# [aspect-devel] inconsistency in use of x, y, and z

Wolfgang Bangerth bangerth at tamu.edu
Wed Mar 11 04:13:31 PDT 2015

```Magali,

> In 2D the dimension of the box are set with X extent and Y extent. However,
> for the initial temperature
> function, the variables that need to be used are x and z.

As Timo pointed out, it's not that you need to use x,z. The default is in fact
x,y, and that remains consistent then, but I recognize that that may not be
what you want to use.

I'm not sure what the best way to address this is. Whatever one does seems to
rest on one or another frame of mind of what the "second" coordinate should
be. The best I could think is to call the extents not
Set X extent = 42
Set Y extent = 108
but something
Set Extent 1 =  42
Set Extent 2 =  108
and then introduce another parameter
Set Extent order = XYZ    // or XZY
which in 2d makes no difference at all and in 3d specifies whether the second
extent is in Y or Z direction.

We do like backward compatibility. I think I could somehow finagle this in a
backward compatible way with some work. Or we could just make an attempt at
documenting that in the box geometry, XYZ extents are unrelated to your choice
of coordinates in function descriptions.

> This is also an issue with the boundary indicators.  To easily move from a 2D
> to a 3D model, the boundary
> indicators for left, right, bottom, top should remain 0-3 and adding the third
> dimension should add the boundary indicators 4 and 5 for the front and back.
>   I know you can get around this by using the words "front",  "back", etc...
> but this seems like a weird way of counting things

That depends on your viewpoint :-) If you're fond of right handed coordinate
systems (XYZ), then the existing order makes perfect sense. It comes from the
order described here:
http://dealii.org/developer/doxygen/deal.II/structGeometryInfo.html
If, on the other hand, you like left-handed coordinate systems (XZY), then
what you suggest makes sense.

Best
Wolfgang

--
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Wolfgang Bangerth               email:            bangerth at math.tamu.edu
www: http://www.math.tamu.edu/~bangerth/

```