National Geospatial Data Clearinghouse. It is a component of a two-dimensional manifold. Compound elements represent higher-level concepts that cannot be represented by individual data elements. Examples include cones and cylinders. Extended elements may be defined by a data set producer or a user community.
Extended elements are elements outside the Standard, but needed by the data set producer.
If extended elements are created, they must follow the guidelines in Appendix D, Guidelines for Creating Extended Elements in the Content Standard for Digital Geospatial Metadata. No ring, inner or outer, shall be collinear with or intersect any other ring of the same G-polygon. Longitude and latitude are specified in decimal degrees with north latitudes positive and south negative, east longitude positive and west negative. This information may be derived from, among other things, remote sensing, mapping, and surveying technologies. Numerous rule sets can be used to distinguish different types of graphs.