# Volume form not what I expected.

Hello everyone,

Let us suppose I want to create a finite element representation of half-sphere of radius one, where the surface is described by a 3-component vector phi = (X, Y, Z). I then wish to compute the volume of this half-sphere.

Here is a minimal example:

# Begin

from dolfin import *

mesh = UnitCircle(7) # I'm aware that this is deprecated, but I can't recall the new syntax

space= VectorFunctionS

class phiexpression(

def eval(self,

values[0] = x[0]

values[1] = x[1]

values[2] = sqrt(abs(

def value_shape(self):

return (3,)

phi = Function(space)

phiexp = phiexpression()

phi.interpolate

V = (1./3)*

initial_

print initial_

# End

Now, this ought to give me 2 pi / 3, but instead I get 1.92919724808

What am I doing wrong? Is this just round-off error?

## Question information

- Language:
- English Edit question

- Status:
- Solved

- For:
- DOLFIN Edit question

- Assignee:
- No assignee Edit question

- Solved by:
- Christopher Laing

- Solved:
- 2013-02-24

- Last query:
- 2013-02-24

- Last reply:
- 2013-02-24

Jan Blechta (blechta) said : | #1 |

That's probably discretization error:

mesh = UnitCircle(70) ... 2.07779064251

mesh = UnitCircle(700) ... 2.09275075086

Excellent, cheers.