To edit favorite base coordinate systems, first launch the **Base
Coordinate System** dialog by clicking the datum button in the **Custom** tab of the **Coordinate System** dialog.

The resulting menu presents a list of favorites, with WGS84 being a
built-in favorite, and the option to choose **Edit
Favorites** to launch the **Favorite
Base Coordinate Systems** dialog. The dialog allows one-click
choice of a frequently used **Base
Coordinate System**, also known as **ellipsoid**, that
has been added to our favorites list.

This dialog is so similar in operation to the **Favorite Coordinate Systems** dialog
that reading the example for that topic covers it as well. See
the **Example:
Adding a Favorite Coordinate System ** topic for use.

**Synonyms** - Cartographers
favor the term **projection** while
programmers seem to prefer **coordinate
system**. This documentation uses the two terms interchangeably,
with the term **projection** tending
to be used more in GIS or display contexts and the term **coordinate
system **tending to be used more when discussing programming, SQL
or standards.

**Bases are Basic** - All coordinate
systems are based upon a model of the Earth's sphere or **ellipsoid** that specifies
the size and shape of the Earth using various parameters such as radius,
eccentricity, center of rotation and so on. Such models have
usually been referred to by cartographers and GIS people as the **ellipsoid**
or **datum **but the more popular
term among computer people now is becoming the **base**,
short for **base coordinate system**.
Manifold tends to use the terms **base**,
**base coordinate system**, **ellipsoid** and **datum**
as interchangeable synonyms since that is how most people working with
spatial data know the terms.

All spatial data in any projection, including Latitude / Longitude,
assumes some **base** even if the
base is not explicitly specified as is often the case with data where
latitude and longitude numbers specify a location. If precision
is required it is important to know what **base**
is assumed because different **bases**
used with exactly the same type of coordinate system and exactly the same
numeric data can result in differences of hundreds of meters in the position
of a location.

We might not care about what **base**
was used if we are creating maps that display all of Europe where it does
not matter if the dots that represent cities vary in position by a few
hundred meters, but in other applications such as guiding an emergency
medical response vehicle to the correct entry portal for a hospital and
not into water in an adjacent lake, or determining whether a specific
real estate parcel falls within a special planning zone or taxation zone,
a few hundred meters can matter very much. See the **Latitude and Longitude
are Not Enough** topic for a visual example of how varying bases
can move the position of exactly the same coordinates.

**Assign Initial Coordinate
System**

**Example:
Assign Initial Coordinate System - **Use the **Contents pane** to manually assign
an initial **coordinate
system** when importing from a format that does not specify the
coordinate system.

**Example:
Change Projection of an Image** - Use the **Change Coordinate System**
command to change the projection of an image, raster data showing terrain
elevations in a region of Florida, from Latitude / Longitude to Orthographic
centered on Florida.

**Example:
Adding a Favorite Coordinate System** - Step by step example
showing how to add a frequently used coordinate system to the Favorites
system.

**Re-Projection
Creates a New Image** - Why changing the projection of an image
creates a new image.

Latitude and Longitude are Not Enough