A method for synchronizing a three-dimensional measurement device having an internal 3D coordinate system O 3DM, X 3DM, Y 3DM, and Z 3DM, with an intelligent guidance device separate from the 3DM, the IGD being a portable computing device equipped with a display device, a gravity direction measurement device, an electronic compass, and having an internal 3D coordinate system O IGD, X IGD, Y IGD, Z IGD, the method comprising: defining a coordinate system O IGD-Earth, X IGD-Earth, Y IGD-Earth, Z IGD-Earth using a direction opposite to gravity as Z IGD-Earth and a direction corresponding to a magnetic north pole as Y IGD-Earth, from a perspective of the IGD by: measuring a direction of gravity using the gravity direction measurement device and defining a vector Z IGD-Earth as opposite to the direction of gravity; measuring a direction of North magnetic pole using the electronic compass and defining a vector Y IGD- Earth pointing in the North magnetic pole direction; and computing a vector product of Y IGD-Earth and Z IGD-Earth defining vector X IGD-Earth; computing and applying a rigid transformation T IGD-To-IGDEarth to transform O IGD, X IGD, Y IGD, Z IGD to O IGD-Earth, X IGD-Earth, Y IGD-Earth, Z IGD-Earth; defining a coordinate system O 3DM-Earth, X 3DM-Earth, Y 3DM-Earth, Z 3DM-Earth using a direction opposite to gravity as Z 3DM-Earth and the direction corresponding to the magnetic north pole as Y 3DM-Earth, from a perspective of the 3DM; and computing and applying a rigid transformation T 3DM-To-3DMEarth to transform O 3DM, X 3DM, Y 3DM, and Z 3DM to O 3DM-Earth, X 3DM-Earth, Y 3DM-Earth, Z 3DM-Earth, such that X IGD-Earth and X 3DMEarth are substantially parallel and point in a first direction, Y IGD-Earth and Y 3DM-Earth are substantially parallel and point in a second direction, and Z IGD-Earth and Z 3DM-Earth are substantially parallel and point in a third direction, thereby synchronizing an orientation of the IGD with an orientation of the 3DM. defining a coordinate system O IGD-Earth, X IGD-Earth, Y IGD-Earth, Z IGD-Earth using a direction opposite to gravity as Z IGD-Earth and a direction corresponding to a magnetic north pole as Y IGD-Earth, from a perspective of the IGD by: measuring a direction of gravity using the gravity direction measurement device and defining a vector Z IGD-Earth as opposite to the direction of gravity; measuring a direction of North magnetic pole using the electronic compass and defining a vector Y IGD- Earth pointing in the North magnetic pole direction; and computing a vector product of Y IGD-Earth and Z IGD-Earth defining vector X IGD-Earth; computing and applying a rigid transformation T IGD-To-IGDEarth to transform O IGD, X IGD, Y IGD, Z IGD to O IGD-Earth, X IGD-Earth, Y IGD-Earth, Z IGD-Earth; defining a coordinate system O 3DM-Earth, X 3DM-Earth, Y 3DM-Earth, Z 3DM-Earth using a direction opposite to gravity as Z 3DM-Earth and the direction corresponding to the magnetic north pole as Y 3DM-Earth, from a perspective of the 3DM; and computing and applying a rigid transformation T 3DM-To-3DMEarth to transform O 3DM, X 3DM, Y 3DM, and Z 3DM to O 3DM-Earth, X 3DM-Earth, Y 3DM-Earth, Z 3DM-Earth, such that X IGD-Earth and X 3DMEarth are substantially parallel and point in a first direction, Y IGD-Earth and Y 3DM-Earth are substantially parallel and point in a second direction, and Z IGD-Earth and Z 3DM-Earth are substantially parallel and point in a third direction, thereby synchronizing an orientation of the IGD with an orientation of the 3DM.