In Part VI: Objects vs. Fields (and Vector vs. Raster), we mentioned that the export format's data model can have an impact upon the map's appearance. In this part, we address the issue of print and/or display resolution in the context of both vector and raster graphics.
The appearance of raster graphics is highly dependent upon the resolution at which they are exported because they produce a pixel-by-pixel rendition of the image, and a higher resolution implies that the image has more pixels. One consequence of this resolution dependence is that text and linework can look jagged if the image is exported at a coarse resolution. Exporting the image at a higher resolution will improve the appearance of these features, but this improvement comes at a price: much larger file sizes (see Figure 1.7.1). Some raster graphics formats use compression algorithms to reduce file sizes, such as JPEG, an image format that is commonly used on the Web. JPEG uses a lossy compression algorithm, which means that some of the data are lost during the compression process (i.e., if you imported the JPEG into a GIS, you would not be able to recover all of the original data). The resolution at which you choose to export your map will be influenced by the context in which that the map will be viewed. If you are going to electronically disseminate your map (e.g., on the Internet or via e-mail), file size will be very important, so you may choose to sacrifice some image quality for the smaller file size or to use a format that employs lossy compression (you will want to make sure that all text and fine linework are legible at this lower resolution). However, if you are sending your map to the printer, you will want to give them a file with the best possible image quality — that 12,000 dpi print quality from the offset printer will go to waste on a map that has been exported at 300 dpi!
Vector graphics, on the other hand, are resolution-independent. In other words, you can enlarge or shrink the graphic objects without losing any detail or clarity. This is an advantage if your map contains lots of type or linework that needs to look sharp. Vector graphics files sizes are generally much smaller than raster images. However, if your map integrates both raster and vector data (e.g., a satellite image overlaid with roads), exporting it in some vector formats may cause the raster data to look pixellated. One solution to this problem is to use a format that can represent both vector and raster data, such as the Adobe PDF format. This format has the added advantage (compared with many other vector graphics file formats) that it can be easily viewed over the Internet.