i. A line, if possible in red, drawn to the most prominent object near

the centre of the field of view. (_See_ P.T., Plate X.) The object so selected is called the reference point. In drawing this line the card should be set so that its longer edge roughly corresponds to the line of the position to be occupied. The lateral angle, in degrees, between the line to the reference point, and a line to all prominent features for ranging upon should be given.