Yeh, I am using the same coordinate system (OSGB-1936). I setup both instrument on the same coordinate system and in GIS, I edit them at the same coordinate system. the starting point of GPS (let's call it GCP1) was E177118.941; N34727.625; H86.568 ). This data from GPS measurements and the rest of points were measured relatively. Then I setup another instrument ( called Total stations) over GCP1 and I assumed the coordinates of this point is 1000, 1000, 1000, then the rest of points measured relatively. So, maybe it is a datum shift because when I added both data in ArcGIS, they are totally in different in location (because they were measure from different instrument) . Any suggestion on how I put them at the same location in ArcGIS please?