Discuss various method used to overcome problems in magnetic compass surveying

Magnetic compass surveying is a type of surveying in which the directions of surveying lines are determined with a magnetic compass, and the length of the surveying lines are measured with a tape or chain or laser range finder. The compass is generally used to run a traverse line. The compass calculates bearings of lines with respect to magnetic needle. The included angles can then be calculated using suitable formulas in case of clockwise and anti-clockwise traverse respectively. For each survey line in the traverse, surveyors take two bearings that are forward bearing and back bearing which should exactly differ by 180° if local attraction is negligible. While a magnetic compass is a device consisting principally of a circular compass card, usually graduated in degrees, and a magnetic needle, mounted and free to rotate on a pivot located at the center of the card (Broadmeadow, 2005).

OCCURANCE OF ERRORS MAGNETIC COMPASS SURVEY

Sagging of chain/calibration is a source of error in the eventual heading readings of a magnetic compass. With many systems, the calibration is tedious and is often referred to as the “Chicken Dance” due to its complexity requiring the user to rotate to multiple positions while tilting the device upside-down. Most of the difficulty of these procedures lies in the fact that the user does not know which points are needed and which are not. In this case, there is the belief that a true and perfect calibration that will work best with all points if only these procedures are followed exactly (Thompson.R et al, 2009).

Misreading, this is the most common personal error in which  the observer can make in compass work is, this is caused by the observer’s eye not being vertically above the compass at the time of the reading. Other common mistakes are reading a needle at the wrong end and setting off local attraction or declination in the wrong direction when the compass is being oriented.

Bending of compass needle, there will be a constant instrumental error in all observed bearings and azimuths, to check for this condition, set up and level the compass, wait for the needle to cease oscillating, and read the graduation indicated at each end of the needle.

Graduated compass bearings, the numerical value at each end of the needle should be the same. If the compass is graduated for azimuths, the readings should be 180° apart. Similarly, if the pivot supporting the needle on a magnetic compass is bent, there will be an instrumental error in the compass. However, this error, instead of being the same for all readings, will be variable.

 Integration and Alignment

When integrating a magnetic compass into a product it can be difficult to find the ‘correct’ spot to minimize the magnetic distortions that can impact the sensors. There are many passive and active electronic components in most systems these days. Even in the systems where only passive components are used, not all of them will be utilized or turned on during the usage of the device. It is these time-varying fields that cause the most issues when the system is integrated. Once the least magnetically disturbing location has been found, it can still be difficult to place the compass in an orientation that aligns with the host device’s A-B-C axes. If the system is not aligned properly, the heading/pitch/roll during static and dynamic situations will be compromised (Thompson.R et al, 2009).

The point at which the line of sight, the horizontal axis, and the vertical axis of a transit meet is called the instrument center. The point on the ground over which the center of the instrument is placed is the instrument point, transit point, or station. A wooden stake or hub is usually marked with a tack when used as a transit station or point. To prevent jarring or displacement of the transit, avoid those stations having loose planking, those having soft or marshy ground, and those having other conditions that would cause the legs of the tripod to move (Department of Land Resources, 2013).

 Directions are similarly determined by the use of a transit, this can be done by measuring the size of the horizontal angle between the line whose direction is sought and a reference line. With a transit, however, you are expected to do this with considerably more accuracy and precision than with a surveyor’s compass.

Some steps are recommended when you are setting up a transit over a station point includes:-

v  Center the instrument as closely as possible over the definite point by suspending a plumb line from a hook and chain beneath the instrument. The plumb string is tied with a slipknot, so that you can adjust the height of the plumb.

v  Move the tripod legs as necessary until the plumb bob is about 1/4 in. short of being over the tack, meanwhile maintaining a fairly level foot plate. Spread the tripod legs, and apply sufficient pressure to the legs to make sure of their firmness in the ground. Make sure to loosen the wing nuts to rid the static pressure in them before retightening.

v  Turn the plates so that each plate level is parallel to a pair of opposite leveling screws. (It is common practice to have a pair of opposite leveling screws in line with the approximate line of sight.) The leveling screws are then tightened to firmness, but not tight. Rotate opposing pairs of leveling screws either toward each other or away from each other until the plate bubbles are centered.

v  If the plumb bob is not directly over the center of the tack, you may loosen two adjacent leveling screws enough to free the shifting plate. Re-level the instrument if the bubbles become off-center. During breezy conditions, you may shield the plumb line with your body when setting up an instrument. Sometimes in windy locations, it may be necessary to construct a wind shield.

v  Setting and leveling the transit rapidly requires a skill on your part that you will learn and develop through consistent practice. You should take advantage of any opportunity that you can to train yourself and increase your skills in handling surveying instruments. Again, when setting up or operating a transit, you should remember the following

METHOD TO OVERCOME MAGNETIC COMPASS SURVEYING ERRORS

The bore sight matrix method is a simple rotation matrix held inside the compass that is only used on heading, pitch, and roll. This can be figured out mechanically ahead of time by the systems integrator. The Invoke Tare procedure has the user align the system with a known magnetic North and has the end device placed flat and level. It is important to realize not to use the compass data to measure north and level during this procedure since this procedure is used to align the compass with the host device, Some procedure allows a user to point to any known heading, pitch, and roll with the host device to calibrate the alignment  (Department of Land Resources, 2013).

Straightening  a bent needle, if a discrepancy still exists, then probably the pivot is bent too, in order to eliminate either of these instrumental errors by reading both ends of the needle and using the average between them. Suppose, for example, that with a compass graduated for bearings you read a bearing of N45°E and a back bearing of S44°W. You would use the average, or the error in the compass should, of course, be corrected as soon as possible. Normally, this is a job for an expert. Remember the cause of a discrepancy in the reading at both ends when there is one. It is more probable that the needle, rather than the pivot, is bent (Broadmeadow, 2005).

The needle should be drawn from the center of the bar magnet toward the end, with the south end of the needle drawn over the north end of the magnet and vice versa. On each return stroke, lift the needle well clear of the magnet if a compass needle is sluggish that is, if it moves unusually slowly in seeking magnetic north it will probably come to rest a little off the magnetic meridian. The most common cause of sluggishness is weakening of the magnetism of the needle. A needle may be demagnetized by drawing it over a bar magnet. But sometimes the cause of a sluggish needle is a blunt point on the pivot. This may be corrected by sharpening the pivot with a fine file. If the compass is not level when a bearing or azimuth is being read, the reading will be incorrect. A similar error will exist if the compass is equipped with sighting vanes and one or more of them are bent. To check for bent compass vanes, you set up and level the compass and then sight with the vanes on a plumb bob cord.

Conclusion

Magnetic compasses provide accurate platform heading and attitude information in a variety of applications and in many operational environments. However, these devices are susceptible to distortion with the presence of magnetic material. There are, however, techniques and best practices to minimize this risk to performance. The proper calibration of a device is critical in its performance capability. Where a device is implemented and how it is aligned into a system can further impact its performance. Continued monitoring of the environment and performance once in operation ensures the magnetic compass provides the best results possible. Manufacturers and users who adhere to these practices can better expect their magnetic compass to perform in otherwise challenging magnetic environments and applications.

REFERENCES

Broadmeadow (2005). Cartographic on land features, British Woodland: Forestry Commission.

Department of Land Resources (2013). Elementary surveying and Map making, Government of India: Ministry of Rural Development

Thompson.R et al (2009). Prismatic compass surveying instruments for planning, U.S.A: Restoration of Native Woodland on Ancient