How to set default latlon by editing the form values?

Where do I go to directly alter the default values of the locations when adding new nodes?

I am trying to create a link from a node with an assigned location to the node/add/nodetype, which is then prepopulated with referring node's lat/lon.

The idea is to (in a custom module) use $_GET to retrieve the values from URL,
then (if numeric etc) put them in on the proper places in the location array.

If not in a module, is there a way to change lat/lon in the theme template instead?