I. Edges: Each edge is tested separately to see if it should be tagged for refinement or derefinement.

     Definition:

c = a user supplied tolerance
f(i) = value of the field variable f at node i
L = length of the edge

     For the edge between nodes 1 and 2, we have

     Criteria:

     1) Junction: Refine if the edge field values straddle c.
         Tag for refinement if

f(1) > c and f(2) < c        or         f(1) < c and f(2) > c
example: For c = 0, refine if f changes sign across the edge.

     2) Constant: Refine if the edgeís field values exceed c.
         Tag for refinement if

f(1) > c or f(2) > c

     3) Maxsize: Refine if the edge length exceeds c.
         Tag if l > c

     4) Delta: Refine if the magnitude of the difference of the field values at the edge ends exceeds c.
         Tag if

| f(1) - f(2) | > c

     5) Lambda Refine: Refine if lambda/dx < c. Where dx  is a scale length (here taken to be the edge length

Generally lambda/dx is a quality measure of the discretization. A larger value of usually indicates a better grid discretization.  There are some special cases.  If  one of the field values is zero as could happen on a boundary, then lambda/dx 1/2.  If f(1) is equal to  f(2) then lambda/dx  is divergent but the algorithm uses a  small number e = .000001 added to the denominator lambda/dx to give a large but finite value of lambda/dx thus indicating a good discretization.