north—it will probably come to rest a little offthe magnetic meridian. The most common causeof sluggishness is weakening of the magnetism ofthe needle. A needle may be demagnetized bydrawing it over a bar magnet. The needle shouldbe drawn from the center of the bar magnettoward the end, with the south end of the needledrawn over the north end of the magnet and viceversa. On each return stroke, lift the needle wellclear of the magnet.Sometimes the cause of a sluggish needle is ablunt point on the pivot. This may be correctedby sharpening the pivot with a fine file.If the compass is not level when a bearing orazimuth is being read, the reading will beincorrect. A similar error will exist if the compassis equipped with sighting vanes and one or moreof them are bent. To check for bent compassvanes, you set up and level the compass, and thensight with the vanes on a plumb bob cord.The most common personal error the observercan make in compass work is MISREADING.This is caused by the observer’s eye not beingvertically above the compass at the time of thereading. Other common mistakes are reading aneedle at the wrong end and setting off localattraction or declination in the wrong directionwhen the compass is being oriented.ESTABLISHING DIRECTIONSBY TRANSITDirections are similarly determined by the useof a transit. This can be done by measuring thesize of the horizontal angle between the line whosedirection is sought and a reference line. With atransit, however, you are expected to do this withconsiderably more accuracy and precision thanwith a surveyor’s compass. Some of the basicprocedures associated with the proper operationof the instrument will be discussed in thefollowing paragraphs.Setting Up the TransitThe point at which the line of sight, thehorizontal axis, and the vertical axis of a transitmeet is called the INSTRUMENT CENTER. Thepoint on the ground over which the center of theinstrument is placed is the INSTRUMENTPOINT, TRANSIT POINT, or STATION. Awooden stake or hub is usually marked with a tackwhen used as a transit station or point. Toprevent jarring or displacement of the transit,avoid those stations having loose planking, thosehaving soft or marshy ground, and those havingother conditions that would cause the legs ofthe tripod to move. The following steps arerecommended when you are setting up a transitover a station point:1. Center the instrument as closely as possibleover the definite point by suspending a plumb linefrom a hook and chain beneath the instrument.The plumb string is tied with a slipknot, so thatyou can adjust the height of the plumb.2. Move the tripod legs as necessary until theplumb bob is about 1/4 in. short of being overthe tack, meanwhile maintaining a fairly level footplate. Spread the tripod legs, and apply sufficientpressure to the legs to make sure of their firmnessin the ground. Make sure to loosen the wingnuts to rid the static pressure in them beforeretightening.3. Turn the plates so that each plate level isparallel to a pair of opposite leveling screws. (Itis common practice to have a pair of oppositeleveling screws in line with the approximate lineof sight.) The leveling screws are then tightenedto firmness, but not tight. Rotate opposing pairsof leveling screws either toward each other oraway from each other until the plate bubbles arecentered.If the plumb bob is not directly over the centerof the tack, you may loosen two adjacent levelingscrews enough to free the shifting plate. Relevelthe instrument if the bubbles become off-center.During breezy conditions, you may shield theplumb line with your body when setting up aninstrument. Sometimes in windy locations, it maybe necessary to construct a wind shield.Setting and leveling the transit rapidly requiresa skill on your part that you will learn and developthrough consistent practice. You should takeadvantage of any opportunity that you can totrain yourself and increase your skills in handlingsurveying instruments. Again, when setting up oroperating a transit, you should remember thefollowing points:1. The plate bubble follows the direction ofthe left thumb when you are manipulating theleveling screws.2. You should always check to see if theplumb bob is still over the point after leveling it.If the plumb bob has shifted, you should recenterthe instrument.3. While loosening the two adjacent levelingscrews, you can shift the transit head laterally.13-8
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