Figure 1. Overview of ERP projection, cylindrical coordinate and
comparison of HHA and REL. Tˆ is the tangent line to circle O
θ,p p
at point p. Circle O lies in the plane perpendicular to gravity,
p
passes through point p, and its center lies on the cylindrical axis
L of ρθz . In (c), we compared how different structure informa-
cyl
tion are represented in REL and HHA.