put the sounder screen up then press the soft key that I think says "split". There are 4 options in here: no split, bottom lock, bottom zoom and dual.

You also need to have the colour pallete on 11 or 12 from memory for the bottom lock to work (either white or blue background with the bottom showing as brown). For what it's worth I hated the HDS10 bottom lock and the way it displays and zooms, hence a nice shiny Furuno 585 sitting front row centre on the dash now, which is what I should have done in the first place.

thanks. ill have a go