﻿ Favorite Base Coordinate Systems

# Favorite Base Coordinate Systems

Called by the star, Favorites button in the Coordinate System dialog's Custom tab,, the Favorite Base Coordinate Systems 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.

## Notes

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.

Coordinates

Projections

Change Projection

Initial Projection

Map Projection

Coordinate System

Base Coordinate System

Favorite Coordinate Systems

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.