Verifying surveying instrument's external orientation during a measurement process, comprising directing the imaging means onto a reference object and detecting a first photographing direction of the imaging means, taking a first image of the reference object in the first photographing direction, memorizing the first image and the first photographing direction as being indicative of the surveying instrument's external orientation, re-directing the imaging means onto the reference object and detecting a second photographing direction of the imaging means, taking a second image of the reference object in the second photographing direction, and comparing a first with a second imaged position of the reference object in the first respectively the second image by image processing as well as the first with the second photographing direction and verifying the surveying instrument's external orientation based on disparities between the first and the second imaged position and/or between the first and the second photographing direction.