What does “Canonical URL Detected” in the log mean?

Question: When I crawl my site with Sitemap Creator I see a lot of messages about “Canonical URL Detected”. What does this mean?

Answer: On some sites a single page may be accessed using multiple URLs. For example, on our site the following two URLs point to the exact same page:
http://www.inspyder.com/
http://www.inspyder.com/Default.aspx

Google is unable be able to tell which of these URLs is the preferred URL for indexing, so they invented the “Canonical URL” standard. The “Canonical URL” of a page is the preferred URL to use when accessing or indexing that particular page.

Sitemap Creator recognizes this tag and only includes the canonical formats in your sitemap. When a canonical URL is found that doesn’t match the URL used to access that page, Sitemap Creator logs it so you know why that particular URL wasn’t included in your sitemap. Without this feature your sitemap would be full of URLs you don’t want Google to index.