The reason for checking the output of GRIDIT is that although the triangulation algorithm is fairly robust, it can produce various errors, some immediately obvious and some not. For instance, when the coordinate datum is poorly positioned, remote from the domain being modelled, consequent error in single precision subtractions in GRIDIT can lead to very obvious misconnections between nodes far removed from one another.
On the other hand, less obvious but serious error can occur if the first few nodes on a boundary are collinear or approximately collinear. Then some or all of the nodes involved, and even some nodes further along the boundary, may have surplus connections to nodes which are not their immediate neighbours in the input node set. Detection and correction of this type of error is possible with the help of the EDITOR, but it is better to prevent it by modifying the arrangement of the corresponding block of nodes in the input file. Since each boundary corresponds to a closed curve, collinear nodes at the beginning of a boundary block can be moved to the end of the block (using a text editor), where they normally cause no problem.
The triangulation algorithm can be confused by certain complicated coastal geometries. When an island lies partly in a coastal bay and both features have relatively few boundary nodes, spurious connections may be set up; the island coastline may be erroneously incorporated into the outer boundary. Another common error occurs along some nearly-straight boundary segments. There are boundary connections that lie outside the boundary and define long,thin triangles that are difficult to see. These errors can generally be detected by using option {.TEST}CHECKNODES in the EDITOR, as will be explained later.
These errors are not peculiar to the Renka algorithm. All methods of triangulation used so far by the authors suffer from some problems similar to those mentioned, so it is advisable to have good error-checking capability, no matter what triangulation method is used.