a diverse level of experience in cartography, geomatics, technical sales & marketing and business development.

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