Morphometric Data - An Invariant Approach to Statistical Analysis of Shapes - page 20

Figure 2.3. Examples of two classes of traditional landmarks. Shown at left are the
landmarks nasale, anterior nasal spine and maxillary foramen. The location of these
landmarks can be precisely given without reference to a coordinate system. Shown at
right are the landmarks glabella and inion. These landmarks can only be found when
the skull is placed in the Frankfort horizontal orientation and the skull is viewed from
the side. Thus, the placement of these landmarks requires a specific coordinate system.
coordinates of either of these points change according to the position of
the skull, but their location relative to each other and to other land-
marks on the skull remains the same regardless of position or
orientation of the skull.
The second class of traditional landmarks consists of precisely
delineated points corresponding to the location of features of some bio-
logical significance but whose definition is dependent upon a
particular coordinate system. Examples include glabella and inion
( Figure 2.3 ) . Glabella is defined as “the most forward projecting point
of the forehead in the midline at the level of the supra-orbital ridges
above the naso-frontal suture” (Bass, 1971). The definition of this land-
mark requires that the skull be positioned in a particular way in order
for that landmark to be located. This means that the location of this
landmark may change depending upon the orientation of the skull.
Even when orientation is explicitly defined (e.g., the Frankfort hori-
zontal plane), the exact orientation may never be duplicated when
collecting data from other subjects, resulting in an additional source of
potential error. For this reason, landmarks that require orienting of
the specimens for data collection are less desirable.