I had tried using the PFM (portable float map) image format that supports floating point values and negatives first to be sure it was correct. Then I switched to TIFF which gave the same results. I copied and pasted that wrong command. I have fixed it above.
Questions and postings pertaining to the usage of ImageMagick regardless of the interface. This includes the command-line utilities, as well as the C and C++ APIs. Usage questions are like "How do I use ImageMagick to create drop shadows?".