The Base Coordinate System dialog is called by clicking the Base picker button in the Custom tab of the Coordinate System dialog and choosing More... from the dropdown menu.
A base coordinate system is also known as a geodetic coordinate system, in that it is a coordinate system using latitude and longitude coordinates that incorporates both a precise description of the Earth ellipsoid used (also known as the datum) as well as the transformation method to convert coordinates in that specific geodetic system to the reference standard in Manifold, the World Geodetic 1984 (WGS84) coordinate system.
(Favorites List) |
A list of favorite base coordinate systems. The factory default favorite is World Geodetic 1984 (WGS 84), which is also the reference standard for conversions between base (geodetic) coordinate systems. |
More... |
Launch the Base Coordinate System dialog that allows a choice from a vast number of systems or specifying a custom system. |
Favorites |
Add, delete, or modify favorite base coordinate systems |
Copy |
Copy the current base coordinate system to the Windows Clipboard. |
Paste |
Paste a base coordinate system from the Windows Clipboard. |
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. Many EPSG coordinate systems will also appear in the Standard tab, using whatever names came into use for those systems before rigorous standardization by EPSG.
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 coordinate systems from the EPSG Geodetic Parameter Dataset published by IOGP. Using EPSG nomenclature, the Base Coordinate System dialog's EPSG tab lists what EPSG calls geodetic coordinate systems. Coordinate systems that EPSG calls projected coordinate systems are listed in the Coordinate System dialog's EPSG tab. |
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 WGS is 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. EPSG refers to base coordinate systems as geodetic coordinate systems. For projected coordinate systems in the EPSG system, use the EPSG tab in the Coordinate System dialog. Each base coordinate system 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 for base coordinate systems, including those (marked with a red ! message icon) that have been deprecated or otherwise are discouraged by the EPSG system. Using EPSG nomenclature, the Base Coordinate System dialog's EPSG tab lists what EPSG calls geodetic coordinate systems. Coordinate systems that EPSG calls projected coordinate systems are listed in the Coordinate System dialog's EPSG tab.
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.
Although in practical use there is very little difference between a base coordinate system and the datum used by that base coordinate system, the EPSG system provides different code numbers for a datum and for the base coordinate system using that datum. This can lead to confusion if we are seeking to find a particular base coordinate system by using the name or number of an EPSG datum. For example, a datum used in 1948 for the United Arab Emirates is listed in the EPSG system as code 6303 with the name Trucial Coast 1948. The base coordinate system that uses that datum is listed as code 4303 and is named TC(1948).
The Base Coordinate System dialog lists EPSG geodetic coordinate systems by EPSG code. We can search in the EPSG tab either by 4303 or by TC and we will find the base coordinate system. However, if we search the EPSG tab by 6303 or by Trucial we will not find an EPSG listing, because EPSG code 6303 is a code for a datum and not for a geodetic (base) coordinate system that uses that datum.
The Base Coordinate System dialog does not have a separate sub-dialog to search for EPSG datums, since a datum can only be used within some specific coordinate system. If we want to invent a new, custom system using an EPSG datum, we must enter that datum's parameters into the Base Coordinate System dialog's Custom tab.
To add to confusion about names, the Standard tab does list a Trucial Coast 1948 base coordinate system, which is just a traditional (that is, non-standardized) name for what EPSG calls TC(1948). The exact same coordinate system will often be found both under the Standard tab, using a traditional name, and under the EPSG tab, using EPSG's standardized code and name.
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.
The usual way of working with NADCON (including HARN and HPGN), NADCON 5, and NTv2 grid transformations in Manifold is to convert between EPSG-named coordinate systems in the Reproject Component dialog, and to choose as the Conversion method in that dialog a grid-based conversion method, if it is available for the reprojection desired. In Manifold dialogs, NADCON (North American Datum CONversion) automatically includes use of HARN (High Accuracy Reference Network) and HPGN (High Precision Grid Networks).
grids.dat
file required - The option to use NADCON 5, NADCON (including HARN
and HPGN), and NTv2 grid transformations, if applicable, will be available
and can be used in the Conversion
setting of the Reproject
Component dialog and when setting NADCON 5, NADCON, and NTv2
base coordinate system transformation methods only if we have installed
the optional grids.dat file or
the equivalent required grids as individual files. The grids.dat
file may be downloaded for free from Manifold's Product
Downloads web page, and may be used with either Manifold System
or the free Manifold Viewer.
The grids.dat file provides in compressed form over 170 grid files, in .gsb, .las, and .los format, plus NADCON 5 grids, for use in NADCON 5, NADCON (including HARN and HPGN), and NTv2 grid-based transformations. Grid files are for Australia, Austria, Brazil, Canada, France, Germany, Ireland, Japan, Netherlands, New Zealand, Portugal, Spain, Switzerland, the United Kingdom, and the United States.
The Base Coordinate System dialog also allows us to choose NADCON, NADCON5, or NTv2 as the transformation method for converting to the WGS84 base coordinate system. That allows us to utilize those three grid transformation methods in a fully custom way, utilizing whatever grid transformation files we want.
When we choose NADCON, NADCON5, or NTv2, we can name a transformation grid file that is in the grids.dat file or, if we do not have the grids.dat file installed or if we wish to use a transformation grid file not in the grids.dat collection, we can browse to and use a standalone grid file in .gsb, .las, or .los format. If we name a grid in the grids.dat file, and either the grids.dat file is not in place, or if the named grid file does not exist in the grids.dat collection, the transformation will fail when we attempt to use it, with the reason for failure written to the Log.
Choosing NTv2 requires us to specify a Grid file in .gsb file format. We can name a transformation grid in the grids.dat file, or we can click the picker button and browse to the grid file desired.
In the illustration above, we have named a specific grid file, bc_27_05.gsb, that we know is in the grid.dat collection. See the Notes below on how to get a list of .gsb files available within the grids.dat collection.
If instead of using files within the grids.dat collection we prefer to use a .gsb file not in the grids.dat collection, we click the picker button to launch a browse session, allowing us to browse our computer to find and pick a specific .gsb file.
Choosing NADCON requires us to pick two grid files, one for Latitude shifts in .las format and one for Longitude shifts in .los format.
In the illustration above, we have named a specific .las and .los grid files, nyhpgn.las and nyhpgn.los, that we know are in the grid.dat collection. See the Notes below on how to get a list of .las and .los grid files available within the grids.dat collection.
If instead of using files within the grids.dat collection we prefer to use .las and .los grid files not in the grids.dat collection, we click the picker button to launch a browse session, allowing us to browse our computer to find and pick a .las file and a .los file.
NADCON .las
files are not the same as
LAS
format files used for LiDAR data.
It is usually easy to tell if we have a NADCON .las
file and not a LiDAR .las
data file, since NADCON .las
latitude shift files are accompanied by like-named NADCON
.los longitude shift files.
Choosing NADCON5 requires us to pick two grid files, one for Latitude shifts and one for Longitude shifts, plus optionally a third grid file for Height shifts if the latitude and longitude grid files are accompanied by a height grid file.
NADCON 5 grid files have very long names. For example, the EPSG 8559 NADCON 5 transformation often used for CONUS (lower US 48 states) work uses the following file names:
Latitude grid: nadcon5.nad83_2007.nad83_2011.conus.lat.trn.20160901.b
Longitude grid: nadcon5.nad83_2007.nad83_2011.conus.lon.trn.20160901.b
Height grid: nadcon5.nad83_2007.nad83_2011.conus.eht.trn.20160901.b
See the Notes below on how to get a list of NADCON5 grid file names available within the grids.dat collection.
If instead of using NADCON 5 grid files within the grids.dat collection we prefer to use NADCON 5 grid files not in the grids.dat collection that we have on hand, we can click the picker button to launch a browse session, allowing us to browse our computer to find and pick the desired files.
Individual grid files we choose
using the browse buttons must
be located within the Manifold installation folder hierarchy, specifically,
within the extras folder,
for Manifold to be able to use them. The standard Windows
dialog used for browsing will allow us to pick a name outside
of the Manifold installation folder hierarchy, but that is only to choose
the name of the file. The transformation will use that name
to try to find a grid file of that name within
the extras folder. Looking
for grid file names only within the Manifold folder hierarchy is essential
to keeping portable installations portable, or otherwise a custom definition
of the base coordinate system using grid files would be tied to the folder
structure of the machine, and not just to the portable installation.
At the present time the browse button allows us to browse our computer for individual grid files, but it does not allow browsing within the grids.dat file. Future builds will add that capability.
We often want to reproject a component using exactly the same base coordinate system used by some other component. That is easy to do with Copy and Paste. In what follows, the source component means the one with the base coordinate system we want to copy, and the target component is the one we want to reproject to use that base coordinate system.
In the Custom
tab of the Coordinate
System dialog for the source component, click the datum button
in the Custom tab of the Coordinate
System dialog and choose Copy.
That copies the base coordinate system in text form into the Windows
Clipboard.
Next, in the Custom
tab of the Coordinate
System dialog for the target component, click the datum button
in the Custom tab of the Coordinate
System dialog and choose Paste.
The copied base coordinate system will be pasted.
SRIDs from databases: When copying base coordinate systems from components that reside within DBMS data sources, SRID:xxx definitions that only make sense in the context of a specific database are converted to JSON text representation prior to copying. The JSON text is more broadly understandable and reusable in other settings.
Using Copy is also great way to get a text version of the base coordinate system used by a component. For example, we can paste it into a Comments component or into Notepad to have a text record of all details of the base coordinate system in use. If we want to compare two different base coordinate systems that seem similar, we can Copy each of them and then Paste into a Comments component or into Notepad as ordinary text, to facilitate comparison of all details.
Many coordinate systems use the World Geodetic 1984 (WGS84) base coordinate system, another name for the default Latitude / Longitude base coordinate system. The Prime longitude parameter specifies the Prime Meridian of the coordinate system.
The default setting of 0 is what typical GIS Latitude / Longitude projections use, where the Prime Meridian runs through Greenwich, England, and positive longitude numbers from 0 to 180 indicate longitude positions to the East of Greenwich, while negative longitude numbers from 0 to -180 indicate longitude positions to the West of Greenwich. The westward "edge" of this projection, the -180 longitude line, runs from the Aleutian Islands chain in the Northern hemisphere down to just off the Eastern coast of New Zealand's North Island in the Southern hemisphere.
The standard setting is designed to work with drawings and images that use modern, standard, Latitude / Longitude coordinate systems, where longitude ranges from -180 to 180, and latitude ranges from -90 to 90. However, we often encounter data sets that use Latitude / Longitude coordinate systems with longitudes ranging from 0 to 360. One such example are the images used in the Example: Convert a 0 to 360 Degree Projection topic, where the images are in a Latitude / Longitude coordinate system where longitude values range from 0 to 360. That example shows how to use the Coordinate System Metrics dialog to assign an offset that shifts the image 180 degrees.
The illustration above shows a typical image in Latitude / Longitude projection that uses a 0 to 360 range for longitudes. The left edge of the image, with values of 0 longitude, when placed in a drawing with a Bing background ends up having the left edge of the image aligned to the 0 longitude line, that is, the Prime Meridian line running through Greenwich, England, that is the central longitude of GIS-standard Latitude / Longitude projections. The image shows a photorealistic atlas of the world as it probably looked during the last glacial maximum.
In the illustration above, we have overlaid a World layer of country outlines, to better illustrate how the image is shifted 180 longitude degrees Eastward, that is, to the right. We can fix that by using Repair Initial Coordinate System to drill down into the Base Coordinate System dialog for that image, and to alter the Prime longitude setting from default 0 to -180.
Changing the Prime longitude to -180 is specifying that the prime longitude, what this data set calls zero longitude, is in fact located in the GIS-standard Latitude / Longitude coordinate system at the -180 longitude. If we make that change and then press OK upward through the dialogs, the image will shift into correct georegistration.
We can turn off the bright green country outlines to better see what the world looked like during the last glacial maximum, when sea levels were so low that a land bridge connected Eurasia to North America. Ireland, England, and much of Indonesia were part of the Eurasian continent, and Australia and New Guinea were joined by dry land.
Note that we used Repair Initial Coordinate System, so what we have done is not a reprojection of the image, but just simply assigning the correct, initial values for the coordinate system. That is normally how a nonzero setting for Prime longitude is used, and not as part of a reprojection using Reproject Component.
Assigning a -180 Prime longitude is equivalent to assigning a -180 Local X offset as done in the Example: Convert a 0 to 360 Degree Projection topic. Images frequently use custom local offsets and scales. In the case of the example topic, assigning the correct coordinate system required changing not just the local X offset but also the local Y offset, and also local X and Y scales as well. When working with drawings, it is usually simpler to avoid altering offsets and scales, if possible, and instead to deal with 0 to 360 degree longitude systems by altering Prime longitude to -180.
What grids are in the grids.dat file? - See the SQL Example: List Transformation Grids topic.
Grid files are cached - For better performance, Manifold caches any grid files used by coordinate transforms. The cache is effectively unlimited in 64-bit Manifold.
Deprecated transformation methods - Manifold maintains an internal database of transformations methods for base coordinate systems that includes the EPSG collection of transformation methods. EPSG marks some transformation methods as deprecated, usually because data submitted to EPSG for a transformation was later found to be incorrect, or because there is a newer version which is better in some way. Manifold will not use deprecated EPSG transforms for default transformation method.
Synonyms - Cartographers seem to 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 in some way, through some conversions, upon a latitude and longitude coordinate system that incorporates 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, because in addition to detailed specification of the Earth ellipsoid a base coordinate system also specifies the conversion method to use to convert coordinates from that base coordinate system into the reference standard base coordinate system used by Manifold, the World Geodetic 1984 (WGS84) 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, but that is a sloppy habit since a base coordinate system is not exactly the same thing as an ellipsoid or datum. An ellipsoid or datum is a specific description of the Earth shape being used, typically involving thirteen different parameters. A base coordinate system uses that ellipsoid within a coordinate system using latitude and longitude degrees as units of measure, and also includes what conversion method should be used to convert coordinate numbers in that base to coordinate numbers in the World Geodetic 1984 (WGS84) reference standard base coordinate system. EPSG, for example, assigns different codes to a given datum as well as to a base coordinate system that uses that datum.
All spatial data in any projection, including Latitude / Longitude, assumes some base coordinate system even if the base is not explicitly specified as is often the case with data when latitude and longitude numbers are used 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 an entire continent, for example, 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
Repair Initial Coordinate System
Favorite Base Coordinate Systems
Example: Convert a 0 to 360 Degree Projection - We often encounter data, both images and drawings, using latitude and longitude degrees that appears to be in Latitude / Longitude projection but which has longitude values from 0 degrees to 360 degrees and latitude values from 0 degrees to 180 degrees, instead of the usual arrangement of -180 degrees to 180 degrees for longitude centered on the Prime Meridian, and -90 degrees to 90 degrees for latitude centered on the Equator. This example shows how to utilize such data by assigning the correct projection.
Example: Assign Initial Coordinate System - Use the Info pane Component tab 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 Reproject Component 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.
SQL Example: List Transformation Grids - In this example we use a few snippets of easy SQL to list NTv2 and NADCON transformation grids that are available within the grids.dat compressed collection of transformation grid files. Grid transformation files can be used when creating custom base coordinate systems, for NADCON / HARN / HPGN and NTv2 high accuracy transformations that use grid files.
Reprojection Creates a New Image - Why changing the projection of an image creates a new image.
Latitude and Longitude are Not Enough