Digital Elevation Model scale

I am trying to analyze the surface of a sample using a technique similar to the Digital elevation model. The pixel value of the image is considered as the elevation from the surface. From this I obtained a 'rasterized' surface. However, the pixel values are from 0-256 for an 8-bit resolution, I can convert to 12-bit and get a better resolution. Can i convert this resolution (0-256) to a length scale (meters)? I was told I can use the pixel length but I am not sure how they relate. The pixel length is related to the x and y axis but I am not sure how I can relate it to the elevation (z-axis). Perhaps I am doing some wrong assumptions. Can anyone point me in the right direction?

[dpb]

"Can i convert this resolution (0-256) to a length scale (meters)? "

Sure, but you have to have an independent definition of what full scale is in actual distance--it can't be obtained without that external bit of information from somewhere.

Or, alternatively, if you can identify a particular location in the image and know what its elevation is, you can back out the scaling from those two pieces of information. Clearly, this is going to be much more susceptible to estimation errors.