Merge pull request cf-convention#521 from JonathanGregory/issue163

Provide a convention for boundary variables for grids whose cells do not all have the same number of sides

ch01.adoc

@@ -120,6 +120,8 @@ There is no relationship between the name of a tie point variable and the name(s
 
 time dimension:: A dimension of a netCDF variable that has an associated time coordinate variable.
 
+vertex dimension:: The dimension of a boundary variable along which the vertices of each cell are ordered.
+
 vertical dimension:: A dimension of a netCDF variable that has an associated vertical coordinate variable.
 
 === Overview

ch07.adoc

@@ -15,7 +15,12 @@ For cases when data pertain to geospatial features with highly variable geometry
 To represent cells we add the attribute **`bounds`** to the appropriate coordinate variable(s).
 The value of **`bounds`** is the name of the variable that contains the vertices of the cell boundaries.
 We refer to this type of variable as a "boundary variable."
-__A boundary variable will have one more dimension than its associated coordinate or auxiliary coordinate variable.__ The additional dimension should be the most rapidly varying one, and its size is the maximum number of cell vertices.
+__A boundary variable must have one more dimension than its associated coordinate or auxiliary coordinate variable.__ 
+We refer to the additional dimension as the "vertex dimension".
+The vertex dimension must be the last dimension in CDL order (the most rapidly varying dimension), and its size is the maximum number of cell vertices.
+The vertex dimension must be of size two if the associated variable is one-dimensional, and of size greater than two if the associated variable has more than one dimension.
+For grids constructed from cells that do not all have the same number of sides (e.g., a grid with some rectangular cells and some triangular cells), the vertex dimension must be at least as large as the maximum number of cell vertices.
+For cells with fewer vertices than the size of vertex dimension, the unneeded elements must appear as the last elements in the vertex dimension and must be assigned the **`_FillValue`**.
 
 A boundary variable inherits the values of some attributes from its parent coordinate variable.
 If a coordinate variable has any of the attributes marked "BI" (for "inherit") in the "Use" column of <<attribute-appendix>>, they are assumed to apply to its bounds variable as well.

conformance.adoc

@@ -362,6 +362,8 @@ If there are two dimensions, the leading dimension (CDL order) must match one of
 * The type of the **`bounds`** attribute is a string whose value is a single variable name.
 The specified variable must exist in the file.
 * A boundary variable must have the same dimensions as its associated variable, plus have a trailing dimension (CDL order) for the maximum number of vertices in a cell.
+The trailing dimension must be of size two if the associated variable is one-dimensional, and of size greater than two if the associated variable has more than one dimension.
+* Any elements of the boundary variable which contain the **`FillValue`** must form a consecutive block at the end of the trailing dimension.
 * A boundary variable must be a numeric data type.
 * A boundary variable can only have inheritable attributes, i.e. any of those marked "BI" in the "Use" column of http://cfconventions.org/cf-conventions/cf-conventions.html#attribute-appendix[Appendix A], if they are also present on its parent coordinate variable.
 * If a boundary variable has an inheritable attribute then its data type and its value must be exactly the same as the parent variable's attribute.

history.adoc

@@ -7,6 +7,7 @@
 
 === Working version (most recent first)
 
+* {issues}163[Issue #163]: Provide a convention for boundary variables for grids whose cells do not all have the same number of sides.
 * {issues}174[Issue #174]: A one-dimensional string-valued variable must not have the same name as its dimension, in order to avoid its being mistaken for a coordinate variable.
 * {issues}237[Issue #237]: Clarify that the character set given in section 2.3 for variable, dimension, attribute and group names is a recommendation, not a requirement.
 * {issues}515[Issue #515]: Clarify the recommendation to use the convention of 4.3.3 for parametric vertical coordinates, because the previous wording caused confusion.