If you want an accurate absolute position, I believe you need to get it surveyed by someone who knows what their doing. I just used Google Earth's number. To change your base's fixed position value, you have to change it with u-center or figure out how to do it via UBX commands on a serial interface.
By default, you'll get those 2 messages and a few more. To turn off the extra, connect with u-center, open the configuration view, go to the messages page, and selection each message that you want to change from the drop down menu and check/uncheck the ports as needed.