A point P is given as (r,θ,h). In terms of the Cartesian coordinate system:
* r is the distance from O to P’, the orthogonal projection of the point P onto the XY plane. This is the same as the distance of P to the z-axis.
* θ is the angle between the positive x-axis and the line OP’, measured counterclockwise.
* h is the same as z.
* Thus, the conversion function f from cylindrical coordinates to Cartesian coordinates is f(x,y,z) = (rcosθ,rsinθ,h).
For use in physical sciences and technology, the recommended international standard notation is ρ, φ, z (ISO 31-11).
Some mathematicians indeed use (r,θ,z).
Cylindrical coordinates are useful in analyzing surfaces that are symmetrical about an axis, with the z-axis chosen as the axis of symmetry. For example, the infinitely long circular cylinder that has the Cartesian equation x2 + y2 = c2 has the very simple equation r = c in cylindrical coordinates. Hence the name “cylindrical” coordinates.
Line and volume elements
The line element is 
The volume element is 
The gradient is 
출처: 위키 링크
