Donald Trump wants to build an "impenetrable, physical, tall, powerful, beautiful, southern border wall" between the US and Mexico. But how tall? How powerful? How beautiful? The Republican candidate's big ideas can be small on detail, and the wall is no exception. The US-Mexico border is about 1,900 miles (3100 km) long and traverses all sorts of terrain from empty, dusty desert to the lush and rugged surroundings of the Rio Grande. Some 650 miles of the border is covered already by a confused and non-continuous series of fences, concrete slabs and other structures. Mr Trump says his wall will cover 1,000 miles and natural obstacles will take care of the rest. So what will he build? This is...
