ASSET Operation
The ASSET is easy to use.
Level the system.
Adjust the tripod until the bubble is centered in the bull's-eye.
Align to South.
First, determine the local magnetic declination for your survey site. You can get this from a USGS topographic map or online. The map on the right gives a general sense of how magnetic north varies from true north. East Declination means that "magnetic north", the direction that your compass needle will point, is east of "true north", and vice versa for West Declination. In the United States, the division between East and West Declinations is approximately at the Mississippi River, and declination becomes greater the further east or west you travel.
Declination
Adjust the outer declination dial for local magnetic declination (as shown in the left photo of Figures 5 and 6). Figure 5 shows an example of an East declination adjustment of 20°: the red arrow (magnetic north) is rotated 20° East of the white indicator bar. The white indicator bar (true north) will then be aligned with 340° (360° - 20°). Figure 6 shows an example of a West declination adjustment of 20°: the red arrow (magnetic north) is rotated 20° West of the white indicator bar. The white indicator bar (true north) will then be aligned with 20° (0° + 20°).
Figure 6: 20° West Declination Example
(For locations east of the Mississippi River)
Figure 5: 20° East Declination Example
(For locations west of the Mississippi River)
Fig. 7: Camera Position To
Minimize Compass Deflection
The metal parts used in modern digital cameras affect the ASSET compass. When aligning your ASSET to true south, position your camera in the manner listed below (see Fig. 7) to minimize deflection of the compass:
- Rotate the camera to directly above the base, with the aluminum between the camera and the compass, just as the ASSET is positioned in the carrying case.
- Tilt the camera to a 45-degree angle (Adjust tilt).
In addition, any magnetic metals near the compass, including wristwatches, hardware, tools, steel tripods, etc, should be removed from the vicinity of the compass during alignment to obtain the most accurate reading.
Figure 8: knurled screw
Loosen the knurled screw (see Fig. 8) and rotate the entire assembly so that it aligns to South. When the red half of the compass needle is directly over the red arrow, the ASSET tool will be aligned to true South (as shown in Figs. 6 & 7, right photos). Tighten the knurled screw.
If you're still having trouble or simply would like more information, www.compassdude.com has some great info on how to use a compass, and in particular, how to adjust for magnetic declination. Also, the U.S. Geological Survey website is an excellent resource for geomagnetics with numerous charts and models of the Earth's magnetic fields.