Divide large image into 16 equally sized rectangles

[mark-in-the-tropics]

Hi everybody
I have a very large Mercator projection which I want to divide into several sections, each of which I will then re-shape to fit the map onto onto a globe of the Earth.
It's easy enough to shrink my image down to say 4096 pixels width, which (in principle) allows the image to be evenly divided into 16 rectangular sections of 256 pixels width. I'm having trouble making the selections. 
The first selection is easy enough, because I have the edge of the image to refer to. The next selection is problematic, because I can't see where my first selection left off. (Hmm, I could have bucket-filled the first selection with black, I guess...)
I can't see how to tell gimp, using numbers, that I want a particular part of the image to be selected out, but I'm sure there must be a way.
Thanks
Mark

[Ofnuts]

Use the image grid with Image > Configure grid where the spacing is the size of your sections, the offset is 0, and for such a large grid you probably want a line style (just Intersections may be hard to spot)

Then View > Show grid and View > Snap to grid and the tools will magically snap at the right spots.

[rich2005]

There is a filter in the gmic_gimp_qt plugin to make the sections (gores)  Map Projection -> Sinusoidal Map and set the number of panels you want.

   

A Windows Gimp 3 version of the plugin https://www.gmic.eu/download.html