the header offset value helps you read in images that have different formats. When you store an image to a file, you also need to store the size of the image, or else the program reading that image won't be able to read the correct size, for example. The image dimensions, along with other image information, is commonly called "metadata." But the point is that you ahve to have the metadata info to read the image. When ENVI writes a file, it stores its metadata in a separate file (the .hdr file), so the image file itself just has the image data. But, many other formats store the metadata and the image data in the same file, with the metadata stored first. If you want to read in the image data, you have to know to skip over the metadata if its in the same file. This is the offset value: it's the number of bytes (or bits? can't remember) to skip over to get to the actual image data. "Z plot range" lets you specify a default range thats plotted in a z-profile window (ie, plotting a spectrum). "Data ignore value" is to allow a user to write code to check for a certain value in an image (the data ignore value) and not process pixels that have that value. I tend to check the ENVI help files for the routine ENVI_SETUP_HEAD when i have a question about header values, even if i'm not writing a program, b/c that routine has a fairly comprehensive and concise list of everything that can be in an envi header.