Simply having alternative text to your images is not enough. The saying that a picture is worth a thousand words really is accurate, and you need to keep the purpose of placing an image in your page in mind.
Your alt tag doesn't need to describe everything in the image; it would be nearly impossibly to accurately depict a picture for the unsighted. Your tag merely needs to convey the information that you intended the image to convey. A good test to determine if a text equivalent is useful is to imagine reading the document over the telephone. What would you say upon encountering this image to make the page comprehensible to the listener?
If I were to place an image of a map on my page, I could be doing it for any number of reasons. On a map of Lake Winnebago, your alt tag will be different if you intend to show where Oshkosh lies on the lake than if you intend to show how big the lake is.
Using the same map as before, assume we are placing the image to illustrate directions to my house.
alt="map"
In this example, the alt tag is "map". While this is technically an alternate representation, it does not adequately describe the content of the image, nor does it convey the information intended in the image.
alt="map from my house to where I work"
This alt tag is a bit more descriptive, and tells the user what the image is about, but does it tell every thing he needs to know? You need to consider the reasons behind including an image. If an image is just a decorative blue square, it is OK to call it "bluesquare" .
If the image conveys information, you need that info in your alt tag. Using "A map from my house to where I work" gets the general idea across, but it would still be hard to find your way based on this information
alt="Take 41 south to Algoma exit. Turn left, continue to University. Park in lot 34."
This is a better alt tag. It succinctly takes the important info out of the image and gives it to the user. I put this image on my page so I won't forget how to get to work, the alt tag contains the information I need to do that without the map.
It is OK that I don't describe the map inch for inch. Just as someone looking at the map to get from my house to where I work won't care where Winneconne lies in relation to Omro, neither does the person reading about the map.
Alt tags are limited to a few characters, and descriptions that require more information than is allowed require special handling. A tag does exist that will allow you to do longer descriptions, but Explorer and Netscape do not support it yet.
The longdesc tag is implemented by placing a textual description of the image in a separate file. This allows you to describe the image in as much detail as you need. The tag calls this file, just as a link tag (HREF) does. In the example below, the long description for picture.jpg is located in the picture.html file
<IMG src="picture.jpg" ALT="The Whole Family:" LONGDESC="picture.html">
Designers have begun placing a "d link" next to images with longdesc tags to accommodate browsers that do not read longdesc tags. The alternative browser crowd recognizes a d link as an alternate to a longdesc tag.
The complete image with longdesc tag and dlink looks like this:
There are a few different methods of implementing a <longdesc> and dlink. The two most used methods are inclusion of all dlinks included in a single file and the use of a different file for each dlink. The pages linked below are good examples of this..
In this page, all photos have descriptions, links to which are given by (D). All the descriptions are in one single file with different <#anchor> links.
In this page, all photos have descriptions, links to which are given by (D). All descriptions occupy their own separate files.
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