HelmertRotationSignConvention Enumeration
Specifies the sign convention used for the rotation angles in a HelmertDatumShift.
NuGet/Assembly: Carmenta.Engine.5.16.2.nupkg (in the CECore assembly)
Syntax
public enum HelmertRotationSignConventionValues
| Value name | Value | Description |
|---|---|---|
| CoordinateFrameRotation | 0 | The sign convention used in for example USA, Australia, and some European countries (Sweden, Finland, the Netherlands, Belgium, Luxembourg, Hungary, Slovenia, perhaps more). It is also used in Clifford J. Mugnier's columns on geodetic datums. |
| PositionVectorTransform | 1 | The sign convention that is recommended by ISO 19111 and used by the International Association of Geodesy, by most European countries, by NATO (standard agreement STANAG 2211), by the cs2cs command in the PROJ cartographic library, and usually in Well-Known Text. |
Remarks
A HelmertDatumShift has three rotation parameters RotationX, RotationY and RotationZ, but there are two opposite conventions for their signs: should a positive angle mean clockwise or counterclockwise rotation? In two dimensions, at least, mathematicians usually say that a positive angle goes counterclockwise, while geographers say that a positive angle (for example, an azimuth) goes clockwise. For a rotation in 3D, one must also specify what a clockwise rotation around an axis would mean: are you looking at the rotation from the positive end of the axis, or from the negative end? Actually, we think that the disagreement comes from another question: is it the geodata that are rotating while the coordinate axes are fixed, or vice versa?
Anyway, Carmenta Engine supports both sign conventions (as does EPSG), but you must remember to specify the correct one (the one intended for the rotation angles). Otherwise, the datum shift may be wrong by several hundred meters. But a Helmert datum shift is often published without telling which sign convention is used. To be safe, always check the datum shift on a published test point!
Sometimes, a Helmert datum shift is published without naming the sign convention, but the transformation formula is given explicitly. To determine the sign convention implied by the formula, it is usually best to compare it to the paragraph on the Burša-Wolf formula, see HelmertDatumShift.
Since Carmenta Engine does not use the simplified Burša-Wolf formula, the exact meaning of the sign conventions in Carmenta Engine is specified under HelmertDatumShift.RotationX, HelmertDatumShift.RotationY and HelmertDatumShift.RotationZ.
Trivia: among the 7-parameter Helmert datum shifts published by EPSG, about 45 percent use the Position Vector Transform and 55 percent use the Coordinate Frame Rotation.
About terminology
Sometimes, people try to distinguish the two sign conventions by calling them the left-handed convention and the right-handed one. Although this would be a simpler and more symmetric naming scheme, the trouble is that some British sources (like the Ordnance Survey) claim that it is the Position Vector Transform that is right-handed, while some American sources (like Clifford J. Mugnier) claim the opposite. So it is best to avoid these terms.
The terms American and European are also misleading, since some European countries do use the Coordinate Frame Rotation.
Also, according to some sources, the difference between Helmert and Burša-Wolf is not whether the rotation matrix is linearized (as described in HelmertDatumShift), but is instead the rotation sign convention. But we think this is a misunderstanding, used only in a few texts.
Platforms
Windows, Linux, Android