# How can I coarsen a mesh in Gmsh when 'Mesh options' include 'Refine by splitting' but nothing about coarsening?

I am new to Gmsh and I am having trouble creating a circle with a coarse mesh. I use

Geometry->Elementary entities->Add->Circle


to create the geometry and then

Mesh->1D


to create a mesh. Then I can can refine the mesh using Mesh->Refine by splitting but there seems to be no option for coarsening the mesh?

For other shapes/dimensions I can easily coarsen the mesh using the .geo file directly. For example, I created a planar rectangle in 3D and use the 'h' parameter in my .geo file to make the mesh finer/coarser. Here is that .geo file:

h = 1;
//+
Point(1) = {-1, -1, 0, h};
//+
Point(2) = {1, -1, 0, h};
//+
Point(3) = {1, 1, 0, h};
//+
Point(4) = {-1, 1, 0, h};
//+
Line(1) = {1, 2};
//+
Line(2) = {2, 3};
//+
Line(3) = {3, 4};
//+
Line(4) = {4, 1};
//+
Line Loop(1) = {3, 4, 1, 2};
//+
Plane Surface(1) = {1};


But my .geo mesh for the circle doesn't to have any place I can use a variable such as 'h' above to refine/coarsen the mesh for the circle:

//+
Circle(1) = {0, 0, 0, 1, 0, 2*Pi};


So how can I create coarser meshes for the circle either in Gmsh or directly using some parameter in the .geo file?

To create a coarser mesh, you can set the characteristic length globally to a larger value, e.g.,

SetFactory("OpenCASCADE");
Mesh.CharacteristicLengthFactor = 2;

Circle(1) = {0, 0, 0, 1, 0, 2*Pi};
Line Loop(1) = {1};
Surface(1) = {1};


Increasing the value of Mesh.CharacteristicLengthFactor results in a coarser mesh; decreasing the value results in a finer mesh.

You can also set the characteristic length factor in the gmsh options. You can find it under Tools -> Options -> Mesh -> General -> Element size factor.

If you need a finer control over the element sizes, you can set the element size for different parts of your domain, e.g., by defining a box in which a certain mesh size is required. See Specifying mesh element sizes in the gmsh manual.