As you may recall from Part VI: Objects vs. Fields (and Vector vs. Raster), we mentioned that map output formats include both raster and vector files. Here, we introduce you to some common export file formats.
EMF (Enhanced Metafile): This format is one of the native file formats used by Windows for copy-paste operations. For this reason, it's a good format to choose if your map needs to be imported into a Microsoft application. However, because it's a Windows file format, it will cause problems for people who work with Macs. One problem you may find with this format is font substitution: if the computer on which you view the image does not have the font that was is in the file, the computer will substitute another font.
AI (Adobe Illustrator): This format is the native format for the Adobe Illustrator software package and is a useful export format for times when you need to carry out additional pre-press operations that your GIS software cannot perform (e.g,. exchanging TrueType fonts with PostScript fonts).
EPS (Encapsulated Postscript): This is a common publishing industry format (also created by Adobe) and may be the only format your pre-press shop or printer can deal with. The quality of this file format is so high that it is quite common for EPS files to be imaged to film or the printing plate. The data in this format are actually stored as a series of PostScript printing language commands that the PostScript device then interprets. This format, like PDF can deal with both vector and raster image formats, but will not perform well (if at all) on a non-PostScript printer.
PDF (Portable Document Format): PDF files are built on the same technology as EPS files (the PostScript printing language commands). Although they do not offer all of the features that are needed for offset printing and that are included in the EPS format, they still offer very good print quality (quite adequate for laserjet or inkjet printers), and they can be distributed and viewed directly over the Internet. This format is a very good option for when you need a relatively small number of copies of your map and do not want to spend the money to get them professionally printed.
TIFF (Tagged Image File Format): TIFF is a high-quality raster graphics format that is commonly used in the printing industry. It provides support for color in both print and electronic media (i.e., in the CMYK and RGB color models) and uses a lossless compression algorithm (i.e., you can recover all of the data after uncompression). One drawback to this format is its relatively large file size (compared with the highly compressed raster image formats commonly used on the Internet).
JPEG (Joint Photographic Experts Group): This format was specifically designed for storing photographs and achieves substantial file size compression (up to 25:1) by permanently removing some of the data from the image (i.e., it uses a lossy compression algorithm). It is best used on continuous tone images (i.e., images that don't have large areas of flat color), because it is easier to trick your eyes into not noticing the data that were thrown away in a continuous tone image. The visual quality of a JPEG can be affected by the 'quality' setting that was used when the image was exported. Choosing medium rather than maximum for this setting can result in the introduction of speckled artifacts in areas of flat color (see Figure 1.8.1).
GIF (Graphics Interchange Format): The GIF format is best suited for displaying images with large areas of flat color (which is often the case with maps). Like TIFF, this format uses a lossless compression algorithm, but because GIF images are limited to 256 colors, GIF images generally achieve a much higher level of compression than TIFF images. This may be a better choice than JPEG for displaying many maps on the Internet. One other Internet advantage that this file format has is that it supports interlacing. With interlacing, the image is not drawn sequentially from top to bottom, but by drawing the even lines first and then the odd rows. This allows the viewer to interpret (at least some of) the image before it is fully drawn, which may be important for Internet users with slow connections.
PNG (Portable Network Graphics): Pronounced "ping", these files also use lossless compression, and were made to be a royalty-free web graphic format (after the owners of GIF required software that made GIFs to pay a royalty). They also can be interlaced for web pages, making a low-resolution image appear immediately and sharpen into a higher resolution image. Another advantage of PNG files is they can support 24-bit color (millions of colors) and transparency.
BMP (Bitmap): The BMP format is one of the oldest raster image formats. Before the advent of color monitors, the file format contained a grid of pixels that had one of two values: on or off (e.g., black or white). With the advent of high color depths, such as 24 bit color, file sizes have grown greatly. Although the BMP format itself is not compressed, graphics programs provide support for simple compression algorithms. One limitation of this format is that it only provides support for color in electronic media (RGB).
A final topic related to raster file formats that is worth mentioning is anti-aliasing. Anti-aliasing is a way of making curves (whether they are a part of text or other linework) look smoother in raster images. The method does this by substituting shades of gray in some areas of the curve (see Figure 1.8.2). One drawback to using this method is that anti-aliased images will look fuzzy when they are printed.
If you are interested in investigating this subject further, I recommend the following:
- Rado, D. and McGhie, J. (2003). "Choosing graphics formats."