# RegionMeasure not measuring my region

This tetrahedron and sphere intersect:

pts = {{0, 0, 0}, {0.8, 0, 0}, {1, Sqrt[3]/2, 1}, {1, -1, 3/4},
cent = {0.7, -0.0335, 0.43}};
regTet = MeshRegion[pts, Tetrahedron[{1, 2, 3, 5}]];
regSphere = Region[Sphere[cent, .1]];
Show[regTet, regSphere]


Using RegionIntersection to pick out the intersection seems to work:

int = RegionIntersection[regTet, regSphere]


and everything checks out with RegionQ:

{RegionQ[regSphere], RegionQ[regTet], RegionQ[int]}
{True, True, True}


Now I can get the measure of the tetrahedron and of the sphere

{RegionMeasure[regSphere], RegionMeasure[regTet]}
{0.125664, 0.054984}


but when trying to get the measure of the intersection:

RegionMeasure[int]
RegionMeasure::reg: int is not a correctly specified region.


It feels like I must be making a simple mistake... but how do I get the area of the intersection of the surface of the sphere and the tetrahedron.

If you work with graphics primitives instead, things work:

tet = Tetrahedron[{{0, 0, 0}, {0.8, 0, 0}, {1, Sqrt[3]/2, 1}, {0.7, -0.0335, 0.43}}];
sph = Sphere[{0.7, -0.0335, 0.43}, .1];

Show @ Graphics3D[{tet, sph}]
RegionMeasure @ RegionIntersection[tet, sph]


0.0207873

• Thanks -- this is a nice work-around. The Region commands seem so tempting... I wish I understood them better. Commented Mar 12, 2018 at 15:56