Which type of georeferencing uses 6 control points?

The type of georeferencing that utilizes 6 control points is known as 2nd Order. In the context of georeferencing, control points are locations on the earth's surface with known coordinates, which are used to align or transform spatial data to match a reference system.

For 2nd Order polynomial transformation, the geometric adjustments to the data require 6 or more control points to accurately establish a relationship between the known coordinates and the target location. This transformation allows for a more flexible fit to the control point data than a first-order fit, which only needs 3 points to establish a linear relationship.

In contrast, the other options do not align with the requirement of 6 control points. A 1st Order transformation requires only 3 control points and fits the data to a linear model. The 3rd Order transformation uses a minimum of 10 control points to create a more complex model, while a 4th Order transformation necessitates at least 15 control points for an even more refined fit. Thus, the 2nd Order option is correct due to its specific requirement of 6 control points, allowing for a balance between complexity and the number of controls needed.