Manifold uses datums in projection calculations. A datum is an ellipsoid that is combined with an offset of the ellipsoid relative to the Earth's center (defined as the center of the WGS84 datum). The definitions of datums available to Manifold are compiled into the product. User-supplied definitions may be loaded from .xml files found in the Config folder. If desired, users can add their own datums to incorporate other datums into Manifold.
Datums are defined by .xml files in the following pattern:
· There can be any number of <datum> … </datum> entries within a single XML file between the <xml> tag at the beginning of the file and the </xml> tag at the end of the file.
· The name string supplied in the <name> attribute is mandatory and must be unique. If the <ellipsoid> tag is used it must contain the name of an existing ellipsoid (user-defined ellipsoids are OK). The <ellipsoid> tag is not mandatory and can be replaced by an alternative specification of the ellipsoid using equivalent sets of tags, such as <majorAxis> and <minorAxis>, <majorAxis> and <eccentricity> or <majorAxis> and <invFlattening> tags.
· When using alternate specifications for ellipsoids, the <majorAxis> tag is mandatory and is the length of the major axis in meters. <invFlattening> is the value of the inverse flattening. The <invFlattening> attribute may be replaced with by the <eccentricity> attribute specifying the eccentricity of the ellipsoid, or by the <minorAxis> attribute giving the length of the minor axis in meters. A value for the minor axis that is greater than the major axis will be rejected.
· <centerX>, <centerY> and <centerZ> tags specify the offsets of the datum center along the X, Y and Z axis with respect to the center of WGS84, and are optional. The offset distances are in meters.
· <rotationX>, <rotationY> and <rotationZ> tags specify the rotation of the datum near the X, Y and Z axis with respect to WGS84, and are optional. The rotation angles are in whatever units are specified by the unit attribute. If no unit is specified, the rotation angles are in degrees.
· <scaleAdjustment> specifies the adjustment of the datum scale relative to WGS84, in units per million. That is, a scale adjustment of 1 means that the scale of the datum is 1.000001 that of WGS84.
To utilize newly created or modified datums place the new or modified .xml file containing the datums into the Config folder for Manifold (normally C:\Program Files\Manifold System Professional\Config) and restart Manifold.
When Manifold launches, the system will scan all .xml files in the Config folder. Any <datum> … </datum> entries found in any of those .xml files will be loaded into the system as available datums.
We've created a file in the Config folder called mydatums.xml that contains a new datum we would like to use with Manifold. The file contains:
When Manifold launches it will see the mydatums.xml file in the Config folder and will read it to see what it contains. It will find a definition for a datum called My Datum that uses an ellipsoid called Manifold 2001. If this ellipsoid is not part of the standard set of ellipsoids in Manifold, it must be defined elsewhere in an <ellipsoid> specification in one of our customizing .xml files.
Manifold cannot use any customizations if the .xml files do not contain XML that is exactly correct. A useful test before launching Manifold is to open any newly created or modified .xml file in Internet Explorer. Internet Explorer will show a correct .xml file in a simple text format. If Internet Explorer cannot parse the .xml file, Manifold won't be able to either.
See the The Earth as an Ellipsoid for more information.