NavList:

Peter Monta you wrote:

I would guess portability.  Theodolites would normally outperform sextants, but in 1910 they may have been too heavy and bulky to consider for a mission of this type.  The small Wild theodolites like the T2, with glass scales and optical micrometer, weren't around until the 1920s.

Peter

Scott had sledge theodolites on wooden tripods, and this is what Bowers used at the Pole.  I can’t remember where I read this.  I think it was in Hinks who analysed Bowers’ work during WW2.   Amundsen, on the other hand, used a sextant and had a choice of a mercury or mirror AH.  He took the mirror in case the mercury froze.  In fact, the mercury didn’t freeze, and this has led some observers to suggest that Amundsen’s famous mirror AH photograph at the South Pole was staged before or after the Pole.  Here is a photo of a Scott Expedition theodolite, note the way all the relevant knobs have been insulated, so the device could be operated ‘mittens off’ if required https://www.flickr.com/photos/64866643@N02/6326231808

I thought I’d seen a photo of Bowers using a theodolite, but I can’t find it now.  All I could find was a photo of Lt Edward Evans, second in command (not PO Edgar Evans who accompanied Scott to the Pole) using one https://www.alamy.com/lieut-e-r-g-r-evans-surveying-with-the-four-inch-theodolite-october-1911-1913-artist-herbert-ponting-image217114495.html  . 
DaveP