Image sitemaps can be useful for sites that use lazy loading
When “lazy loading” images on a page in a way that doesn’t include defined image elements, it’s recommended to have back-up in the form of structured data or an image sitemap. That way, Google will know to associate those images with the page even before they’re loaded.
Use Sitemaps Ping, Last Modified and Separate Sitemaps to Index Updated Content
To help Google index updated content more quickly, ping Googlebot when a Sitemap has been updated, use Last Modified dates in Sitemaps, and use a separate Sitemap for updated content so it can be crawled more frequently.
Specify Timezone Formats Consistently Across Site & Sitemaps
Google is able to understand different timezone formats, for example, UTC vs GMT. However, it’s important to use one timezone format consistently across a site and its sitemaps to avoid confusing Google.
Include Most Recently Changed Content in Separate Sitemap
Rather than submitting all of your sitemaps regularly to get Googlebot to find and crawl newly updated pages, John recommends adding recently changed pages into a separate sitemap which can be submitted more frequently, while leaving more stable, unchanged pages in existing sitemaps.
Use the Last Modified Date to Provide a Hierarchy of Changes Made to A Site
John recommends using the last modified date in sitemaps in a reasonable way to provide a clear hierarchy of the changes that have been made on a site. This helps Google to understand which pages are important and ensures they focus on crawling these first.
“Discovered Not Indexed” Pages May Show in GSC When Only Linked in Sitemap
Pages may show as “Discovered Not Indexed” in GSC if they have been submitted in a sitemap but aren’t linked to within the site itself.
Google Has a Separate User Agent For Crawling Sitemaps & For GSC Verification
Google has a separate user agent that fetches the sitemap file, as well as one to crawl for GSC verification. John recommends making sure you are not blocking these.
Internally Link Pages Together to Increase Discoverability & Reduce Reliance on XML Sitemap
Internally linking pages together helps Googlebot to discover the pages on your site more easily, and reduces the reliance on using XML sitemaps for URL discovery.
XML Sitemaps Should Include URLs on Same Path Unless Submitted Via Verified Property in GSC
XML sitemaps should contain URLs on the same path. However, URLs in sitemaps submitted via GSC can be for any valid property within your GSC account.