Curve Expansion type The reference table containing the codes, names, and descriptions of the valid curve expansion methods. United States Geological Survey, Surface Water User Group EQTN 1 true equation The curve is expanded by computing an equation of form y = a * (b + x) ** c + d. LINR 2 true linear The curve is a linear expansion between curve point-pairs. LOGX 3 true logarithmic The curve is expanded by logarithmic interpolation between curve point-pairs. PEQT 4 true polynomial equation The equation is expanded by computing a polynomial equation of the form y = a + bx + c( x) ** 2 + d(x)**3 +. e(x)**4 + f(x)**5, which can have any number of degrees.