What follows is a short overview of some simple techniques for viewing, manipulating, editing and changing images on the Internet. For most of these actions no specific software needs to be purchased. Several viewers and types of image processing software are freely available on the Internet. The examples below are based on the use of Netscape 4 (not Internet Explorer).
Many image formats are supported by browsers. However, TIFF is not (yet) supported by web-browsers, only by way of plug-in software. In most cases, a TIFF file needs to be converted first into JPEG or GIF. To open any image from a hard disc or CD-ROM in Netscape in, for instance, JPEG, GIF etc. format: click in Netscape on File - Open Page, then Choose File from directory on hard disc/CD-ROM/disc.
Let us assume you want to compare two (or more) map images found in different digital sources. This could, for instance, be one image in a website, and another image on your hard disc or CD-ROM, or two images from different websites (URL's). Open one image in the Netscape window and resize this window to half its width. Then, open a new Netscape window (File - New) and open the second image in this window and resize it accordingly until the two windows fill the screen. By opening more windows and resizing them, more images can be seen simultaneously, though they may not be on the same scale.
Comparing in more advanced ways is possible with most of the viewers decribed below, once you have downloaded the relevant images.
Point your mouse at the image you want to download, click the right mouse button, and select Save Image As... Then specify a name and directory on your (hard) disc where the image will be saved. After saving, this image can be viewed in a viewer or image processing program.
Viewers and image processing software
Any image processing software can be used to view images. Most computer systems, such as Microsoft, supply simple viewers and image processing software as standard. The programs described below are only a few examples of the many software programs available for viewing and editing images.
ACDSee is freely available and recommended for its ease of use.
In ACDSee, several windows can be opened simultaneously, allowing you to compare images (perhaps of different formats), pan, and change the zoom-factor for each window. The program has an interesting feature for showing self-demos (which you can use as a screensaver as well), and for making a gallery of thumbnails, which allows you to see all images in one directory.
MrSID, whose stand-alone viewer can be downloaded free of charge.
However this viewer can only handle .sid files, i.e. files that are compressed by their (priced) compression program. Both the Library of Congress and the Koninklijke Bibliotheek (the National Library of the Netherlands) provide maps on their Websites in the .sid format. These can be downloaded and viewed with the (free) stand-alone MrSID viewer. This viewer has more functionalities than the www-version. Dynamic panning and zooming allow you to compare map details. Printing is also much easier. Using this viewer, files can be saved as TIFF. Other viewers or image processing programs cannot display .sid images.
Thumbs Plus, which offers a free evaluation version (4.01). With ThumbsPlus it is easy to view, organize and edit images.
IMAGE PROCESSING SOFTWARE
PaintShopPro, where you can download a free 30-day trial program (or longer). PSP allows you to view, edit and manipulate, and save images into different formats. It is a commonly used, good piece of software.
Adobe PhotoShop 4/5
This is quite an expensive program which is used by professionals. It has extensive functionalities for zooming and manipulating images. As a result, it is not easy to learn all its options.