# GRASS Help System

## COORDINATE SYSTEMS AND PROJECTIONS

A map coordinate system is either a geographic coordinate system (like UTM or latitude/longitude) or an arbitrary coordinate system (like the x,y coordinate system initially assigned to imported satellite imagery) used to reference the locations of objects in a two-dimensional space.

A map projection describes the manner in which the spherical surface of the earth is represented on a two-dimensional surface. It attempts to minimize distortion in area, shape, distance, and direction.

All of the data stored beneath the same GRASS "location" directory must use the same map coordinate system and map projection. This means that latitude/longitude data, Universal Transverse Mercator data, State Plane coordinate system data, and unrectified (x,y) imagery data, should be stored under different GRASS "locations" and "mapsets".

Often, one wishes to combine data that are available in different coordinate systems and/or different map projections. To do this, users must convert these data to a common map coordinate system and projection.

The commands listed below are useful to those importing and exporting data to and from GRASS. They can be used to perform map coordinate system and map projection conversions.

- m.datum.shift - r.mapcalc - v.transform
- m.gc2ll - r.in.ll
- m.ll2gc
- m.ll2u
- m.u2ll
- m.region.ll
- m.tiger.region

Satellite imagery data are generally made available in an x,y coordinate system. In order to analyze imagery data in combination with other GRASS data layers, it is necessary that all these data be rectified to the same map coordinate system and projection. Several GRASS programs are used to rectify imagery data.

- i.points - i.rectify - i.rectify.blk

General information on use of imagery in GRASS is accessible by selecting imagery discussion.

For more information on a specific command, select a command and press ESC.

Read the section on Geometric Rectification under the Image Processing topic area for a more detailed description of the image rectification process.

See also the references for: "Map Projections - A Working Manual" (Snyder, 1989), "The Department of Defense World Geodetic System 1972" (Seppelin, 1974).

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Automatically created on: Wed Mar 30 04:38:58 2005