Regarding the .pdf "Guergarrat to NKT with GPS":
What datum was used for the grid references provided?
As I understand it, there are hundreds of different datums (different ways to compensate for the world not really being round). The GPS receiver can be changed from datum to datum but usually defaults to the datum common in the country of origin (purchase).
My reason for asking is that I believe a gps receiver set to a different datum to that used to set (and publish) the way-points will place you somewhere different. I assume the error can be large or small depending on which (wrong) datum is being used.