BREVE ANÁLISIS DE ITALIA

Troops in the field need to be able to find their way from one location to another. In particular, they need to be able to find their way to an ambush site, to a rally point, to a rest camp, or other specific location. Ability to do this is critical to Resistance forces.

Map Reading

A map is essentially a picture of the Earth as seen from overhead. However, it is a very specialized picture, an abstraction. It does not show the Earth as it actually looks, but instead presents only those features important to the map user. A road map will show roads and towns, and possibly rivers, bridges, railroad crossings, and other features important to the motorist. A hydrographic map, instead, will show things of interest to the mariner, such as water depth, location of reefs and wrecks, lighthouses, and location of shipping channels. An airways map will show airports, beacons, surface elevations, air routes, and other features of importance to a pilot. That is, these different maps

emphasize those features important to the user, and suppress all the rest. Regardless of the type of map, however, it is still a picture of the Earth, as the user would see it from the appropriate altitude, with only the important features marked.

One point must be kept in mind about maps. The Earth is (approximately) a sphere.

Maps are flat. It is impossible to portray the spherical Earth accurately on a flat map. Every map is in some way a compromise, designed to represent a region as accurately as possible. On the Earth, lines of longitude converge at the poles. On the commonly used Mercator map projection, however, the lines of longitude are parallel. Near the equator, this isn't a serious problem. Near the poles, however, there is serious distortion, with features stretched in an east-west direction.59 While the Mercator projection distorts regions near the poles, it has the significant advantage that a compass course crosses all the meridians at the same angle (a so-called “rhumb line” course). For small distances, the deviation from a true “great circle” or minimum-distance course is not significant, and plotting a rhumb-line course on a Mercator map is much easier than plotting a true great circle course.

From the standpoint of the Resistance fighter, there are three kinds of maps that will be important. For urban operations, a street map is required. For rural operations, a topographic map is required. Each one, it its own way, presents those features the user is interested in, and suppresses all the rest. For instance, on a street map, color may be used to indicate the size or importance of the street, rather than the actual color of the paving. Parks, reservoirs, public buildings, and other urban features may be shown, but not in their true color. In addition, the Resistance fighter may use aerial or satellite photos.

For the typical operations of a Resistance unit, the problem of representing a spherical Earth on a flat map is not significant. City street maps and topographic maps are adequate for the purposes to which they will be put.

Topographic Maps

An example of a topographic map is shown in Figure 10-1. This map is a section of West Virginia. A topographic map shows roads, towns, railroads, political subdivisions, rivers and lakes. It also shows a scale of distances. What makes it a topographic map is that it shows elevations as contour lines. Each contour line is noted as to its height above a reference. In the case of this map, the reference is sea level. Contour lines provide important information about the “lay of the land.” If they are close together, the land is steep. If far apart, the land is gently sloping or level. If closed, they indicate a hill (or a hollow). V-shaped contours indicate a valley. Topographic maps can be helpful in identifying suitable locations for ambushes and similar activities.

Figure 10-1. Example of Topographic Map (U.S. Geological Survey)

Street Maps

An example of a street map is shown in Figure 10-2.60 The map shows streets, important buildings, parks, and other urban features. Street maps may or may not have a distance scale. Almost always, however, they will have grid markings like those shown at the edge of the map. These allow the user to find some feature in an index, then locate it on the map using the grid references. For instance, a high school is shown in Grid Square M-10.

Figure 10-2. Example of Street Map (Dayton, OH)

Aerial & Satellite Photos

An example of a satellite photo is shown in Figure 10-3. This is an image of Chicago, IL, obtained from Google Earth. Satellite photos vary in the resolution available, depending on the source. Those from NASA are usually of fairly low resolution. Those from commercial sources may have resolutions as good as half a meter. High resolution photos, up-to-date, can be expensive.

However, for many purposes those available from sources such as Google Earth may be satisfactory. The Resistance should plan to make use of available satellite imagery in planning raids, ambushes, and similar activities.

Using Maps for Navigating

There are two questions you may need to answer regarding land navigation. The first is “Where am I?” That is, can I locate my present position on the map? The second is, “How do I get to where I want to be?” Note that this second question does not require that you know where you are. It only requires that you know how to get to a specific place. As will be shown later, it is possible to navigate from a known starting point to a known destination without knowing where you are at every step of the way.

Global Positioning System (GPS)

GPS is a satellite-based navigation system. It uses twenty-four satellites orbiting the Earth in six different planes at an altitude of 20,200 km. The satellites have a twelve-hour period. The Coarse Acquisition (C/A) mode signal of GPS is at 1575.42 MHz. It provides an accuracy of 75 – 100 meters. The Precision-code (P-code) signal is at 1227.6 MHz. It has an accuracy of 16 meters. When augmented by Differential GPS, the GPS accuracy can be a few inches. GPS provides 2-D and 3-D coverage throughout the world.

P-code is encrypted by the US government to deny it to enemy nations. C/A can be degraded in wartime to deny precise targeting information to an enemy.

GPS is not the only satellite navigation system in existence. Russian GLONASS and European Galileo may also be available. However, receivers for these systems are not as readily available as are receivers for GPS.

Keep in mind that GPS may not be available to the Resistance. It may be degraded severely or turned off completely, if the U.S. government is involved in fighting elsewhere, or for other reasons that have nothing to do with your fight against your government. Other means of

not be available, however, it is important for the Resistance fighter to know other means of navigation.

Compass and Map Navigation

A magnetic compass points to the North magnetic pole, which is located in northern Canada, south of the true North Pole. The difference between magnetic north and true north is called “declination.” There are maps that give the declination for specific areas on the Earth. The book by Seiden contains an example. Since most maps are based on true north, it will be necessary for the Resistance fighter to correct for declination when using a magnetic compass.

Compass Types

There are two types likely to be of interest to the Resistance fighter: the baseplate compass and the lensatic compass.

Baseplate Compass

A baseplate compass is shown in Figure 10-4. The rotating dial can be used to offset the compass to account for declination. If at your location the declination is West (that is, the compass needle points west of true north), the rotating dial should be rotated clockwise a number of degrees equal to the declination. Conversely, if declination is East, the rotating dial should be rotated

counterclockwise. Then when the needle is contained within the outline on the baseplate (and in the

right orientation), the numbers on the rotating dial read degrees from true north. Figure 10-4. Baseplate Compass

Lensatic Compass

A Lensatic compass is shown in Figure 10-5. The essential features are a sighting wire and a sighting notch. In use, the compass is held near eye level. The user sights on a landmark by looking through the notch and lining up the sighting wire with the landmark. The lens then allows the

user to read the bearing to the landmark.61 The lensatic compass can be used to orient a map, however it is not as convenient to do this as it is with a baseplate compass.

Figure 10-5. Lensatic Compass (from FM 21-6)

Some Lensatic compasses do not have an adjustment for magnetic declination. If so, then some arithmetic is required. If the declination is West (or minus), add the amount of the declination to the measured bearing. If the declination is East ( or plus), subtract the declination from the measured bearing. This will give the true bearing, which can be plotted on a map.

Using Compass and Map

The baseplate compass is intended to be used with a map. The map is placed on a level, flat surface. The compass is corrected for declination. The edge of the compass is aligned with true north on the map. The map is then rotated until the compass needle is within the outline. Once this is done, the map is oriented to true north. The numbers on the rotating dial of the compass then

correspond to bearings on the map.

With the baseplate compass, it is assumed you know where you are and can locate that point on the map, but you need to know the direction to get to your destination, which is also on the map. Orienting the map to true north will allow you to determine the bearing to follow to get to your destination.

The lensatic compass can conveniently be used to locate yourself on a map, by a process known as “resection.” This is shown in Figure 10-6 (taken from FM 21-25). The idea is to “shoot a bearing” from your position to at least two prominent landmarks that can be located on the map (hills, towers, smokestacks, etc.) That is, you get the bearing from your (unknown) position to the landmark. You then draw on the map a line from the landmark back to you, at the “back azimuth” from the

landmark. The back azimuth is computed as follows. If the direct bearing (including correction for declination) is less than 180 degrees, back azimuth is the direct bearing plus 180 degrees. If the direct azimuth is greater than 180 degrees, the back azimuth is the direct azimuth minus 180 degrees. Plot back azimuths from at least two landmarks on the map. You are at the intersection of the back

azimuths. Ideally the landmarks should be about 90 degrees apart as seen from your location, to provide the greatest accuracy.

Figure 10-6. Locating yourself by resection.

Following a Bearing

Regardless of which method you use to locate yourself on the map, you then need to follow a bearing to reach your destination. The best method is to locate some landmark (hill, prominent tree, building) that is right on the bearing you want to follow. Simply head for that

landmark. Even if you have to deviate from your course because of minor obstacles, when you reach the landmark you are on course, and can then repeat the process.

If there is no prominent landmark in the direction in which you want to go, but you are at some prominent landmark, you can reverse the process. From time to time, shoot a bearing back to your starting point. The back azimuth from that point will be the bearing you want to follow. If the back azimuth you measure is wrong, you know you have deviated from your desired course. You can then correct your course by moving sideways until you get the correct back azimuth.

If there are no landmarks available, one option is to send one member of your party ahead, using radio or hand signals to keep them on the correct bearing. Once they have gone far enough, have them wait while the rest of the party catches up.

Bearing and Distance Navigation

Navigating using compass and map allows you to know where you are all the time. You know the direction you must follow to reach your destination.

However, there is an alternative. Historically it has been known as ded (from deduced) reckoning. It amounts to keeping track of the distance and direction you travel on each leg of a multi- leg journey, especially if you have to deviate from the direct course to your destination to avoid obstacles or for other reasons.

Assume you are to reach a destination that is 600 yards away from your starting point, at a bearing of 40 degrees. From the starting point, take the compass bearing on which you intend to travel, which may be different from 40 degrees, because you want to avoid some obstacles. As you move, measure distance or time on that bearing. When you change direction, repeat the process. If on foot, count paces, or clock minutes and multiply by your rate of march. In a vehicle, use the odometer. The results can be recorded in a table such as that in Table 10-1.

Table 10-1. Record of bearings and distances.

(Use Start and Stop columns for starting and stopping times)

The bearings and distances can obviously be plotted on a map. At the end of each leg, you will know where you are relative to your destination. However, a map is not needed. Figure 10-7 shows the bearings and distances plotted on graph paper. The direct distance to the destination is shown as a dashed arrow. The legs from the Table are shown as solid arrows. At the end of the fifth leg, with a protractor and ruler you can read off directly the bearing and distance to the destination: 200 yards at 82 degrees. However, while graph paper is convenient, it is not needed. The path can also be drawn on plain paper. It can even be scratched in the dirt. So long as it is drawn to scale, a plot of bearings and distances is sufficient to navigate from a starting point to a point at a known distance and bearing, or to navigate back to the starting point, regardless of deviations from a direct path.

Figure 10-7. Plot of bearings and distances

Urban Navigation

The map and compass navigation used in rural or open area is of little value in urban areas. Travel in cities is via streets and alleys. A street map is more valuable than a compass. Electronic street maps of many cities are available for GPS units. These maps can be downloaded into the GPS, which then provides the user with his present position in the city in terms of street location rather than latitude and longitude. Even GPS may be of little help, however, since tall buildings block the satellite signals. Street signs may be far more useful than either a compass or a GPS in cities. Resistance fighters operating in cities must be able to use street maps to navigate from where they are to where they are needed.

Aerial and satellite photos are also useful, but they lack street names. However, street names can be marked on the photos prior to an operation, and they will give a more accurate

Summary

Resistance forces will need to navigate from one point to another. If GPS is available, it is the best method to use. In the absence of GPS, Resistance force members should be trained to use map, compass, and ded reckoning.

References

http://www.armystudyguide.com/content/army_board_study_guide_topics/land_navigation_map_reading/index.shtml Department of the Army, Map Reading and Land Navigation, FM 21-26, 1987.

Gallagher, James J., Combat Leader’s Field Guide, Chapter 13, Mechanicsburg, PA, Stackpole Books, 1994, ISBN 0-8117-2425-5.

Hotchkiss, Noel J., A Comprehensive Guide to Land Navigation with GPS, Herndon, VA, Alexis Press, 1994, ISBN 0-9641273-2-6.

Paul, Don, The Green Beret's Compass Course, Woodland, CA, Path Finder Publications, 1991. (Instructions on ded reckoning.)

Seidman, David, The Essential Wilderness Navigator, New York, McGraw-Hill, 1995. (Essentially a map-and-compass instruction book.)

http://www.monsterguide.net/how-to-use-a-lensatic-compass.shtml, a web site that gives instruction on use of a lensatic compass.

Appendix To Chapter 10

As shown in the main text, you can navigate using bearings and distances, making a scale drawing of your path “made good.” However, with the use of some mathematics, even a drawing can be eliminated.

In the example, the object was to reach a destination 600 yards away, at a bearing of 0 degrees. You could do this by moving 459 yards due north and then 385 yards due east. These are calculated, using sine and cosine functions as:62

Distance north = 600*cos(40)=459 Distance east = 600*sin(40)=385

Thus we can set up a table similar to the one in the main text (omitting the unused “time” columns), where “Northing” is the distance made good to the north, and “Easting” is the distance made good to the East. Northing is calculated as the leg length multiplied by the cosine of the bearing angle for that leg; Easting is calculated as the leg length multiplied by the sine of the bearing angle. If the travel is to the south or to the west, the sine and cosine functions will automatically attach the proper negative signs.

Table 10A-1. Calculated distances and bearings

The first five rows show the distance made good to the north and the east on each leg. These are calculated as the leg distance multiplied by the cosine and the sine of the bearing angle for that leg, respectively. The total distances made good north and east are shown in the seventh row. The intended distance is shown in the eighth row, and the distance remaining in both north and east

directions, that is, distance desired minus distance made good, is shown in the ninth row. Bearing and remaining distance are shown in the tenth row. These are calculated as:63

Bearing = arctangent(Easting remaining/Northing remaining) (Note that the desired angle is measured from North)

Distance = Easting remaining/sine(Bearing). (Note that sums may not equal detail because of rounding.)

This exact calculation should be compared with the graphical calculation. The two are within the accuracy of the graphical calculation. For many practical navigation problems, the

graphical ded reckoning method is “good enough,” provided sufficient care is taken to make the drawing to scale. However, the mathematical calculation is more accurate and avoids the need to make a drawing.64

The mathematical calculation can be done on a laptop with a spreadsheet, if this is