Measurement of Directions and Elevations

Definition

Measurement of directions and elevations is the process of determining the horizontal orientation of survey lines with respect to a reference line or meridian, and the vertical position of points with respect to a datum or reference level.

In simple terms:

Direction

tells the angle of a line in the horizontal plane.

Elevation

tells the height of a point relative to a known level.

Direction is usually measured by bearings, azimuths, or angles, and elevation is measured by leveling methods, trigonometric methods, or instruments such as a level, theodolite, or total station.

Main Content

1. Direction Measurement in Surveying

Direction is the horizontal angle a survey line makes with a reference line such as the north-south meridian.
It is commonly expressed using bearings or azimuths.

In surveying, direction measurement is necessary to show the exact orientation of a line on a map or in the field. Bearings may be measured from north or south toward east or west, while azimuths are generally measured clockwise from the north direction from 0° to 360°. For example, a line having an azimuth of 60° lies in the northeast quadrant, while a bearing of N 60° E gives the same direction in a different form.

Direction measurement is useful in establishing control networks, plotting survey points, and preparing cadastral and engineering plans. It ensures that all lines in the survey are correctly oriented relative to each other and to true north or magnetic north.

2. Elevation Measurement in Surveying

Elevation is the vertical distance of a point above or below a chosen reference datum.
It is determined by leveling, direct measurements, or indirect methods like trigonometric leveling.

Elevation measurement helps in finding the relative height of different points on the earth’s surface. A point may be above mean sea level, below ground level, or at the same level as another point. The most common reference used is mean sea level (MSL) or any local benchmark established for the project.

In engineering projects, elevation data is required for designing drainage systems, road gradients, building foundations, and earthwork calculations. For instance, if a point on a road has a higher elevation than another point, the slope of the road can be calculated and adjusted for safe movement of vehicles and proper drainage.

3. Instruments and Reference Systems

Direction is measured using instruments such as a compass, theodolite, and total station.
Elevation is measured using dumpy level, auto level, tilting level, leveling staff, and digital level.

The accuracy of direction and elevation measurement depends on the instrument and the reference system used. A compass gives magnetic direction, while a theodolite gives precise horizontal and vertical angles. Total stations combine both direction and elevation measurement in one instrument, making modern surveying faster and more accurate.

Reference systems are equally important. Direction may be referenced to:

True north

Magnetic north

Grid north

Elevation may be referenced to:

Mean sea level

Bench marks

Local datum

For example, if a building site uses a benchmark with known elevation, all other spot heights on the site can be calculated relative to that benchmark.

Working / Process

1. Establish the reference line or datum

First, a reference for measurement is selected.
For directions, this may be true north or magnetic north.
For elevations, a benchmark or datum level such as mean sea level is used.

2. Measure the horizontal direction

The survey instrument is set up and aligned properly.
Bearings or azimuths of survey lines are observed.
The angle is recorded carefully, along with the line length if needed.

3. Measure the vertical elevation

Leveling staff readings are taken using a level instrument.
For inaccessible points, vertical angles and distances may be used.
Reduced levels or height differences are computed and checked for accuracy.

Advantages / Applications

Helps in accurate plotting of survey lines and construction layouts.
Useful in determining slopes, gradients, and relative heights of land surfaces.
Essential for road, railway, canal, pipeline, and building design.
Supports topographic mapping, contouring, and land development projects.
Improves precision in engineering surveys by combining horizontal and vertical measurements.

Summary

Measurement of directions and elevations is a basic and essential surveying activity used to fix the position of points in both horizontal and vertical planes.
Directions are measured by bearings or azimuths, and elevations are measured with respect to a datum using leveling or trigonometric methods.
Accurate measurement is necessary for correct planning, design, and execution of engineering works.
Important terms to remember: bearing, azimuth, datum, benchmark, elevation, leveling, true north, magnetic north.