Drawings and images are reprojected from a current coordinate system into a new coordinate system using mathematical formulae, called conversion methods, which vary depending upon the starting and ending coordinate systems. Conversion methods also are called paths. In most cases of the millions of different starting and ending combination possible within Manifold, only a single, standard conversion method is available and is used by default. In some cases, however, more than one conversion method can be used.
Manifold includes many transformation methods for converting one coordinate system into a different coordinate system. Most such methods use mathematical formulae for the recalculation and do not need any other data. However, three transformation methods, NADCON 5, NADCON (including HARN and HPGN), and NTv2 transformations, in addition to mathematical formulae also use data files that provide parameters for specific areas. Those data files are called grid files. Transformation methods that use grid files provide enhanced accuracy in the regions covered by the grid files.
Most transformation methods convert a coordinate system based on whatever Earth ellipsoid is used within the source and target coordinate system. However, the Earth is sufficiently lumpy that in many regions the smooth surface of an Earth ellipsoid does not model the actual surface of the Earth in that region with sufficient accuracy to allow coordinate system recalculations at the accuracy desired. In such regions, grid files containing an array of numbers that describe the uneven surface of the Earth in the region can be used with NADCON, NADCON 5, or NTv2 transformations to provide higher accuracy conversions from one coordinate system into a different coordinate system.
Grid files contain numbers that describe the lumpy surface of the Earth in a specific region, so a grid file for one region cannot be used for a different region. Grid files are also designed for use with a specific transformation type, either NADCON, NADCON 5, or NTv2. Grid files created for use with NADCON transformations, for example, cannot be used with NTv2 transformations. NADCON (also including HARN and HPGN) and NADCON 5 are used primarily in the United States. Many countries outside the United States use NTv2. Manifold supports all three, NADCON 5, NADCON, and NTv2, to enable use of all grids worldwide.
Many grids have been published by governments for use in various countries, regions, provinces, and even for local settings such as counties and even individual cities. Grids are not always easy to find, so Manifold has assembled a collection of grids within a single file, the grids.dat file, which covers all grids known to the EPSG database. 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 grids.dat file is provided as a separate file since it is approximately 380 MB in size: if we are not working with projections in a country that uses grid transformations, we can skip downloading the file and keep our Manifold installation significantly smaller.
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 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 the Product
Downloads web page. Place the grids.dat
file in the ~\extras folder of
the Manifold installation.
In addition to grids known to the EPSG system that are published within the grids.dat file, there are grids with such niche constituencies that they do not appear even in the exhaustive EPSG database. Occasionally new grids pop up as local governments publish new grids. When grid files are published in standard formats like .LAS / .LOS for NADCON and .GSB for NTv2, Manifold can use those when they are placed in the ~\extras folder of the Manifold installation.
When we use a NADCON (including HARN and HPGN), NADCON 5, or NTv2 conversion method, Manifold searches for the required grid file by name, checking first within the grids.dat file if that has been installed. Manifold also checks for individual grid files by name, using .LAS and .LOS extensions for NADCON grid files and .GSB extension for NTv2 grid files. If an individual grid file is found using the name required by the chosen conversion path, Manifold will use that grid file instead of the version in the grids.dat file. That allows us to use a newer or alternate version of a grid file that has been published within the grids.dat file.
When Manifold looks for required grid files, if the files are not found or if they cannot be loaded for any reason (such as lack of permissions, corrupted data, access prevented by an antivirus package, etc.), the transformation logs the name of the file that failed to load into the Log Window and returns NULL values for the coordinates.
Using individual grid files, that is, not using the collection within the grids.dat file, is also a way to save space. If we only use NADCON, NADCON 5 or NTv2 transformations in one specific region that requires only a single grid file, if we have that grid file we can put it in the in the ~\extras folder of our Manifold installation and skip downloading the entire grids.dat file. As a practical matter, a 380 MB file is no big deal in GIS, so most users will simply download the grids.dat file for the convenience of not having independently to find and to download individual grid files.
See the Product Downloads web page for download and installation instructions for the grids.dat file.
The
Conversion button allows us to choose a non-default conversion method,
if available, for recalculating the reprojection from the Current
to the New coordinate system.
If the current and new coordinate systems are the same, or if they
are the same except for coordinate system metrics and/or axes ordering
(XY or YX), the conversion path will be shown as default
and cannot be altered. In such cases a short description will appear
stating why default is the only
choice.
In most cases, the conversion picker
button will be disabled, since alternate conversion methods are generally
defined for a limited set of combinations from Current
to New coordinate system. In
most cases, only coordinate systems defined in the EPSG system will have
conversion paths other than the default. Conversion paths that use NADCON
5, NADCON (including HARN and HPGN), and NTv2 transformations are available
only between Current and New coordinate systems that are
both specified using EPSG codes.
One such combination is reprojecting a component from a Current coordinate system of MGI (EPSG: 4312) to a New coordinate system of ETRS89 (EPSG:4258). Both of these coordinate systems are used in Austria. For that conversion path, alternatives to the default conversion are available and the conversion picker button will be enabled.
Press the
conversion picker button to launch the Conversion
dialog, to see alternatives.
The Conversion dialog shows that for this particular path there are five options in addition to the default conversion method. The numbers in the right hand column show the accuracy provided by the different conversion methods. Smaller numbers indicate better accuracy.
Conversion paths marked with a red message
icon have been deprecated or otherwise are discouraged by the EPSG system.
In the illustration above, two of the options are marked with a
red message icon to indicate they have been deprecated in the EPSG system.
Click on a conversion path in the list to see the definition of that conversion path in the lower panel of the dialog. In the illustration above, we have clicked on one of the deprecated conversion paths to see the definition for that path. We can see from the comments in the definition that it has been replaced by the EPSG 3914 conversion method.
The EPSG 3914 conversion method does not appear in the list because it is not a usable conversion path from the Current to New coordinate systems currently selected. This is a fairly typical situation for deprecated paths, where the path was deprecated because it contained an error, for example, in the datums used.. When the error is corrected, the new path, with new EPSG number, is no longer applicable because one or both of the corrected datums are no longer the same as used in the Current or New coordinate systems.
Click on the NTv2 method to see the grid file it uses.
To use a particular method, we click on it to choose it and then we press OK.
The conversion method then appears in the Reproject Component dialog. Pressing Update Component will reproject the component from the Current to the New coordinate system using the Conversion method specified.
Choosing a different coordinate system for the New coordinate system will preserve whatever was the previously selected conversion path, if it is still available for the new Current to New pair.
Coordinate Systems / Projections
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