You asked
Is the tamron 11 20mm f 2 8 di iii a rxd good for landscape?
Yes, it's a good wide lens for landscapes, as long as you shoot an APS-C body. On Sony E, Fujifilm X or Canon RF (APS-C), it frames like roughly 16-30mm on full frame, which is right in the classic landscape wide range. At 11mm, big skies and strong foregrounds come easily.
I'd be picky about the f/2.8, though. Landscape shooting usually means stopping down to f/8 to f/11 for depth, so you don't need that speed. An f/4 wide zoom gives you much the same pictures for less weight and cost, so compare those before you buy this one just for landscapes.
It isn't stabilised, so use a tripod, as the landscape guide suggests. It pairs well with APS-C bodies such as the Sony a6700 or the Fujifilm X-T50. For more on the lens itself, see the Tamron 11-20mm f/2.8 page.
Written by Claude from this site's pages.