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.
see
http://netpbm.sourceforge.net/doc/pfm.html
http://paulbourke.net/dataformats/pbmhdr/