Page 9 - 4798

Fig. 1.2. The vertical direction is defined as the direction of the force of gravity

1.2 Measuring Distances and Angles
Horizontal distance is measured along a level surface. At every point along that
length, the line tangent to the level surface is horizontal. It can be measured by tape
or Electronic Distance Measurement (EDM). A true horizontal distance is actually
curved, like the surface of the earth.
A vertical distance is measured along the direction of gravity and is equivalent
to a difference in height between two points. When the height is measured with
reference to a given level surface, like mean sea level, it is called an elevation. An
instrument called level, which is used to observe the rod at different points, can
measure elevation. The relative vertical position of several points separated by
long distances can be determined by a continuous series of level rod observations.
This procedure is called leveling.

Fig. 1.3. A true horizontal distance is actually curved, like the surface of the earth

7