[gdal-dev] Confusion regarding GeoPDF composition.xml units

Andrea Giudiceandrea andreaerdna at libero.it
Mon Sep 21 23:36:25 PDT 2020

Nyall Dawson wrote
> Actually it's a bit more complex than this, because regardless of the
> DPI specified for the page, the width/height have to be specified in
> size in inches * 72! (i.e. there's a hardcoded 72 dpi assumption
> somewhere).

I don't know how composition.xml works, but if you wonder why in PDF page
the width/height have to be specified in "size in inches * 72" you will find
a hint in PDF 32000-1:2008 [1] especially at paragraphs Page
Objects, Table 30 and 8.3 Coordinate Systems.

"The length of a unit along both the x and y axes is set by the UserUnit
entry (PDF 1.6) in the page dictionary (see Table 30). If that entry is not
present or supported, the default value of 1⁄72 inch is used. This
coordinate system is called default user space"

"Note 2: The default for the size of the unit in default user space (1⁄72
inch) is approximately the same as a point, a unit widely used in the
printing industry. It is not exactly the same, however; there is no
universal definition of a point."

"UserUnit: A positive number that shall give the size of default user space
units, in multiples of 1⁄72 inch. The range of supported values shall be
implementation-dependent. Default value: 1.0 (user space unit is 1⁄72

See also [2] and [3].

Hope this will help.

Best regards.

Andrea Giudiceandrea

[2] https://stackoverflow.com/a/21164534/8779560
[3] https://github.com/qgis/QGIS/issues/33465#issuecomment-644566633

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