Northing and of the and Easting Solution 1.differential levelling is used to measure the elevation difference between two or more points with reference to a datum. 2.theodolite is used to measure the angles in horizontal and vertical planes. 3.corrected horizontal distance,height difference,corrected slope distance respectively. 4.GPS stands for Global Positioning System. 5.number of satellites required to obtain an acceptabel level of accuracy while taking gps readings is 3..