isochasm                a locus of points where auroras are sighted
                        with equal frequency.
almucantar              (astron.) the circle of all points at a
                        specified altitude on the celestial sphere.
aclinic line            the locus on the earth's surface,
                        approximating the equator, where a compass
                        needle does not tend from the horizontal. 
agonic line             the locus of all points on the earth's surface
                        whose magnetic declination is zero.  [< Gk
                        /go~nia~/ angle]
thalweg                 a imaginary line along the bottom of a valley
                        or riverbed.
hachure                 the same as a contour line?