At this time I would like to suggest my approach to riving or splitting large planks from the sides of a large tree like that above, destined to become a squared hewn timber.
I would roll this log up on good stout bed pieces at least 4-- 6"by6"'s placed about 8 feet apart, making sure that they are level, and parallel to one another.
After layout of the timber on the ends of the log, I would extend the lines to the top and after removing some bark I would snap a chalk line full length on both sides. I then would take the hewing axe and flatten as much of a surface as possible without scoring on both outside faces. After this was completed I would measure in at least 6" on both sides, and snap another line this would be the splitting line. Like Ken above I would work along this line with steel and wood oak wedges to open up a preliminary riving line.
I would then turn the log over and repeat the process on the under side. If you place the wedges carefully you should be able to turn the log easily, but the bed pieces need to be long enough.
Carful observation of the splitting lines would have to be part of the process.
Using consistently wider wedges should eventually split off the a large plank on both sides.
This completed smaller planks then could be rived on the other 2 sides.
traditional hewing would finish out the surfaces, in the end producing some lovely wide heavy planks, some narrower ones, and 1 lovely hand hewn timber.
I would welcome and invite comments on my suggestion