Consider the following example.
IN here, we have angles 1 and 2 measured with respect the horizontal line that passes through point A. Note that angle 2 is related to points A and C. In this case, angle is under the horizontal line that passes through A. Then, angle 2 is the angle of depression from point A to point C. Every angle that is under the horizontal line would receive the name of angle of depression. In the same manner, since angle 1 is above the horizontal line, then it is an elevation angle from point A to point C. In fact, every angle that is above the horizontal line, would receive the name of elevation angle.
Coming back to our problem, we can see that angle 2 is formed by the horizontal line that passes through the radar and the line that connects the plane to the radar. This angle would be above the horizontal line of the radar. Then, according to our previous explanation, this means that angle 2 is an elevation angle from the radar to the plane.