## Geographic Coordinate Systems (GCS)

Tutorial Contents

### What is the Geographic Coordinate System?

A **Geographic Coordinate System** (GCS) is a system that uses a three-dimensional spherical surface to determine locations on the Earth. The GCS is a spherical or ellipsoidal coordinate system for measuring and communicating positions directly on the Earth at latitude and longitude.

The GCS consists of **latitude **and** longitude** lines. Each line of longitude runs north–south and measures the number of degrees east or west of the prime meridian. Values range from “**-180°**” to “**+180°**”. Lines of latitude run east–west and measure the number of degrees north or south of the Equator. Values range from “**+90°**” at the North Pole to “**-90°**” at the South Pole. The equator is at an angle of 0 degree latitude.

A geographic coordinate system is a method for describing the position of a geographic location on the earth’s surface using spherical measures of latitude and longitude. These are measures of the angles (in degrees) from the center of the earth to a point on the earth’s surface when the earth is modeled as a sphere. When using a spheroid (ellipsoid), latitude is measured, extending a line perpendicular to the earth’s surface to the equatorial plane. Except at the equator or a pole, this line will not intersect the center of the earth.

In the geographic coordinate system, the sphere is divided into equal parts, usually called degrees; some countries use grads. A circle is **360°** or **400** grads. Each degree is subdivided into 60 minutes, with each minute composed of **60** seconds.

### Uses and Applications

Users with global datasets often use geographic coordinates to store and manage their data on a global framework but project the data into a local planar coordinate system for editing and analysis.

**Map projections** also use latitude and longitude values from geographic coordinate systems to reference parameters, such as the central meridian, the standard parallels, and the latitude of origin.

### Geographic Coordinate Systems ArcGIS

**ArcGIS** data frame’s has a default unknown coordinate system. If you change the data frame’s coordinate system, all layers that have coordinate systems will be projected on the fly to the new coordinate system.

#### Specifying GCS in ArcMap

**Steps:**

**1**. Open **ArcMap**, Right-click the data frame **Layers **and choose **Properties **to bring up the Data Frame Properties dialog box.

**2**. Click the **Coordinate System** tab and navigate to the Geographic Coordinate Systems, and select your desired coordinate system for your map display.

3. Finally, click **OK**.

#### Defining a Shapefile’s Coordinate System by specifying a new GCS

**Steps:**

- In
**ArcCatalog**, click the**shapefile**whose coordinate system you want to define. - Click the
**File menu**and click**Properties**. - Click the
**XY Coordinate****System**tab. - Click
**New**and click**Geographic**. - Type a
**name**for the new geographic coordinate system. - Type the appropriate semi-major and semi-minor or inverse flattening values and type a name for your custom spheroid and datum.
- Type the appropriate radians per unit and type a name for your custom units.
- Type the appropriate degrees, minutes, and seconds defining the prime meridian and type a name for this line of longitude
- Click
**OK**. - Then, click
**OK**on the Shapefile Properties dialog box.

#### Convert GCS to PCS in ArcGIS

In this process, first complete the **Georeferencing**, then you can perform to convert **Geographic Coordinate System** to **Projected Coordinate System**.