[Geotiff] GeoTIFF spec - ModelTransformTag is not invertible
Grissom, Ed
ed.grissom at intergraph.com
Fri Feb 2 08:02:29 PST 2007
GeoTIFF'ers
I have an issue with one small part of the spec, that I would like to
get resolved. This deals with the ModelTransformationTag and is in
section 2.6 of the spec.
The example for 'baseline GeoTIFF' is:
==============================================
For Baseline GeoTIFF, the model space is always 2-D, and so the matrix
will have the more limited form:
|- -| |- -| |- -|
| X | | a b 0 d | | I |
| | | | | |
| Y | | e f 0 h | | J |
| | = | | | |
| Z | | 0 0 0 0 | | K |
| | | | | |
| 1 | | 0 0 0 1 | | 1 |
|- -| |- -| |- -|
Values "d" and "h" will often be used to represent translations in X and
Y, and so will not necessarily be zero. All 16 values should be
specified, in all cases. Only the raster-to-model transformation is
defined; if the inverse transformation is required it must be computed
by the client, to the desired accuracy.
==============================================
Note that the main diagonal contains a zero at the intersection of the
third row and the third column. If I am not mistaken, a zero on the
main diagonal means that the matrix is NOT invertible. I've tested
this with our matrix inversion code, and substituting a one for the zero
gives a matrix that can be inverted.
Since the paragraph immediately below this example mentions matrix
inversion, why does the example show a matrix that CANNOT be inverted ?
In my estimation, this is a typo and a '1' should be at the intersection
of the third row and third column.
However, the next section of the spec deals with how to translate a
"point-n-scale" description to a matrix, and shows the Z-scale of the
point-n-scale (which is typically zero for 2-D) as being inserted at
this point in the matrix. However, any value can be inserted here
without changing any outcomes since the input "K" values for 2-D are all
zero. In other words, any value here gives the same results except when
inverting, and then any non-zero value allows inversion where a zero
will not.
Intergraph has been writing matrices with a 1 at this position for the
ModelTransformTag for many years. Recently, however, we came across a
file from a customer which had a 0 there and it blew up our matrix
inversion code.
Any arguments why this should not be changed ?
What is the process for getting the spec changed or at least amended ?
Thanks for any info or discussion.
--
ed grissom
ed.grissom at intergraph.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/geotiff/attachments/20070202/e435a581/attachment.html>
More information about the Geotiff
mailing list