Production Rules of CSDGM

A production rule specifies the relationship between a compound element, and data elements and other (lower-level) compound elements. Each production rule has a left side (identifier) and a right side (expression) connected by the symbol "=", meaning that the term on the left side is replaced by or produces the term on the right side. Terms on the right side are either other compound elements or individual data elements. By making substitutions using matching terms in the production rules, one can explain higher-level concepts using data elements.

The symbols used in the production rules have the following meaning:

=	is replaced by, produces, consists of

+	and

[|]	selection - select one term from the list of enclosed terms (exclusive or).
	Terms are separated by "|".

m{}n	iteration - the term(s) enclosed is(are) repeated from "m" to "n" times

()	optional - the term(s) enclosed is(are) optional

Examples:

a = b + c		"a consists of b and c"
a = [b | c]		"a consists of one of b or c"
a = 4{b}6		"a consists of four to six occurrences of b"
a = b + (c)		"a consists of b and optionally c"

Interpreting the production rules:

The terms bounded by parentheses, "(" and ")", are optional and are provided at the discretion of the data producer. If a producer chooses to provide information enclosed by parentheses, the producer shall follow the production rules for the enclosed information. For example, if the producer decides to provide the optional information described in the term:
```
(a + b + c)
```
the producer shall provide a and b and c.

Only for terms bounded by parentheses does the producer have the discretion of deciding whether or not to provide the information.
The variation among the ways in which geospatial data are produced and distributed, the fact that all geospatial data do not have the same characteristics, and the issue that all details of data sets that are in work or are planned may not be decided, caused the need to express the concept of "mandatory if applicable." This concept means that if the data set exhibits (or, for data sets that are in work or planned, it is known that the data set will exhibit) a defined characteristic, then the producer shall provide the information needed to describe that characteristic. This concept is described by the production rule:
```
0{ term }1
```

This page excerpted from Peter Schweitzer's on-line version of the CSDGM