Equipment
Spring Balance
Plumb Bob
Range Finder
Compass
Clinometer
The Magnetic Compass
Examples Of Usage
A compass can be used for:
- Measuring the angles in a traverse used for navigation or control purposes.
- As compasses have relatively poor direction accuracy, they are typically used for reconnaisance surveys.
Factors Which Influence The Use Of Compasses
- A clear line of sight between the user and the measured point is needed.
- A well defined object is required to obtain the maximum precison.
- The accuracy is highly dependent on other magentic influences, such as electric motors and natural anomalies caused by local geology.
Survey Methods Used For
Compasses are used for the measurement of angles.
The Magnetic Compass is an instrument which indicates the whole circle bearing from the magnetic meridian to a particular line of sight. It consists of a needle or disc magnetised so that it will align itself with the direction of the Earth's magnetic flux, and some type of index scale so that numeric values for the bearing can be determined.
There are two main types of magnetic compasses used in the field by surveyors navigators and orienteers: the Sunnto type and the prismatic type, as well as compass-theodolites. There are others like the gyro-compass which are used in inertial navigation systems, however they will not be addressed here.
Both the Sunnto type and the Prismatic type are held in the hand for use, and are therefore subject to poor centring and an unstable platform. The effects of this are reduced over long sight lines, which, when combined with the vagaries of the magnetic meridian, combine to make the compass a reconnaissance or inventory tool only. Neither the instruments nor the basis upon which they work are sufficiently stable for any sort of precision work.
The magnetic bearing is related to true bearings as follows:
d = Magnetic declination (positive when clockwise)
qt = True bearing
qt= Magnetic bearing
qt = q t + d
Variations in Declination
The geophysical phenomena that generate the Earth's magnetic flux are still not fully understood. It is known that magnetic north moves quite considerably over time, and has even reversed polarity in prehistory. Some of the phenomena that effect the direction of magnetic flux (and hence magnetic north) are known as variations in declination and are as follows:
| Variation | Cause | Amount of Dd |
|---|---|---|
| Secular variation | Rotation of magnetic pole around geographic pole | In 1933 - 8 ° E |
| Diurnal variation | Effect of sun during the day | up to 10' |
| Irregular variation | Sunspot activity |
up to 5° |
Conclusion: Magnetic north is generally too unreliable for use as a survey datum!
Local Attraction
The needle of the compass can also be 'attracted' by metallic objects close to the point of observation. These objects cause local aberrations in the direction of magnetic flux, and give rise to an effect known as local attraction. These local disturbances in the Earth's magnetic field are often due to large iron masses, electric cables, fences, cars and so on. They tend to occur locally, and if detected can sometimes be compensated for in survey procedures. Magnetic anomalies caused by underground minerals are a problem for surveyors, but form the basis of many mineral exploration techniques so the news is not all bad.
Where a closed traverse consisting of compass bearings and distances has been performed around a parcel of land (see later) it is possible to compensate for the effects of local attraction and to distribute 'angular misclosure'.
This will be covered in more detail later but in summary the procedure consists of:
measuring forward and back bearing of each line
computing angles and angle misclosure (misclosure = [180°(n - 2)] - S angles) {p(n-2) - S angles} )
adjusting each angle by adding to each
recomputing bearings from adjusted angles.
(The bearing of one line must be known or assumed).
The presence or otherwise of local attraction can be determined from the difference between a ‘forward’ bearing and a ‘reverse’ bearing observed from, and to, a station. If I was to measure from Point A to Point B, and then from Point B back to Point A the difference in the bearings should be 180°. Any variation in this in excess of what would be expected from random error would be most likely due to local attraction. Needless to say both forward and reverse bearings are always observed when using a compass for traversing.
