Volume 3, Issue 5
14 May 1998
and Mickey Jojola
"That Others May Live..."
Meet at 0800 to carpool at Fire Station on So. 14 (marked route 337). The
fire station is 9.7 miles south on route 337 (south 14) from I-40 exit
Bosque Trail is 2.2 miles from trailhead to Manzano Crest Trail (170).
The trail begins as a wide, rocky path and quickly becomes steep. About
3/4 of a mile up, a switchback momentarily levels the trail. Then trail
ascends up the north side of the canyon. Cave spring, a cattle watering
spot, can be seen in the canyon bottom. A short distance from the spring
a spur trail to the north lead to a cave in the canyon wall. Farther up
the trail, large stands of oak and New Mexico locust divide the canyon
into small meadows. The trail emerges into a larger meadow just below the
Manzano Crest Trail (170) and continues southwest through the clearing.
Turning south, it comes along the ridgeline. At this junction with the
Manzano Crest Trail is a half-section of private land that originally
belonged to the Rea-Formwalt family. The old homestead cabin ruins are
Bosque Peak is to the east of the trail junction and can be accessed from
either side in this open area. Continue south along the Manzano Crest
Trail for about 2.5 miles to junction with trail 176 (Trail Canyon Trail)
which descends through Canyon de la Vereda. Trail 176 is 2 miles long.
Return on the gravel road to Bosque Trail parking area.
|Hike of the Month||Bosque Peak via Bosque Trail and Trail Canyon
Trail Loop||0800, May 23/24, 1998|
|Trailhead: Exit 175 from I-40, south on 337 (south 14) to SH 55 south to
Tajique. From Tajique take FR 55 9 miles to trailhead. (see below for carpool
|R.T. Distance: 6 miles||Elevation Min/Max: 7440/9610|
|Hiking Time 3.5 hours||Hazards: area laced with unmarked man-made and cow
trails that can be confusing. |
|Topo Maps: USGS Bosque Peak, Capilla Peak or Forest Service Manzano Mountain Wilderness Map & Guide|
||by John Mindock and Mike Dugger
ORIENTEERING - PART 2
This lesson will concentrate on declination and its role in land navigation.
The north which applies to roads, section lines, and other topographic map features is based on lines that run between the geographic north and south poles (i.e., the ends of the line which represents the rotational axis of the earth). These are known as lines of longitude, and actually are "great circles" that all intersect at the poles. This so-called "true north" is depicted on the bottom of a topographic map as a line segment with a star atop it, representing the North Star's apparent location above the North Pole. For you nit-pickers Polaris, the North Star, does not lie exactly on the rotational axis of the earth but is off by perhaps a degree. It is possible to look up tables of the precession of Polaris around the Pole axis, and determine the correction to apply at any given hour, on any day of the year.
The poles of earth's magnetic field do not coincide with the geographic poles - the northern magnetic pole is somewhere in Canada above the U.S. Great Lakes region. Thus, the magnetic north to which a compass needle points is not the same as true north. In addition, the magnetic field lines are not always straight between the magnetic poles, like the projection of a great circle of longitude is on the surface of the earth. Magnetic field lines curve. The difference between where the compass points and true north is depicted on the bottom of a topographic map as a line segment with a half-arrowhead, displaced from the true north line segment by the appropriate degrees "declination," as described below. The earth's magnetic field is not particularly strong, and other objects which generate a magnetic field can affect compass readings, such as knives, belt buckles, radio batteries, watches, pens, railroad tracks, electrical lines, and so forth. There are also geological features that are magnetic, such as the Malpais volcanic deposits south of Grants and northwest of Ruidoso, New Mexico.
The difference between true north and magnetic north is called the declination. Technically, this is the angle between the geographic meridian and the local magnetic meridian. Since a compass points about 10.5 degrees eastward of true north in central New Mexico (as of today), it is labeled "east" declination. Said another way, our declination is 10.5 degrees east. The amount of declination has been decreasing slightly for the last few decades, so the declination listed on older topographic maps is larger (by as much as two degrees) than its current value. Trivia buffs may be interested to know that the earth's magnetic field can actually reverse over time, but this occurs at geologic time scales. Hopefully, none of our searches will last that long! For the rest of this document, we'll assume 10 degrees east declination to keep the math simple.
Translating Compass To Map Bearings
The following situation is typical. Searchers in the field determine compass readings to landmarks, relative to magnetic north, by sighting on them with their compasses. These readings need to be transferred onto maps, which have lines and borders parallel to true north. Imagine that you are walking along a fence or road that marks a section line, and points toward true north. What would your compass say? Since the compass needle will be pointing 10 degrees off to the east, you would conclude that you are walking a bearing of 350 degrees. But on the map, this direction is 0 degrees, or true north. Therefore, to translate the magnetic bearing determined from the compass to a map bearing, you would add 10 degrees to get 360 or 0 degrees, true north. The MAGnetic reading, PLUS DEClination, equals the TRUE or map reading. So, when working with east declinations, we add magnetic plus declination to get the map bearing. Equivalently, if given a bearing determined on a map with respect to true north, we would subtract the declination to determine what our compass should read relative to magnetic north.
A variety of algorithms, mnemonic devices, and formulas have been devised to help people remember whether to add or subtract declination. It is best to understand the concept, then pick any convenient contrivance solely as a double-check. You could visualize the lines of magnetic field pointing off to the east of true north (again, a function of our position on the earth), and imagine what your compass would read if you were following a section line, as in the above example. Those inclined to remember formulas could memorize MAG+DEC=TRUE. Yet another is "east is least, and west is best," indicating that one should subtract declination from map bearings to get magnetic bearings for east declinations, and add for west declinations. There are many others. Use whatever works for you.
Translating Map To Compass Bearings
In this case, you are asked to follow a heading of X degrees true north. You need to derive the heading relative to magnetic north so you can use your compass to follow it. Since in our part of the country MAG+DEC=TRUE and MAG=TRUE-DEC, the proper heading to set on your compass is X-10. Of course you may need to make an adjustment (add 360) if X-10 is a negative number.
There is one more direction reference at the bottom of a topographic map, depicted as a line segment with "GN" atop it. This is related to the UTM grid mentioned in the previous mini-lesson. The difference between grid north and true north depends upon which latitude you are at, and is less than one degree throughout New Mexico. GN is generally not referenced in SAR work (even when UTM's are used).
Exercises - Orienteering Part 2
- What imaginary lines is True North based upon?
- Where is the Magnetic North Pole located?
- What is the meaning of "declination"?
- 78 degrees magnetic is what number of degrees relative to true north, in central New Mexico?
- 8 degrees relative to true north is how many degrees magnetic, in central New Mexico?
It's been a fairly hectic month here at Newsletter Central,and we apologize for this skimpy newsletter. I also apologize to those who didn't receive their April newsletters in the mail. I have these newsletters for you, and will be mailing them along with the May issue. I hope to have sufficient contributors lined up by June to make that issue a touch weightier than this one.
||by Tom Russo
Susan Corban, Mickey Jojola (and, of course, "Jake") and Don O. Gibson gave a
PSAR presentation to a group of third, fourth and fifth graders at Edmund
G. Ross Elementary school. By all accounts it was a successful event, and we
look forward to more of these in the near future.
The information in this newsletter was gathered from many sources and presents facts as we believe them to be true. This newsletter is not meant to be an official document, but a means to disseminate team information.
||by Tom Russo