In Mapping we require accurate Ground based Control Points as reference points for our mapping project, this increases the accuracy from 2.5m to 0.5-1m accuracy.
One way of obtaining GPS Co-ords is to utilise the RTKLIB
project with One Talent GNSS Yuan10 GPS Receiver (<10cm accuracy)
connected to a Raspberry Pi USB slot
http://www.onetalent-gnss.com/ideas/usb-hw-receivers/yuan10 (100 Euros)