Copying this from the wiki:

Indigenous for Android has a checkin post type which passes the checkin information as additional parameters in the Geo URI. The payload property is 'geo'. It's not doing a JSON post, so you can also immediately upload pictures as well. Additional properties for the venue/location can be added later. The same technique is used to post my geocache logs, see https://indieweb.org/geocaching

geo:51.5258325,-0.1359825,0.0;name=london;url=https://hwclondon.co.uk

This is supported by:

• IndieWeb for Drupal. An example can be found here: https://realize.be/checkin/1762
• WordPress Micropub Plugin