A geodetic feature is one whose measurements account for the inherent distortion of projected space. Geodetic features are useful when you want to create a feature that spans a large distance, such as a flight path across an ocean.

The features you draw in ArcMap are not geodetic (they are planar) unless you create them using either the Construct Geodetic command or one of the following geoprocessing tools: Bearing Distance To Line, Table To Ellipse, or XY To Line. Geodetic features do not account for changes in terrain, though.

By their nature, maps and geospatial data contain distortion. The act of taking a 3D spherical surface, such as the earth, and projecting it to a flat 2D space warps the spatial relationships between locations on the original surface. To complicate matters, the earth is neither perfectly spherical nor perfectly smooth. It is flattened at the poles and it bulges at the equator. Map projections compensate for these irregularities, but they still contain spatial distortion. For more information on projections and coordinate systems, see What are map projections?.

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Geodetic features contain densified geometry, which is a shape created by a series of connected vertices. For example, a densified curve is composed of a succession of points along the entire path of the curve, whereas a Bézier curve is smooth. Densified geometries require more resources to store and manipulate and are more difficult to edit than standard geometries. Editing or moving geodetic features makes them non-geodetic, so if you need to change or move a geodetic feature, re-create it instead.

## Types of geodetic features you can create

• Geodesic line—The shortest line between any two points on the Earth's surface on a spheroid (ellipsoid). One sample use for a geodesic line is when you want to determine the shortest distance between two cities for an airplane's flight path. Another example is the creation of the path between the point of impact and the point of origin of a missile. This is also known as a great circle line if based on a sphere, rather than an ellipsoid. The geodesic line type allows you to create lines only. In addition, you can create a multi-segment line which is a series of geodesic lines that make up a single line feature. You can use a multi-segment line when you want to create an airplane's flight path with waypoints, such as an air route with multiple stops that make up a full route.
• Geodesic circle—A shape whose edge is defined as a particular geodetic distance from a fixed point. Depending on the coordinate system in which it is displayed, it may not appear to be a circle. You might use this if you are creating a range ring of a weapon system, such as to show a weapon's effective range. Geodesic circles can be used to create either lines or polygons.
• Geodesic ellipse—A shape whose sum of geodetic distances from a fixed pair of points is a constant. You could use this to create a signal error ellipse. This is also known as a geodesic circle when the major and minor axes are the same length. The geodesic ellipse type allows you to create lines or polygons.
• Great elliptic—The line on a spheroid (ellipsoid) defined by the intersection at the surface by a plane that passes through the center of the spheroid and the start and end points of a segment. This is also known as a great circle when a sphere is used. The great elliptic type allows you to create lines only.
• Loxodrome—A loxodrome is not the shortest distance between two points, but instead defines the line of constant bearing, or azimuth. Great circle routes are often broken into a series of loxodromes, which simplifies navigation. This is also known as a rhumb line. The loxodrome type allows you to create lines only.

5/6/2011