3.2.2. Scale and coordinates
The scaling and georeferencing requirements for the target
surface will vary with the intended use of the 3D data/model and
should be considered prior to image acquisition. An object or surface
can be fully reconstructed in 3D without any scale or position
information but, to extract oriented and scaled data, additional
control data must be provided. Scale can be added to a model
simply by knowing the distance between two points on an input
image or on the model. These distance measurements for scale are
best taken over the width of the target area rather than smaller,
isolated lengths. Greater accuracy in scale and full georeferencing
to local or geographic coordinates requires three or more ground
control points, usually collected with survey-grade, carrier phase
differential GPS or total station surveying. These ground control
points should be distributed widely across the target area, not
neglecting the margins. An alternate option for georeferencing is
the use of high-precision camera locations for the input images, but
current GPS units in hand-held cameras are insufficient for the
typical accuracy requirements of high resolution structural mapping
and paleoseismology projects conducted over a few hundred
meters or less
3.2.2. Scale and coordinatesThe scaling and georeferencing requirements for the targetsurface will vary with the intended use of the 3D data/model andshould be considered prior to image acquisition. An object or surfacecan be fully reconstructed in 3D without any scale or positioninformation but, to extract oriented and scaled data, additionalcontrol data must be provided. Scale can be added to a modelsimply by knowing the distance between two points on an inputimage or on the model. These distance measurements for scale arebest taken over the width of the target area rather than smaller,isolated lengths. Greater accuracy in scale and full georeferencingto local or geographic coordinates requires three or more groundcontrol points, usually collected with survey-grade, carrier phasedifferential GPS or total station surveying. These ground controlpoints should be distributed widely across the target area, notneglecting the margins. An alternate option for georeferencing isthe use of high-precision camera locations for the input images, butcurrent GPS units in hand-held cameras are insufficient for thetypical accuracy requirements of high resolution structural mappingand paleoseismology projects conducted over a few hundredmeters or less
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