which is which?

Of course, my way is correct and everyone else is wrong! :-)

Dr. Tomas Soler of the U.S. National Geodetic Survey gave an elaborate

exposition in Bulletin Geodesique regarding the left-handed vs.

right-handed rotations of the various transforms. Apparently, somebody

misunderstood the distinction in Paris, and ergo, the "European" way of

doing things. All of this became possible because of the American SECOR,

then TRANSIT, and currently NAVSTAR (GPS)geodetic satellite systems. If

the Americans that invented the contraptions and put the stuff into orbit

for everyone else to get access to define it a particular way ... guess

which way is correct?

Whether it's infinitessimal rotations with arc seconds, radians, or

whatever ... there's two basic 7-parameter datum transformations - the

Molodensky that uses the geocentric coordinates of the datum origin and the

Bursa-Wolfe that does not use the datum origin. The sign of the rotations

is right-handed as the Americans do it or left-handed as the continental

Europeans do it.

As long as a test point is published, it's pretty easy to distinguish what

the sign of the rotations are. Without a test point, you're going to have

to rely on the nomenclature whether you like it or not.

C. Mugnier

LSU

>

>

>

>

>

>

>

>

....

>

>

>

>

>

Probably the best way to document a rotation method is: use the accepted

terms "coordinate frame" and "position vector".

These terms are also used in other disciplines like kinematics (robotics).

But there are more difficulties.

One datum transformation method is laid down in the ISO 19111 standard.

This

is an approximated 7-parameter Helmert transformation with position vector

rotation. See also ISO/IEC 18026 - Annex B.

PROJ uses about the same method, only the scaling method differs. PROJ does

a scalar multiplication, ISO a matrix multiplication.

With the commonly used parameter ranges, the differences between the

scaling

methods are less than microns, so not important.

As far as I know, the Bursa-Wolfe transform is an approximation to the

Helmert transform. The Helmert transform has sines and cosines in the

rotation matrices, whereas Bursa-Wolfe (and ISO 19111) use the angles

themselves (since sin(a) ~ a, sin(a)*sin(b) ~ 0, and cos(a) ~ 1 for small

angles).

If you read section B.6 of ISO/IEC 18026, then you'll notice that a

Bursa-Wolfe transform can be done with a position vector rotation model OR

with a coordinate frame rotation model. Just what one likes the best; be

sure to use the correct sign of the rotation angles. Therefore: Bursa-Wolfe

is NOT equivalent with this or that rotation model.

A well known expert repeatedly states that the Australians use the same

datum transform rotation model as the Americans.

This is NOT true!

The order of the rotations differs (XYZ vs. ZYX). See the Australian GDA

Technical Manual. By the way, if the rotations are approximated, then the

order is not important.

Again, the differences in rotation order for real life numbers are

literally

microscopic.

This, and your (mr. Rittri) remarks merely demonstrate how silly it is to

refer to a datum transformation method as an American, Australian,

European,

whatever regional model.

If you want to document the way for instance an application transforms,

give

the complete formulae, not just an ill-defined name.

Why not referring to the EPSG coordinate transformation method numbers?

These clearly define the most used datum transformation methods and

projection methods.

_______________________________________________

Proj mailing list

[email protected]

http://lists.maptools.org/mailman/listinfo/proj

_______________________________________________

Proj mailing list

[email protected]

http://lists.maptools.org/mailman/listinfo/proj

Programming list archiving by: Enterprise Git Hosting