{\pdfimageresolution=72 \externalfigure[ch01dia1.png]}
when you set pdfimageresolution there isn't any unit either, the unit is implicit
What about a new pdftex primitive: \pdfimagepixel, which would force using a unit. Then the above would be done by \pdfimagepixel=1bp This subject is dear to me because (1) I'm writing a textbook on dimensional analysis, and (2) almost none of my students use units in their assignments, and often therefore make absurd mistakes. That's what happened to the NASA Mars probe that crashed into the surface after travelling all the way. The engine makers and NASA had different ideas of what units the thrust was in (I think NASA had converted to SI units, whereas the manufacturer used 'English' units). -Sanjoy `Never underestimate the evil of which men of power are capable.' --Bertrand Russell, _War Crimes in Vietnam_, chapter 1.