(This functionality is available
with the Transformations module)
LISCAD supports two types of transformations. One is used to transform the complete file from one projection system to another. The other is used to transform selected points in a file from one co-ordinate system to another.
Projection Transformations
Projection transformations can either be transformed directly from one projection to another provided that the projections share the same ellipsoid parameters, or the user can establish a transformation scheme to nominate the type of transformation technique to use between the nominated systems.
3 Parameter Shift
Applies a direct shift to a set of points based on their co-ordinate type. This transformation is generally considered to be the least accurate.
Molodensky 3 Parameter Shift
Molodensky's transformation method uses an average origin shift (at the centre of the Earth) and the change in the parameters of the two ellipsoids. Our transformation does not use the full version of this formula but the "Abridged Molodensky formula". This method is often used in devices such as hand-held GPS receivers since it provides a simple transformation with medium accuracy.
Bursa-Wolf
This is a transformation of Earth centred Cartesian co-ordinates between two systems. The transformation is defined with a scale, three rotations and three translations.
Bursa-Wolf and Molodensky-Badekas are generally considered to be the next accurate transformation after the grid file.
Note: A Bursa Wolf transformation is the same as a 7-parameter Similarity transformation except the rotations are reversed.
Molodensky-Badekas
This is a transformation of Earth centred Cartesian co-ordinates between two systems like the Bursa-Wolf except the co-ordinates of both systems are first shifted into a common system via centroidal co-ordinates. The transformation is defined with a scale, three rotations, three translations and a centroid co-ordinate.
Bursa Wolf and Molodensky-Badekas are generally considered to be the next accurate transformation after the grid file.
Note: A Molodensky-Badekas transformation can also be defined with reversed rotations like the Bursa-Wolf.
Grid File
A file containing a grid of co-ordinate shift values from which transformations between two datums can be interpolated. The interpolated shifts are then applied to the original co-ordinates to obtain the transformed co-ordinates. This transformation is generally considered to be the most accurate as it can model any distortions between the two systems in the transformation.
Grid files are provided in two formats, National Transformation version 2 (NTv2) or North American Datum Conversion (NADCON).
NTv2 format has a single file with the ".gsb" extension.
NADCON format has two files, one each for the latitude and longitude shifts, with ".las" and ".los" extensions respectively.
General Rules:
Grid File: High accuracy
Bursa Wolf or Molodensky-Badekas: Medium accuracy
Molodensky Shift: Low accuracy
Shift: Lowest accuracy
This is only a general rule and any of the transformations could provide a high accuracy if used appropriately. e.g. If a shift was calculated for a small area.
Co-ordinate Transformations
Co-ordinate transformations are performed by the user selecting control points in both the Control system (the co-ordinate system transforming to) and the equivalent points in the Local system (the co-ordinate system transforming from). Once the co-ordinate transformation control has been defined it is then possible to select the type of transformation to perform.
Conformal / 3D Conformal
The conformal transformation maintains orthogonality between the X and Y axes during the transformation. This can be performed either as a 2D or 3D transformation.
Semi-Affine
The semi-affine transformation compensates for differences in scale between the X and Y axes when comparing the local system against the control system. It is typically used to remove any stretch or shrinkage along either the x or Y axis.
Affine
The Affine transformation compensates for scale differences in the X and Y axes as per the semi-affine, but also compensates for any difference in orthogonality between the local and control systems.
Unscaled / 3D Unscaled
The unscaled transformation is essentially the same as the conformal transformation but the scale factor in the local system is maintained at 1.0. This results in a shift and rotation being applied to the local co-ordinates.