A cylindrical shaft is drilled in the ground in which the column is directly embedded. The diameter and depth are determined by your engineer given the structural requirements and the soil conditions. Concrete is poured around the outside and beneath the base of the column.

A square or rectangle hole is dug into the ground. The column is place in the center of the hole and concrete is poured around the outside and beneath the base of the column. Reinforcing steel should be installed prior to the placement of the concrete. Length, width, and depth will be determined by your engineer given the structural requirements and the soil conditions.

Spread foundations are most commonly used when one of these conditions are encountered:
1. Poor soil 
2. High water table 
3. Bed rock

Typically, once the soil report confirms that a spread foundation is the best option, your engineer will use the  report's data to design the most suitable foundation for the structure being built. 

The spread foundation is designed to disperse vertical pressures, while a cubed footing deals with lateral pressures. The two main differences which accomplish this are the dimensions of the footing and the orientation of the sign faces. 

The spread dimensions are typically much wider and longer, and because the loads are spread over a much larger area the concrete needs to be reinforced with rebar. Care must be taken to insure proper orientation of the sign to the footings chosen. In most cases (this should always be confirmed with your engineer) the faces will run parallel to the longer dimension of the cubed footing and perpendicular to the longer dimension of the spread.