Storing coordinates

Arcs, label points, annotation, tics, links, and the coverage extent contain x,y coordinate information. By contrast, polygons, nodes, routes, sections, and regions are composed of arcs (defined topologically) and do not contain coordinates.

Coordinate precision

The storage precision of x,y coordinate data is often important. Coverage coordinates can be either single-precision (6 to 7 significant digits) or double-precision (typically 13 to 14 significant digits) real numbers.

Coordinate system definition

A coverage's spatial information is stored in a coordinate system defined by the coverage projection. This can affect many operations, such as moving between map projections. The following example describes a coverage's coordinate system as latitude-longitude (that is, a geographic coordinate system) stored in decimal degrees (DD):

Projection	Geographic
Z-units	Meters
Units	DD
Spheroid	Sphere
X-shift	0.000
Y-shift	0.000
Densify	No
Generalize	No

Example of a coverage's coordinate system description

The coordinate system for a coverage, as well as other types of datasets, is stored in a PRJ file.

If you know a coverage's coordinate system, but that coverage does not yet have a PRJ file, use the Define_Projection tool.

Processing tolerances for a coverage

A number of tolerances are used when processing a coverage. These are used to reduce the number of coordinates in an arc, snap nodes, ensure accurate tic registration, and support other important operations on coverages. Many tolerances are tracked in a special file for each coverage (named TOL for single-precision coverages and PAR for double-precision coverages). Use the Tolerance tool to set or modify coverage tolerances.