The Base Coordinate System dialog is called from the datum button in the Custom tab of the Coordinate System dialog.
Choose the Edit Base Coordinate System option to launch the dialog. If we like, we can choose from the Favorites list presented in the menu, with WGS84 being a built-in favorite, or choose Edit Favorites to launch the Favorite Base Coordinate Systems dialog.
The Base Coordinate System dialog allows us to choose a base coordinate system from three options:
Standard - A long list of base coordinate systems known by text names, either as a result of well-established tradition in cartographic circles or by formal government or standards group designation in various countries.
EPSG - The gold standard. A comprehensive list of base coordinate systems from the EPSG Geodetic Parameter Dataset published by IOGP. EPSG base coordinate systems are precisely, unambiguously defined with a level of care unprecedented in international cartographic and geodesic practice. EPSG base coordinate systems are known formally by their EPSG codes but also have a text name to provide easier discussions in more casual settings.
Custom - The ability to define a custom base coordinate system starting with a transformation method taken from a list of configurable transformation methods and providing custom parameters allowed by that transformation method.
The Base Coordinate System dialog will launch with the current base coordinate system in use automatically selected in the lists of known systems, with the parameters of the current base coordinate system loaded into controls in the Custom tab. The tab selected when the dialog launches will be the tab containing the list of systems within which the current system is listed. If the starting system is not found in any list, the dialog launches with the Custom tab.
Standard |
A long list of base coordinate systems known by their names, either by well-established tradition or by government designation in various countries. |
EPSG |
A comprehensive list of base coordinate systems from the EPSG Geodetic Parameter Dataset published by IOGP. |
Custom |
Specify a custom base coordinate system by choosing a standard transformation method from a list of configurable transformation methods and then providing parameters for the custom base. |
(Filter Box) |
Our best friend when sifting through long lists. Enter text, such as mercator into the filter box and only those bases which include that text in their names will be displayed. |
(list pane) |
Click on a base coordinate system in the list to select it. |
(lower pane) |
The dialog opens with the current base coordinate system displayed in JSON format in the lower pane, as seen in the illustration above. Details for a chosen base coordinate system will be displayed in JSON format in the lower pane, as seen below. |
The Filter box provides much-needed help when trying to find a desired base coordinate system in a very long list. The dialog displays only those bases which contain in their names the text entered into the Filter box. If WGSis entered into the Filter box the list will display only those base coordinate systems with WGS in their names.
The EPSG tab provides a long list of base coordinate systems from the EPSG Geodetic Parameter Dataset published by IOGP. Each has a text name and the official EPSG code in parentheses. The best way to find a desired EPSG base is to use the Filter box to find it by entering the EPSG code into the Filter box.
The EPSG tab includes all EPSG codes, including those (marked with a red ! message icon) that have been deprecated or otherwise are discouraged by the EPSG system.
Clicking on a base coordinate system will select it by highlighting it and will display details in the lower pane. Text comments from the EPSG database are provided as ordinary text while the base coordinate system definition is reported in JSON format. To understand EPSG commentary, consult documentation published by IOGP.
Manifold provides the ability to create a custom base coordinate system by choosing a standard transformation method from a list of configurable transformation methods and then providing parameters for the custom base. Most custom base coordinate systems are simply variations on a relatively limited list of frequently utilized transformation methods so this approach can cover a very wide range of possibilities should a base coordinate system be required that is not included in the many EPSG codes plus standard base coordinate systems.
A common use of Custom is usually to specify a base coordinate system for planetary objects like Mars, the Moon and so forth, usually treated as spheres, that is, with Eccentricity of zero.
Provide a Name for the new system.
In the To WGS84 box choose a transformation method
Specify desired parameters. Parameter boxes will be displayed as options for the selected transformation method.
Press OK.
Name |
Choose something more useful and self-documenting than the default of "Custom Base Coordinate System." That will help remind us what we did should we use this project or data years later. |
To WGS84 |
A list of configurable transformation methods, which represent the mathematical transformation to be used to transform the particular base into WGS84. Choosing one of those will configure the parameter boxes to provide allowed options. |
(Parameter boxes) |
Configuration parameters allowed by the selected transformation method.
When editing parameters for a custom base coordinate system, the system will detect invalid parameter values and will suggest correcting them before the dialog is closed. |
Custom base coordinate systems are created by specifying custom parameters for a configurable transformation method. Manifold provides a list of standard transformation methods in the To WGS84 box. To choose one of those we click on the down arrow icon at the right of the box.
Doing so opens up the list of available configurable transformation methods. We choose a method by clicking on it to highlight it.
Above we have selected Polynomial (6) as the To WGS84 transformation method. Once we choose a configurable transformation method we can customize the base coordinate system by specifying parameters of interest for that transformation method.
Manifold will automatically provide option boxes for parameters that may be customized for a particular system. Option boxes have indicators what units of measure are used, for example, m for meters and deg for degrees. Enter values to customize the transformation method to create the specific custom base coordinate system desired.
When a starting transformation method provides too many options to fit at once into the display a scroll bar will appear to allow us to scroll through all of the options. The Polynomial (6) transformation method uses almost two pages of configurable parameters.
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
Favorite Base 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.
Example: Detecting and Correcting a Wrong Projection - A lengthy example exploring projection dialogs and a classic projection problem. We save a drawing into projected shapefiles and then show on import how a projection can be quickly and easily checked and corrected if it is wrong.
Re-Projection Creates a New Image - Why changing the projection of an image creates a new image.
Latitude and Longitude are Not Enough