# The law of triangulation of unit disk
**URL:** https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651
**Category:** General Discussion
**Created:** [April 7, 2022, 3:33am UTC](https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651 "2022-04-07T03:33:18Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Qianrui](https://community.freefem.org/u/Qianrui)
#### Post date: [April 7, 2022, 3:33am UTC](https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651/1 "2022-04-07T03:33:18Z")
The triangulation of unit disk is not consistent with that of rectangular area,
for example
> mesh Th=square(64,64,[-4+8_x,-4+8_y]),
the spatial step size is
> 8/64=1/8,
So the error convergence order is
> log(errorh)-log(errorh/2) /log2;
However, according to the division of the circle
> real r=1;
> real a=0,b=0;
> border C(theta=0, 2_pi){x=a+r_cos(theta); y=b+r\*sin(theta);}
> // Build the mesh from boundary
> mesh Th = buildmesh(C(10));
> How can we get the error convergence order without knowing the size of the spatial step?
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [April 8, 2022, 9:30am UTC](https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651/2 "2022-04-08T09:30:43Z")
Remark, the mesh size on border is 2\*\pi\*r / 10 so the mesh size is close to
2\*\pi\*r / 10. Now be careful because the border is not a circle but a polygone with 10 sides.
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### Author:  [@Qianrui](https://community.freefem.org/u/Qianrui)
#### Post date: [April 8, 2022, 11:17am UTC](https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651/3 "2022-04-08T11:17:14Z")
> [@frederichecht](#):
>
> Remark, the mesh size on border is 2\*\pi_r / 10 2∗π∗r/10 2_\pi_r / 10 so the mesh size is close to
> 2_\pi_r / 10 2∗π∗r/102_\pi\*r / 10 . Now be careful because the border is not a circle but a polygone with 10 sides.
Thank you. Let me ask you another question. Can we only divide triangles? Can I use rectangular elements?
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### Author:  [@frederichecht](https://community.freefem.org/u/frederichecht)
#### Post date: [April 8, 2022, 3:23pm UTC](https://community.freefem.org/t/the-law-of-triangulation-of-unit-disk/1651/4 "2022-04-08T15:23:32Z")
You can only divide triangle with trunc function but no quad element in freefem++