1. Method of ground-based surveying on a site ( 2 ) which comprises an unstable zone ( 60 ; 20 ) and at least one control point (FCP 1 , FCP 2 , FCP 6 , FCP 7 ; CP 1 -CP 3 ) placed outside said unstable zone, in which method at least one surveying device (TS 8 , TS 9 ; ST 1 -ST 3 ) is used to acquire positional data by sighting at least one control point (FCP 1 , FCP 2 , FCP 6 , FCP 7 ; CP 1 -CP 3 ), characterised in that it further comprises the steps of: providing at least one sighting point (UP 3 -UP 5 ; A-C) within said unstable zone ( 60 ; 20 ), using said surveying device(s) (TS 8 , TS 9 ; ST 1 -ST 3 ) to acquire positional data by sighting said at least one sighting point (UP 3 -UP 5 ; A-C) that is located within said unstable zone ( 60 ; 20 ), and combining the positional data acquired from: i) said at least one control point (FCP 1 , FCP 2 , FCP 6 , FCP 7 ; CP 1 -CP 3 ) placed outside said unstable zone and ii) at least one sighting point (UP 3 -UP 5 ; A-C) within said unstable zone ( 60 ; 20 ), to produce a positional reference for said surveying device(s).