LIFE WITH THE MAPSTAR/CRITERION FLUXGATE COMPASS The digital fluxgate compass in your MapStar/Criterion measures the earth's magnetic field in much the same way as a magnetic hand compass. In other words, it measures horizontal angles (azimuths) relative to the magnetic poles. The sensor has a ring that floats freely in fluid inside a cylindrical housing. The floating ring keeps the sensing element horizontal, which allows you to take accurate azimuth readings even when the instrument is tilted as much as 15 degrees from the horizontal plane. The magnetic poles are not located at the same place as the geographic north and south poles. Moreover, they move in a predictable direction a small amount annually. A compass always points toward the magnetic pole. The direction in which the compass points is called a magnetic meridian. "True" or geographic north relative to magnetic north varies somewhat depending on your location on the surface of the earth. The value of the variation between the magnetic north and geographic or true north is known as declination.It is important that you determine the correct declination value for the area that you are working in and enter this value into the MapStar/Criterion. This must be done prior to using the instrument to collect data that will be used to generate true geographic coordinates, as in applications such as Laser-Walkabout. A declination value can be set and modified in the System Menu under Compass Offsets. Examples of Declination Values Magnetic declinations are east if the compass measures east of true north and west if the compass measures west of true north. Denver, Colorado, which is in the western part of the continental US, has a declination value of 10.9 degrees east which is represented as a positive value. Chicago has a declination value of 1.9 degrees west (negative 1.9 degrees) . The zero declination line runs generally north and south somewhere west of Chicago. Obtaining Declination Values Declination information may be obtained by a variety of methods. It may be acquired through the United State Geological Survey by phone, modem or map. LTI also offers a DOS program called MAGCALC which will calculate values based on Lat/Lon location and time. Your LTI dealer/representative will be able to supply you with additional information. Compass Inaccuracies Due To Local Magnetic Attractions Local attractions are caused by objects of iron, steel, nickel and other ferrous materials. Electric utility lines may also affect compass readings. Direct current power utility lines will cause a standard offset and alternating currents will cause an unstable reading of the compass measurement. The effect that local attraction has on the MapStar/Criterion compass depends upon the proximity of the material to the compass and on the mass and strength of the local attraction. Small metallic objects carried on the person of the operator may affect the compass reading by as much as several degrees. In some work situations the strength of the interference could prevent the compass from providing a useful azimuth. Usually the potential for local attraction can be determined by visually inspecting the site or from local knowledge of any buried utilities. Some objects that may potentially affect the compass performance that should be avoided are: Portable radios Hatchets Batteries Pin flags Data recorders or computers Nails Metal watch bands Magnetic antenna mounts Home made yokes Tripods, tribrachs Proximity to automobiles Overhead powerlines It is recommended that the battery and data recorder be located a minimum of 35 centimeters (13 inches) below the compass. Hanging the battery on your belt will ensure that this distance is maintained. Field Methods for Checking Local Magnetic Conditions The following simple tests can be used in the field to test for local magnetic interference. (i) If you are working on a street, and you know it's orientation relative to true north, take a shot down the street. Nearly 80% of the streets in the US are oriented in a north-south or east-west direction. The azimuth from the MapStar/Criterion should match the known direction of the street. (ii) Choose a point about 100 meters away (e.g. a pole) and shoot to it. Note the azimuth. Then rock the gun back 12" - 18" and shoot again. The second azimuth should be within 1/10th to 5/10ths of a degree of the first azimuth. If it is, you are very likely within an anomaly free area. (iii) Check agreement between a "foresight" and a "backsight". Shoot to your prospective next station point and note the azimuth. Move to the next station and shoot back to the original station. The azimuths should be 180 degrees different plus or minus a few tenths of a degree. Testing Calibration of the MapStar/Criterion Compass If one of the previous checks turns up a discrepancy, perform this more definitive test. Choose a flat open area of at least 100 sq. meters. Ensure the area is as free from magnetic anomalies as possible. In the North-South direction, set out 3 stakes in a line spaced at 50 m. Using the center stake as a reference, move around the perimeter of the field. Stop at approximately 45 degree intervals and line up opposing stakes to create a set of four "180 degree" checks. Move to the center stake and record azimuths to all eight points. Check for agreement in the same manner as (iii) above. Re-Calibration of the Criterion Compass (MapStar users can utilize the on-board function) We now have over 700 units in operation in a variety of harsh field conditions. In some instances, we have seen the compass sensors' performance gradually degrade, to the point where they are no longer making measurements to within specification. The reasons for this degradation range from "hard" use, to bias from magnetic anomalies, to simply being transported to northern latitudes, where the horizontal component of the earth's magnetic field is weak. With no "on-board" warning that your compass is out of spec, this condition is often discovered too late by obtaining poor results when the data is processed in the office. Very frustrating! That is why we strongly encourage our users to perform one or more of the simple tests listed above as a part of their typical data collection routine.In response to the cases where the compass sensors are going out of calibration, we now have available to all our distributors (and clients) a calibration fixture. This package consists of a specially modified tripod, an instrument cradle, a special calibration eeprom and a software program written by the compass manufacturer to communicate with the sensor. We also provide (and require) qualified training to an individual to ensure proper operation of the equipment. This kit is identical to the one we use here at the factory, and provides on-site re-calibration of the Criterion's compass. We would at this time like to institute a policy for re-calibration of the Criterion compass sensors. The primary responsibility lies with you, the user. You must be able to detect that your compass is out of spec. Please use the definitive test outlined above. Once you have determined there may be a problem, contact your local LTI dealer or your factory representative. We suggest the following schedule to test the calibration of your compass, and then re-calibrate if necessary: Poor results, dropped, damaged, etc. Immediately "Hard" use in rough field conditions Every 2-4 weeks Regular use in moderate conditions Every 3 months Light, occasional use Every 6 months We at LTI are committed to superior product quality and performance, as well as customer satisfaction. We continue to seek and appreciate your feedback on our equipment. It can only help us attain the highest level of service and support.