More details:

You can use $apply to update the EPSG:21781 and EPSG:4326 coords on the scope. $apply triggers a dirty-check cycle so you probably want to call it at the end of the contextmenu event listener.

The jsonp calls can go outside $watch. The $http callbacks (success and error) are executed in "Angular context", so there's no need to call $watch in $http callbacks.

Also, it should be possible to rely on promises to chain the jsonp calls and update the UI in one pass for altitude and epsg2026.

Also, you may need to abort previous jsonp requests. $http abort support was recently added to Angular.