PPI Limitation in photoshop CS5???

Question

Hi,
I am working with very high resolution images (21,739 pixels per cm).  Not a typo! The images are of zebra finch brain sections that were scanned using an Olympus Nanozoomer (microscope) with a 20X object. The scanning resolution was a cool 0.46um per pixel, and my typical images are ~30,000 x 30,000 pixels (1-2GB uncompressed).
In order to accurately display these images to scale using a google earth like approach (Zoomify), I need to adjust the final pixels/cm setting (Image --> image size) to 21,739 pixels/cm. Unfortunately, they seem to open at 72dpi (for some reason). The problem arises when I try to change my image to a resolution greater than ~12,000 pixels/cm. It automaticallyt defaults to 0.393 pixels/cm, which I understand is the minimum pixel/cm.
Is there an easy way to overcome this?? I'm not resampling, just changing the resolution for display purposes.
Very annoying. . . Thanks!
Pete

Answer 1

I agree that resolution is irrelevant for the actually web display, but that doesn't mean that it isn't relevant for setting the scale of the image (e.g. a scalebar). Google earth has to know what the relationship is between pixels and distance (i.e. ppi) in order to properly set the scale bar.So, yes, we're basically talking about metadata here - a variable in the file header that relates image pixels to real world units. What I'm trying to do is force photoshop to set this variable to 21739 pixels/cm, and it won't let me. It has no problem with 10,000 pixels/cm, but when I set it to 21739 pixels/cm it defaults to 0.393 pixels/cm. Just looks like a limitation of photoshop. Guess the good folks at Adobe never thought anyone would require such a large number of ppi.
Looks like the "easiest" solution will be to build the relationship between pixels and the real world into my web display application, and leave the image at whatever photoshop sets it. The bummer is that the original scans (a proprietary jp2-like file format called NDP) included the correct ppi in the meta-data, but the value must have been deleted when the files were being converted first to binary (archival), and then to TIFF (for image processing). 
Oh, well. Thanks for all your help. . . .