Topographic maps and spot color maps typically use four or more specific colors such as black, brown, blue, and green, which differ from the standard four-color process (CMYK) used in general printing. Due to the unique nature of these spot colors, many digital map publishing tools first convert them into monochrome EPS or PS PostScript files during prepress stages.
Version checking is a crucial step before printing. The traditional method involves printing separate monochrome proofs for each color plate. However, this approach has limitations—it makes it difficult to check alignment, element relationships, overprinting, and accurate color simulation. The main issue is that the layers in EPS vector files (or when converted to grayscale images) are not transparent, and overprints are retained, leading to incorrect color separations. As a result, color inkjet proofing becomes challenging and error-prone, so most workflows rely on monochrome inkjet checks instead.
In practice, we can use Photoshop's spot color channels to convert individual monochrome EPS files into grayscale images. These grayscale images are then combined into a single color PSD file, allowing us to simulate color proofs quickly and efficiently.
**Converting EPS Files to PNG Grayscale Images**
1. **Convert EPS (PS) to PNG Grayscale via GSView**
(1) Set the unit to millimeters in "Options / Units".
(2) Define the image dimensions. For a 1:50,000 scale map, you might set the size to 1600 x 1200 pixels. Use the menu "Media / UserDefined" to input the width and height. The conversion formula is: required value = actual size in mm / 0.35278.
(3) Adjust display settings:
- Resolution: 30 dpi
- Zoom Resolution: 300 dpi
- Depth: Default
- TextAlpha: 1
- Graphics: 4
(4) Print to file: Open the EPS file, go to "File / Print / Print Setup", select "Pnggray" under "Device", set resolution to 500 dpi, choose "Print to File", and save as a PNG grayscale file. Repeat for each version.
2. **Convert EPS (PS) to PNG Grayscale via Acrobat Distiller**
Adobe Acrobat Professional’s Distiller module allows batch conversion of EPS files to PDF. Once converted, open the PDF in Acrobat and save it as a PNG file. You can adjust the resolution during the conversion process as needed.
**Customizing Spot Color Channels in Photoshop and Matching Spot Color Images**
By customizing spot color channels in Photoshop, you can accurately overlay and align spot color images, ensuring proper color representation and visual consistency. This technique helps streamline the prepress workflow and improves the accuracy of color proofs.
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