You can FTP a large file to ftp.globalmapper.com. Use a username of 'upload' and a password of 'upload' to log in.
There are lots of XML formats, including several supported by Global Mapper like GPX, KML, and Garmin's Map Catalog format. Basically XML is a format meta-descriptor that people can use to create actual formats from. Land/XML is another XML-based format, although it is not yet supported by Global Mapper.
For your surface file, lots of applications support Arc ASCII Grids, BIL, DEM, or DXF mesh files, all of which Global Mapper can export (in addition to lots of other less common formats).
Let me know if I can be of further assistance.
Thanks,
Mike
Global Mapper Support
support@globalmapper.com