What is a Sitemap Index?

Sitemap index files and sitemap files are devices that can help search engines crawl websites. A sitemap is a listing of pages within the website. The sitemap may be in a format, such as HTML, that is easily readable by humans, but it can also be in extensible markup language (XML) format, specifically for search engines usage. A sitemap index is usually an XML file that lists a number of XML sitemap files. A webmaster can submit sitemap index files to search engines, and so provide specific information on the sites she’d like the search engines to visit.

The sitemap index and the sitemap files that are used by search engines contribute to search engine optimization (SEO). SEO consists of a variety of techniques that aid search engines in locating and ranking web pages. The sitemap index can be an important piece in SEO because it points to the individual sitemaps that tell search engines which pages the webmaster would like included in the search engine results, along with the priority and frequency of updates, if this information was added.

The individual sitemap file must include certain required XML tags. These required tags are the urlset, url and loc tags. The urlset tag set opens and closes the file. The url tag set acts as the parent tag for each listed uniform resource locator (URL), or web address of the page. The loc tag provides the web address for the sitemap.

Optional XML tags for sitemaps provide information that can assist search engines in indexing the site per the webmaster's preference. The optional tags are lastmod, changefreq, and priority. Lastmod refers to the date the page was last modified. The changefreq XML tag describes how often the page is expected to be updated. The priority tag describes the importance of this page in relation to other pages on the site. The webmaster can check each individual search engine’s guidelines for information how it uses the optional tags.

The requirements for the sitemap index file are similar to those for the sitemap file. The file must open and close with a sitemapindex tag. Other required tags include the sitemap tag for each listed sitemap and the loc tag, which describes the location of each sitemap. The sitemap index file has one optional tag — the lastmod tag — that tells the date the sitemap index was last updated.

Though the same sitemap and sitemap index requirements usually apply to any organization, biomedical research organizations may make use of a biositemap. A biositemap is special type of sitemap that is used by these organizations to help group their research information on the Internet. This allows the information to be more readily discovered and used by researchers.