3 Coordinate Systems and Reference Systems
Learn how geographic and projected coordinate systems, datums, and map projections determine how GIS locations are represented, aligned, and measured.
How coordinate reference systems work
A tells a GIS how coordinate values relate to locations on Earth. It combines a coordinate system with a geodetic or reference frame, and a projected CRS also specifies a map projection. Choosing a suitable CRS helps data from different sources align and makes measurements meaningful.
Geographic coordinates
A identifies places on a model of Earth using angles, usually latitude and longitude in degrees. Latitude measures north–south position from the equator, while longitude measures east–west position from a reference meridian. For example, a location might be recorded as N, W.
Because a GCS describes positions on a curved surface, its coordinates are not ordinary flat-map distances. A difference of of longitude represents a shorter ground distance near the poles than near the equator. Latitude and longitude are useful for identifying global locations, but usually are not the best coordinates for local distance or area calculations.
Projected coordinates and distortion
A represents locations on a flat plane, typically using easting and northing measured in meters or feet. It is created by mathematically projecting coordinates from a geographic coordinate system onto a plane. Since Earth’s curved surface cannot be flattened without distortion, every projection changes at least one property, such as area, shape, distance, or direction. No single projection is best for every purpose.
Choose a projection according to the property the work needs to preserve:
An preserves area and is useful for comparing the sizes of regions.
A preserves local angles and shapes. It can be useful for some navigation and mapping tasks, but may distort area.
An preserves distances from specified points or along specified lines, not necessarily everywhere.
Web Mercator is widely used for online map display, but its area and distance distortions make it unsuitable for many measurement tasks. For measuring features in a limited area, a local UTM zone or another suitable regional projection is often more appropriate.
Datums and reference frames
A defines how a mathematical model of Earth is positioned and oriented relative to Earth. It includes or is associated with a , an approximate mathematical shape of Earth. Different datums can assign slightly different coordinates to the same physical place, so datasets may fail to align even when both use latitude and longitude.
NAD27 and NAD83 are different North American datums. Data referenced to one may need a to align properly with data referenced to the other. The size and direction of the coordinate shift depend on the location and the transformation used. Some modern reference frames also change over time, so high-precision work may require a reference-frame realization and coordinate epoch.
Choosing and changing a CRS
Choose a CRS according to the area being mapped, the map or analysis purpose, the required measurements, and any CRS specified by authoritative source data or project standards. Use an appropriate for area comparisons, and select a suitable local projected CRS for local distance or engineering work. Geographic coordinates may be suitable for displaying a global location. Check the CRS name or authority code, area of use, units, and —not only whether coordinates appear on screen.
Two GIS operations that are easy to confuse have different effects:
Define or assign a CRS: tells the software what the existing coordinate values mean; it does not change those values. Assigning the wrong CRS can place data incorrectly.
Transform or project data: calculates new coordinate values in a target CRS. If the source and target datums differ, an appropriate may also be needed.
GIS software can often display layers in a common map CRS by transforming them on the fly. This is convenient for viewing, but does not necessarily alter stored data. For consistent editing, measurements, or analysis, check that the data use suitable CRSs and that any required is explicitly selected.