Although many programs exist, there are two main types. Type 1 is used
primarily for precise non-metric data entry (written code descriptions of prominent
identifying features) which Whitehead (1990) extended to include a metric 2
dimensional image if needed (Kelly 2001). Type 2 requires less data entry and uses
metric 3-dimensional models to digitize natural features (Kelly 2001, Adams et al. 2006).
Type 1 is often preferred but can be sensitive to user experience and photograph angle
and quality (Kelly 2001). Type 2 is useful because the camera angle does not matter as
much as it does for Type 1 because the program aligns with body features (eye, head,
or shoulder) when analyzing an individual (Kelly 2001).