@fresco,
Also, the use of the words "rough ground" indicates an uneven distribution of weight on the ground, thereby imparting an inherent instability. The word "smooth", another relative term, is another indication that the unasked question is possibly "Should this ladder be taken down immediately? " There are too many unknown variables such as the combined surface area of the top corners of the side rails that contact the wall, materials ( and thus friction ) of the wall and the ends of the ladder, direction and speed of wind , total surface area of the ladder that is exposed to the wind, soil compress ability, width between the centers of the ladder rails contacting the ground, combined surface area of the bottom of the rails, etc. to begin calculation.