Cibola SAR Land Navigation training
Date: September 11th, 1999 0900 - 1700(appx)
Location: St. Chad's Episcopal Church and Bear Canyon Trailhead
Instructors: Paul Donovan, Tom Russo
General Description:Approximately two hours of classroom time will be devoted to explanation of map features, compass use, and the use of map and compass together for wilderness navigation. Following some exercises to practice use of these skills, we will proceed to the field. Participants will be shown a topographical map with waypoints marked on it, and will be expected to copy the waypoints onto their own copies of the map and plan a route which will allow them to visit the waypoints and use their map and compass skills to navigate along this route.
Required map: Please come prepared with a copy of the Sandia Crest USGS quadrangle map.
Map symbolsThe following table of map symbols shows you some of the features which USGS topographic maps may contain. A fuller list is available in the US Geological Survey pamphlet "Topographic Map Symbols," which may be obtained for free wherever USGS maps are sold.
Recognizing Topographical FeaturesElevation features are described on maps by use of contour lines. A contour line on a map is the line you would trace out on the terrain if you were to walk along a path of constant elevation. Making the mental translation from contour lines on a map to the terrain around you takes practice, and we will spend some time today doing just such translation practice.
Put simply, here's how some commonly observed terrain features translate into contour lines:
- Peaks - closed curves of decreasing size. Inner curves are at higher elevation than outer curves, and the peak is within the innermost curve.
- Valleys - these appear as a series of "V" shaped curves. The point of the "V" always toward the head of the valley, and when there are streams in the valleys the "V" points upstream.
- Saddles - The space between peaks can be recognized by its hourglass shaped contour lines.
- Gentle slopes - slow rises in elevation over a long horizontal distance are represented by widely-spaced contour lines.
- Cliffs - since they represent dramatic changes in elevation over a short horizontal distance, cliffs are represented by closely-spaced contour lines.
Image source: "Be Expert with Map and Compass" by Bjorn Kjellstrom
Map coordinate systems and gridsUSGS quad maps all contain grids of one sort or another. Perhaps the most well-known outside SAR circles is the Latitude/Longitude grid (Lat/Lon), but more often we use the Universal Transverse Mercator system (UTM) in SAR work.
The Lat/Lon systemIn the Lat/Lon system the features on the surface of the earth are mapped onto a sphere, and a pair of angles is used to identify the points on the earth. The meridians of longitude are 360 equally spaced great circle arcs connecting the north and south poles. The meridian which passes through Greenwich, England is arbitrarily called "0" longitude, and meridians to the east or west of this meridian are measured in degrees east or west. Here in Albuquerque we are at approximately 106 degrees west longitude. Parallels of latitude start at the equator, which is 0 degrees latitude, and are basically slices parallel to the equator; they are also measured in degrees, and the angle referred to is the one between a line connecting the center of the earth to the surface of the earth at the equator and another line connecting the center of the earth to the surface of the earth at the point in question. Here in Albuquerque we are near the parallel of latitude designated 35 degrees north.
The lat/lon system is cumbersome to use for SAR work. There are 360 degrees used for latitude (0-180 East and 0-180 West), and there are 180 degrees used for longitude (0-90 North and 0-90 South). Each degree is divided into 60 "minutes" and each minute into 60 "seconds." The biggest problem for the map user is that lines of longitude converge at the poles and also a difference of "3 minutes" between two points cannot readily be converted to a distance, since this distance depends crucially on the distance from the equator.
The UTM systemThe UTM system is a rectangular coordinate system. The globe is divided up into "zones" of 6 degrees longitude with the first zone running from 180 degrees west longitude to 174 degrees west longitude. The central meridian in each zone is assigned the arbitrary "Easting" coordinate of 500 kilometers, and all points within the zone are assigned coordinates based on their distance from the equator ("Northing") and from the hypothetical 0 point of Easting coordinate; so at the equator and at the central meridian the coordinate is (500.0,0). Since zones are less than 1000 kilometers wide there is no point which is actually given the coordinate 0,0, and all UTM coordinates are positive. Zones are also divided into sections designated by a letter. Here in Albuquerque that is what the "S" stands for before our UTM coordinates, but this is redundant information: it merely denotes the range in which the northing coordinate falls.
Different ways of reporting UTM coordinatesMost GPS units report UTMs in meters rather than kilometers, and it is common to see on your GPS display something like this:
13S 0360639 3885020This is to be read as a location in UTM zone 13, section S, with easting coordinate of 360369 meters and a northing coordinate of 3885020 meters; that means that the given position is 139631 meters west of the central meridian (which has coordinate 500000) of zone 13, and 3885020 meters north of the equator. In kilometers this would be 360.369 easting and 3885.020 northing.
Even though only the digits down to 100 meters are significant, when reading UTM coordinates from a GPS display to base camp, rattle off every digit that is displayed, regardless of precision. This is to ensure that no errors are introduced into the coordinates by teams who interpret their own coordinates and round them off. Leave issues of precision to base camp, and instead concern yourself with getting information to them without introducing error into it yourself. This [easting followed by northing] is the preferred method for reporting UTMs to basecamp, even though some other organizations, notably the military, use a different, abbreviated method, the Military Grid Reference System (MGRS), described below.
A quick glance at a map shows that [even the xxx.x kilometer format]
is sending more information than is typically necessary. In general, on a 7.5
minute quad map only the kilometer and tens of kilometer digits change, and so
one could abbreviate the reported figure even further by leaving off
the hundred and thousand kilometer digits. This is a technique routinely used
in the military, and in this (MGRS) reporting system one would report the
location above as "606850" --- the first three digits being the ten kilometer,
one kilometer and one hundred meter digits of easting, and the second three
digits being the same figures for the northing coordinate. Figuring out what
digits to use is fairly easy when you look at a topo map: the UTM coordinates
are printed along the edge of the map like this:
One last point: when reporting UTM coordinates, one reports the easting first, then the northing. This is easy to remember when reading them off GPS units, because the format displayed is the correct format to read off to base camp for most GPS units (I have seen some low-end units do it backwards, though). To remember this when using a map, just Read Right Up (i.e. Read left to Right along the horizontal edge to get easting, then Up along a vertical edge to get northing).
Geodetic DataThe Earth is not actually spherical, and this creates a problem in mapmaking. The Lat/lon and UTM coordinates of a point on the surface of the earth are actually dependent upon the way that the Earth differs from a sphere. This is only a problem when there are multiple measurements for how the Earth is shaped, and naturally there are mutiple measurements.
Until very recently, all USGS maps were made with coordinates based on some measurements of the shape of the Earth made in 1866. This resulted in what was known as the North American Datum of 1927, or NAD27. But recently the USGS has updated their data, and now uses the North American Datum of 1983 (NAD83) on its maps. It is not possible to "mix-and-match" UTM coordinates taken off of maps with different data. An NAD27 map might show a particular pair of coordinates corresponding to a point on the earth some 200 yards away from where the same coordinates would be on a map of the same area with NAD83. This point was hammered home to us last year when we tried to work in an area which straddled two USGS quads and we used quads that had been made with different data --- and we were puzzled about why UTM coordinates read from the maps were not working out the way we expected them to. Always check your map datum when comparing coordinates obtained from two different sources (GPS/Map, Map/Map, Map/team-in-the-field-reporting-position, etc.).
The mixing-and-matching of map datum is most often a problem when using maps along with GPS receivers. Most GPS receivers use the WGS84 datum (neither NAD27 nor NAD83!) out of the box, and have to be reset through the menu system to use a different datum. BEFORE YOU LEAVE BASE CAMP, you should make sure that you are using the same datum that is used on incident base's maps! This has been a problem in recent missions, and you must absolutely be aware of it.
One last thought: while it could be considered unnecessary radio chatter, you might consider reporting your geodetic datum along with your UTM coordinates when calling in a position to base camp. This reduces the possibility that they not be aware of the difference between your datum and the one on their map; it doesn't eliminate it, of course, but it makes sure that the mistake of transcribing an NAD27 UTM coordinate onto a map with NAD83 grid lines without a conversion isn't your mistake.
Using a Compass
Parts of a compassThere are several different kinds of compasses, but they have many common features. The base of an orienteering compass is a rectangular piece of transparent plastic. On the ends and sides there are often scales of inches, miles, etc. that relate to the common scales on maps. A certain distance on the map is equivalent to an actual distance on land as determined by the scales. On the base is an arrow, called the "direction of travel" arrow or DOT. The DOT is used to depict where you are going or where you are pointing the compass.
The bezel is a raised circular transparent mechanism having marks on the edge representing the number of degrees. Inside its perimeter are a set of parallel lines. The middle line among these usually has some sort of arrow, pointing to the north mark on the edge. Let's call the middle arrow the "northward" arrow.
Inside the bezel is the magnetic needle, with one end which will point to magnetic north. It is suspended at the center and is usually balanced so it doesn't rub against the bezel. The bezel is also usually filled with a liquid to damp the motion of the needle, so that it settles quickly after some disturbance. The needle is usually colored red and white or red and black. The important point is that the red part of the needle always points toward the north pole of the local magnetic field. Note that this is not the same as saying that the needle always points toward the Earth's magnetic north pole. The difference is that due to perturbations in the Earth's magnetic field, it does not look like a simple dipole or bar magnet, with North at one end and South at the other. The Earth's magnetic field has curvature. We'll talk more about this when we discuss declination.
Determining the bearing to a landmarkThe proper technique for holding a compass depends upon what type of compass you have. For an orienteering compass without fold-up mirror or any other sort of sighting mechanism, the best method is to place your elbows comfortably at your sides, and keep them against your sides. To obtain a bearing to a landmark, face the landmark squarely with your feet comfortably apart. Hold the compass in front of you with your elbows close to your sides, with the compass level and the direction of travel arrow pointing directly away from you, perpendicular to the plane of your shoulders. In order to get consistent readings from the compass, it is important to re-create this position faithfully. Turn your whole body to modify the direction you are pointing, rather than moving your hands or arms. Holding the compass in this manner will result in more repeatable measurements and help to decrease errors in your bearings. Now rotate the bezel of your compass until the "north" (red or luminous) part of the needle is within the orienting marks. You can now read the magnetic bearing to the landmark off of the bezel at the direction of travel arrow.
A sighting compass must be held up to your eye so that you may look through it. Some of these have a folding cover with a mirror on the inside. When used, the cover is opened to tilt above the bezel, and there is a notch on the cover for sighting. The idea is to look at your target through the sighting notch and use the mirror to see when the magnetic needle is properly in place. Make sure to hold it as level as possible so the needle doesn't drag, and that any alignment marks such as lines on the mirror or notches on the bezel are properly lined up.
To obtain the bearing to a landmark, simply sight toward the landmark and rotate the bezel until the north-pointing end of the needle lines up with the alignment marks in the bezel. Then read the bearing to the landmark off the edge of the bezel.
Sometimes it is useful to know the "back bearing" from a landmark to your current location. The easiest way to do this is to find the bearing of the landmark, then turn the compass around and read the back bearing off of the bezel at the tail end of the direction of travel arrow. The back bearing is also easily determined from your bearing by simply adding or subtracting 180 degrees. Depending on what's comfortable for you, another way to determine back bearing is to simply use the bezel. Twist the bezel until the southward -pointing end of the magnetic needle (usually black or white) is lined up with the northward arrow of the bezel. The reading which is now indicated by the arrow or tick-mark on the bezel is the back bearing.
Exercise: finding bearings to local landmarksOnce we get to the practice area you'll see that we have laid out markers pointing at prominent features nearby. Go to each one in turn and determine the bearing from the marker to the landmark.
Walking a bearingIt sounds simple, but there are some practical considerations when you decide to walk toward a landmark you have chosen. For example, how can you make sure that you stay on course? What if there are some obstacles in the way? You could walk with your compass out in front of you set to the desired direction of travel, and keep looking down at it to stay on course. A better way is to pick some distant object that you can see that is in the direction that you want to go, and walk toward it. Keep looking at the object frequently, since its appearance may change as you get closer, or you may lose sight of it if you drop into a low area. When you get to the object, repeat this exercise until you get where you want to go. If there are obstacles (streams, cliffs, rocks, etc.) in the way, you can walk around them to get to the object you picked out from your last point. Then go to the other side of the object and repeat this process.
Exercise: The Three-Point Compass WalkThis is a simple field exercise we will do to practice walking bearings. We'll find an area that's open enough to work in, but wooded enough for it to be a challenge. Mark your starting position by dropping a coin (the value of the coin should be proportional to your confidence that you can find it again). Pick a random bearing, set your compass to that bearing, and walk it for a random distance, say 100 feet. Remember that distance. Stopping after this distance, add 120 to the bearing you've been walking, then set your compass to the new bearing and walk for the same distance as before. Stop, add 120 to the bearing again, and walk the same distance once more. You should be no more than a few paces away from the spot where you dropped your coin.
Magnetic anomaliesSince compass needles are really just lightweight magnets, compass measurements can be thrown off by nearby metal objects. Be sure to keep the compass well away from things like your radio, your car, that barbed-wire fence you're standing next to, railroad tracks, the power lines nearby, etc. You also need to keep metal objects such as belt buckles, knives, and pens away from the compass.
There are other phenomena associated with terrain that can affect compass readings, too. Tailings from mines where iron or other magnetic ores were gathered can affect compass readings. There are also geological features that are magnetic, such as the Malpais volcanic deposits south of Grants and northwest of Ruidoso, New Mexico.
Navigation with a map and a compass
Magnetic declinationThe map and compass can be used together to tell you precisely where you are and how to get where you want to be. But without keeping a few things in mind you might as well not have either.
As we mentioned earlier, the compass needle does not actually point at the northern end of the Earth's axis, that is "true north," but rather at the north pole of the local magnetic field. Most maps, however, are drawn in a projection which puts meridians of longitude parallel to the sides of the map --- that is, the vertical edges of the map point to true north. The angle between a line drawn from any point on the map to the north pole, and a line drawn from that same point to the local magnetic north is known as the magnetic declination, and is currently about 10.5 degrees East in our area. In other words, when your compass is reading zero degrees along your direction of travel, you're actually traveling on a true heading of 10.5 degrees true. Similarly, if you measure on your map that you have to follow a true heading of 10.5 degrees, you must know that when you use your compass to follow that path you have to set it for a heading of zero degrees!
Many people have come up with mnemonics to help remember whether to add or subtract declination to convert from magnetic to true bearings. One reliable tool that you can't forget is the declination diagram printed on the map. Here's an example of one, taken from a USGS training website (NOT a map of our area!):
One approach for dealing with declination is to draw magnetic north lines onto your map. To do this, set your compass to the declination --- thirteen degrees in the case above, and set it on the map with the north line of the bezel parallel to a true north line on the map (ignore the needle for this, just use the markings on the case). Now your direction of travel arrow points along magnetic north. Using the edge of your compass as a straightedge, draw a magnetic north line. It is best to draw several of these lines, across the entire map. Now you can read magnetic bearings directly off of the map by making measurements relative to your magnetic north lines instead of the true north lines, obviating the need for any formulas at all. But be mindful of one thing: magnetic declinations change over time, and the declination printed on the map might not be the declination which is actually affecting your compass today; the change is small over a year, but some maps were printed 10 years ago or more. The declination you must take into account is today's declination, because that's the one your compass sees. So if you draw in magnetic north lines, make sure you're doing so with the right declination.
Another caution which can be important in other parts of the country: the declination diagram is not always to scale, especially if it is depicting small angles. In the case of small angles the figure might be exaggerated, but the numbers printed nearby will be correct. Sometimes map users are told to extend the magnetic north line on the declination diagram to obtain magnetic north lines on the map, and most of the time that's OK, but watch out for printed statements nearby that the diagram is "for obtaining numerical values only." And remember, too, that the declination diagram might be outdated. For these two reasons it's probably better not to use the diagram directly to draw your magnetic north lines.
Exercise: finding true bearingsWith each of the magnetic bearings you obtained in the bearing exercise above, determine the true bearing by applying declination.
Resection to locate position on the mapIn order to determine what direction to go in order to get where you want to be, you must first know where you are. Sometimes this is easy, such as when you can unambiguously identify a feature on the map, and you know that you are standing right next to it. In other cases the map and compass can be used together to locate your current position on the map. This process is sometimes referred to as "triangulation" but is more precisely called "resection."
In order to locate yourself on the map by performing resection, the basic idea is to compare your topographic map to what you are looking at, and identify terrain features that you are sure you can both see and associate with a feature on the map. Just by looking at the map and the terrain, you should have a general idea of your location. Terrain recognition is important to pinpoint your location more accurately. Now you determine bearings to these features, and draw lines on the map corresponding to the bearing to those features. The use of at least three lines is recommended, and they should cross in a small triangle. It is ideal to choose landmarks all around you, but sometimes this is not practical, as when on one side of a mountain range, with nothing distinguishable in the other direction. Choose landmarks as far apart as possible. Your best guess at your position will be in the center of the triangle that you draw. This process is illustrated schematically below:
Exercise: Terrain identification and resectionNow that you have true bearings to all the landmarks we pointed out for you in the two exercises above, it's time to figure out where we are. The first step is to identify each of those features on your map; this is probably the hard part, and we'll probably be spending a good bit of time on this. The next step is to draw lines from the feature on the map which make the same angle with the true north lines the bearing you determined dictates they should. Where the lines intersect is where we are. Congratulations, you've done a resection!
Estimating distanceIt is easy enough to use pacing to estimate short distances in the field. However, another very useful skill is the ability to estimate longer distances in the field, and how these compare to distances on the map. This comes with practice. Mastery of this skill will help enormously in terrain recognition, since in addition to the shape of the object, some clues about how to uniquely identify the feature can be gained from estimating about how far away it is, and seeing if this is consistent with the map. The best way to practice is to carry a topographical map of the area while you are hiking. Pick out objects that you will be hiking to, and see how long it takes you to get there. Stop and look around while hiking, and see if you can pick out near and far objects on your topographical map.
Walking a bearing taken from a mapOk, you've marked two points on your map, one representing your starting point and the other representing the place you want to be. What now?
The easiest way to set yourself up to walk to your destination is to set your compass to the right bearing. Draw a straight line between starting point and destination, set your compass so that the DOT arrow points from starting point to destination. Now rotate the bezel of your compass so that the parallel lines inside are lined up with the magnetic north lines you've drawn onto your map. Your compass is now set so that if you turn yourself until the north-pointing part of the compass needle is lined up with the alignment marks on the bezel, then you will be walking the correct magnetic bearing to your destination.
Once you have set your compass to the correct bearing, you can forget the map again and just follow the bearing as we discussed above: pick out a landmark that lies along your intended direction of travel and walk towards it.