The -a switch

• Imposes a maximum triangle area.
• If a number follows the `a', no triangle will be generated whose area is larger than that number. For instance, -a2.5 specifies that no triangle should have area greater than 2.5.
• If no number is specified after the `a', an .area file (if -r is used) or .poly file (if -r is not used) specifies the maximum area of each triangle.
• An .area file contains a separate area constraint for each triangle, and is useful for refining a finite element mesh based on a posteriori error estimates.
• A .poly file can optionally contain an area constraint for each segment-bounded region, thereby enforcing triangle densities in a first triangulation.
• You can impose both a fixed area constraint and a varying area constraint by invoking the -a switch twice, once with and once without a number following.