Appendix H: Map and Compass Skills
Contents |
Map and Compass Guidelines
Selecting Maps
TBSP recommends the following maps for patrollers. Basic map characteristics should include:
- 1:24000 scale
- Waterproof paper
- UTM and Lat/Long coordinate systems
- Large enough to include surrounding peaks (For resectioning)
- By convention, patrol maps use NAD27 CONUS
Order nifty waterproof maps of our patrol areas, with UTM and Lat/Long grid lines at mytopo.com.
Selecting a compass
A good compass with declination adjustment, sighting mirror and clinometer (Suunto MC 2G or Silva Ranger Type 15CL, aka CL515) costs $55-$70. Buy a good one. The following discussion assumes you have followed this advice. Consequently, it does not deal with the procedures and calculations required to correct for declination if your compass does not have a declination adjustment feature. (If you insist on using a compass without declination adjustment, these procedures can be explained in the field, but they are confusing and difficult to remember. Don't say we didn't warn you.)
Declination adjustment
The Magnetic North Pole lies to the south of true North Pole, which causes an error in compass readings known as declination. The value of declination for a particular area is found on the lower left corner of all USGS maps. For our purposes, declination of 17o east is sufficient for all maps. To adjust your compass, simply insert the small key into the screw on the bottom of your compass and turn the screw until the mark at the rear of the orienting arrow lines up with 17o east declination. Now, unlike those who have compasses without declination adjustment, you will never have to worry about declination again.
Map bearings
- Mark (or note) your position and destination on the map.
- Place the long side of the compass base plate on these two points so that the cover is towards your destination. If necessary, the cover of the compass or a ruler can be used as an extension of the base plate.
- Turn the bezel until the meridian lines on the compass capsule are parallel with the meridian lines on the map (or the left or right border of the map) and N on the bezel is on the north side of the capsule with reference to the map.
- The bearing from your location to your destination can now be read at the index pointer (centered luminous mark near the hinge of the compass).
To transfer this bearing to the field, simply turn your body until the needle is centered on the orienting arrow and the colors match (red with red and black with black). [After dark, turn your body until the luminous point on the needle lies between the luminous points on either side of the orienting arrow.] The sighting line through the mirror indicates your direction of travel.
Orienting (setting) the map
To orient your map (only necessary when in the field), follow the same procedure described above, but leave the compass on the map and turn the map instead of your body. Once the map is oriented, you can compare field topographical features with those depicted on the map.
Taking field bearings
- open the cover to approximately 60o.
- place the lanyard around your neck and raise the compass to eye level while holding it level with both hands, and hold it as far from your body as possible so the lanyard is tight.
- close one eye (always close the same eye) and sight through the notch in the cover (mirror), turning your body until the landmark is precisely above the notch, AND the vertical centerline of the mirror appears to intersect the axis of the needle.
- while maintaining the above position, look into the mirror and turn the bezel until the needle is within and parallel to the orienting arrow as observed in the mirror.
- read the bearing at the index pointer (centered luminous mark near the hinge of the compass).
Finding your location using two or more landmarks
- Take a bearing on a known landmark as described above.
- to transfer this bearing to the map, place the compass on the map and open the cover wide so the long edge of the compass intersects the known landmark.
- turn the compass edge around the known landmark until the meridian lines on the capsule are parallel with the meridian lines (or the left or right border) of the map, and the N mark on the bezel is on the north side (on the map) of the capsule.
- starting from the known landmark, draw a line along the edge of the compass in the direction of your approximate location.
- repeat the process using a second landmark approximately 90o from the first landmark.
- your location is the point where the two lines intersect.
- for improved accuracy, repeat the procedure using a third landmark. A line from the third point should intersect at approximately the same point
Using the clinometer
We use the clinometer to measure slope angle, a critical indicator of avalanche propensity, and alpha angles, which indicate whether you are in a potential avalanche run out zone:
- turn the bezel until it reads 270o at the index pointer.
- position the compass so the long edge is parallel with the surface to be measured and the base plate aligns with the vertical plane (this allows the clinometer pointer to turn freely).
- open the compass cover to approximately 30o.
- steady the compass with both hands and sight along the top edge of the base plate. Looking into the mirror,
read the angle indicated by the clinometer pointer.
GRID COORDINATE SYSTEMS
Patrollers must know how to determine their location on a USGS or Forest Service map and how to move from one location to another using a map and compass. Patrollers also must know how to direct helicopters to a pre-determined landing sites, and be able to use a map and compass to reach such sites. In order to be able to express their location to various rescue personnel, patrollers must learn three systems of map coordinates:
- Universal Transverse Mercator (UTM) which is used by many search and rescue organizations,
- Public Land System (PLS), also called "Township and Range," which is used by the Forest Service; and
- geographic coordinates (latitude and longitude), which are used by rescue helicopters and other aircraft.
Coordinates for all three systems are found on every USGS map we use. Some search and rescue organizations and air medical services use computer software to convert among various coordinate systems, but not all organizations have software or personnel capable of converting among all three systems. Consequently, we must be able to express our location in all three grid coordinate systems. Most map and compass training will be done in the field. Field training will concentrate on locating positions and plotting courses using landmarks and a map and compass. We will not spend much time on grid coordinate systems in the field. The following sections describe each of the three grid coordinate systems used to determine specific map locations. Examples are based upon the USGS 7.5 minute Norden Quadrangle map. You should attempt to master this material before you begin field training in order to gain a more complete understanding of this critical subject. It may seem overwhelming at first, but if you go through each system several times and do the sample problems you should be able to master all three systems. Instructors will attempt to answer any questions you have regarding the written material.
Universal Transverse Mercator (UTM)
Basis of system
Distance is measured in meters north and south from the equator and east from a central meridian.
From the wikipedia entry for UTM: The point of origin of each UTM zone is the intersection of the equator and the zone's central meridian, but to avoid dealing with negative numbers the central meridian of each zone is set at 500,000 meters East. http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system
Map marks
UTM grid coordinates are the series of black numbers on the white border of the map next to the blue grid ticks. For example, in the upper left margin of the 7.5-minute Norden Quadrangle appears the number 4361000N. The blue grid tick to the right of this number marks a distance 4,361 kilometers (4,361,000 meters) north of the equator. In the left end of the top margin of the map, just below the words "GEOLOGICAL SURVEY" is the number 727000E. The blue grid tick to the left of this number marks a distance 727 kilometers (727,000 meters) east of the principal meridian. Other similar numbers on the map eliminate the 000m and express the distance only in kilometers.
Using the map marks to describe a location
Forget all of the superscript numbers. Henceforth, 4361000N will be referred to as 61, and 728 as 28. To learn how to describe your location, let's use an example from the Norden 7.5 minute map:
- locate the summit of Donner Peak in the middle of your Norden 7.5 minute map.
- draw an imaginary north-south line from your position to the top or bottom margin of the map, and note the blue UTM grid tick numbers on either side of the point where your imaginary line intersects the map margin. The numbers should be 31 and 32. [Note that while the blue grid ticks are always present, often the numbers are missing and you have to extrapolate them from nearby grid tick references. For example, at the bottom of the map, the reference to 32 is missing, and at the top, the reference to 31 is missing.]
- Do the same exercise using an east-west imaginary line to the left or right margin of the map, noting the blue grid ticks above and below the point where your imaginary line intersects the map margin. You should fall between grid ticks 54 and 55.
- Now draw a line between grid tick 31 on the top and grid tick 31 on the bottom of the map. If you don't have a straight edge long enough (and you won't in the field), fold over one edge of the map and use it as a straight edge to connect the two "31" grid ticks. Be certain that the edge of the map is flat or your line will be off. Next, do the same exercise to connect grid ticks 32, 53 and 54. [We strongly suggest that for training purposes you draw the entire UTM grid on your Norden 7.5 minute map.]
- Note that the lines you have drawn have created a square box around the Donner Peak reference point. [In the future, you only have to create this "box," and it is unnecessary to draw complete lines between the grid ticks.] Notice that the lines you have drawn are not parallel to the edges of the map, but are slightly skewed. This is because the UTM grid system is not based upon True North, but your map is. From now on, you will focus on the "box" you have just created.
- within the box draw a line, parallel to the left and right sides of the box, from the summit of Donner Peak to the nearer of the top or bottom margin of the box. [In this case, the bottom margin.]
- divide the distance along the bottom margin into ten equal parts, and record where your line intersects the box margin to the nearest tenth. You should find that your line intersected the bottom margin of the box 8 tenths of the distance from grid line 31 (the left side of the box) to grid line 32 (the right side of the box), and thus you should record 318. [It is actually 31.8, but the decimal point is eliminated.] [Try using your UTM grid reader to speed this process.]
- next, draw a line, parallel to the top and bottom margins of the box, from the summit of Donner Peak to the nearer of the left or right margins of the box. [In this case, the right margin.]
- again, record where the line falls to the nearest tenth. You should find that the line intersects the right box margin 3 tenths of the distance from grid line 54 to grid line 55, and thus you should record 543.
- then simply combine the two three digit numbers into one six-digit number, giving the east-west coordinate before the north-south coordinate. In our example 318 and 543 would produce "UTM 318543 on the 7.5 minute Norden Quadrangle." (Note that we reference the particular map because you might get a slightly different reading using a 15-minute map of the same area.)
PRATICE PROBLEM 1: Try determining the UTM location of the triangle marking the tablet atop Anderson Peak, located roughly 1 3/4 inches from the bottom margin and 6 1/2 inches from the right margin. See answer 1 on last page of this appendix.
Public Land System (Township and Range)
Basis of system
Figures 2 and 3 of this article describe the Township and Range system used by the Forest Service. Because we patrol pursuant to volunteer agreements with the Forest Service and use their radio system, we must know how to communicate in a language the Forest Service's radio dispatcher understands. Once you understand the basic Township and Range system, return to this page to learn how to more precisely express a particular location using this system.
Map marks
Township, Range and Section boundaries are marked in red on USGS maps. If the line is dashed, it means its location is approximate. Township (T.) and Range (R.) designations are found along the margins of the map wherever there is a Township or Range boundary (as contrasted with a Section boundary). Township and Range designations also appear in the body of the map where appropriate to mark a deviation in the boundary. Section numbers appear in red in the center of each Section. For example, the Peter Grubb Hut is located in Section 2, T. 17 N., R. 14 E. Using the map marks to describe a location
To describe a particular point within a section:
- find the Township and Range, and the section
- divide the Section into four equal quarters, also know as quadrants.
- subdivide the quadrant in which you are located (or wish to describe) into four sub-quadrants.
- describe the tract in which you are located in terms of the sub-quadrant, quadrants, Section, Township and Range. This method describes your location in a 40-acre tract (a quarter-mile by quarter-mile square).
- for example: the Peter Grubb Hut is located in the "Southwest 1/4, (of the) Northeast 1/4, Section 2, Township 17 North, Range 14 East." Because this is too vague a description for someone trying to find you, when dealing with the USFS, you should give this description, but also give a UTM or geographical coordinate depending on which rescue agency you expect to respond. It wouldn't hurt to give all three. Alternatively, you can measure the distance from the east or west boundary and the north or south boundary of the section, using the scale at the bottom of the map:
- use a ruler, piece of paper or a stick to measure from the left or right (east or west) boundary of the section to your location. Record the distance in feet.
- do the same from the top or bottom (north or south) boundary of the section.
- e.g., the Peter Grubb Hut can be described as being in "Section 2, Township 17 North, Range 14 East, at a point 1500 feet West of the Eastern boundary of the section and 2400 feet South of the Northern boundary of the section." A mouthful, but they will find you.
PRATICE PROBLEM 2: Try describing the location of Anderson Peak in both "Township and Range" methods. See answer 2 on last page of this appendix.
Townships are numbered north and south of the base line.
Ranges are numbered east and west of a principal meridian. The designated abbreviation would read: Township 3 South, Range 45 East.
- Each Township is 6 miles square, and divided into 36 Sections. Each Section is 1 mile square.
- One Section contains 640 acres which can be subdivided into smaller tracts.
- A Section can also be subdivided into (16) 40-acre tracts which can be designated by cardinal directions.
- A legal description of this parcel of 40 acres(beginning with the smallest designation first) will read: ýSE 1D4 NE 1D4, Section 36, T. 3 S R. 45 E.ý
Geographic Coordinates (Latitude and Longitude)
Basis of system
Geographic coordinates use the degree as the basic unit of measurement, with subdivisions into minutes and seconds. The origin of this system is off the west coast of Africa, where the Greenwich meridian (0o longitude) intersects the equator (0o latitude). Latitude is measured north or south of the equator up to 90o at the poles, and longitude is measured west or east of the Greenwich meridian up to 180o in the Pacific Ocean. The contiguous states, Hawaii and most of Alaska are in latitude north and longitude west. Therefore, "north" and "west" may be dropped when giving geographic coordinates in the U.S. In the U.S. latitude and longitude cannot be confused with each other because the range of numbers used to describe each are so different. However, by convention latitude is normally given first.
Map marks
Each corner of a USGS map contains a latitude and longitude reference. Latitude references are on the left and right margins and longitude on the top and bottom margins. An easy way to remember which is which is that San Francisco is at (roughly) latitude 38, so all of our latitude references will be near 38. [Or remember that in the U.S. the longer number is for longitude (3 digits versus 2 digits for latitude). The border of each USGS 7.5 minute map is divided into three 2.5 minute sections, marked by a minute reference (or minute and 30 second reference if it falls between minutes) and a black tick extending into the body of the map. For example, one-third of the way up the left or right hand margin of the 7.5 minute Norden Quadrangle is the reference 17'30", which is really 39 °17'30". Similarly, one-third of the way across the top or bottom margin from the left border is the designation 20', which is really 120 °20'.
Using the map marks to describe a location
Plotting approximate latitude and longitude on a topo map in the field is a relatively simple process. There are two ways to do it, mathematically and geometrically. Fortunately, on the maps we use the two methods are virtually identical, and produce a third method which we call Quick and Dirty (Q&D). We will describe the Q&D method first, but remember that it will only work on our maps. [Note: The easiest method involves use of the Lat/Long Scale we provided. Skip ahead to p. 15, subsection (d) and learn this method first, then come back and learn the other methods.] For all three methods, first use a straight edge to transfer the point you wish to describe to the nearest horizontal and vertical edges of the map, as you would in calculating UTM coordinates. You must now estimate the linear distances between your points and the nearest lesser latitude or longitude references on the borders of the map and express those distances in minutes and seconds. All three methods are based on the fact that the distance between geographical coordinate ticks on the map are 2'30", as described above. Because there are 60" in one minute, 2'30" equals 150". Consequently, if we can divide distance between geographical grid ticks into 150 equal parts, we can describe the points we have transferred to the map borders to the nearest second.
Quick and Dirty method
Fortunately, on our maps, the distance between geographical grid ticks for both latitude and longitude is easily divisible into 150 segments. The distance between grid ticks on the top and bottom borders (longitude) is approximately 150 mm, so each millimeter equals 1 second. The distance between geographical grid ticks on the left and right borders (latitude) is approximately 7.5 inches, so each inch equals 20 seconds, and each 1/4" equals 5 seconds. To determine geographical location, simply find your position in seconds, then convert to minutes and seconds as necessary. For longitude, simply measure in millimeters along the top or bottom border from the next lowest mark to your intersecting point. This is the number of seconds to add to determine longitude. For latitude, simply measure the number of inches from the next lowest latitude mark to your intersecting mark and multiply by 20 (seconds per inch) to get the number of seconds to add to the next lowest latitude reference. To determine the latitude and longitude of the Peter Grubb Hut (PGH):
- first draw lines to intersect the nearby top and left borders of the map.
- the distance from the next lowest geographical grid tick (20', or 120o20') to the mark you placed across the top border of the map is approximately 120 millimeters.
- 120 mm x 1 second/mm = 120 seconds. 120 seconds ÷ 60 seconds/minute = 2.0 minutes.
- add the 2 minutes to the 120o20' (next lowest longitude mark) and the longitude of the Peter Grubb Hut is 120o22'.
To determine the latitude of the Peter Grubb Hut:
- measure the distance from the nearest latitude mark below your transferred mark to your mark. This distance is approximately 6.25 inches.
- 6.25 inches x 20 seconds/inch = 125 seconds.
- to convert the seconds to minutes and seconds, divide your answer by 60 seconds/minute. 125 seconds ÷ 60 seconds/minute = 2'5".
- add the 2'5" to the latitude mark you measured from (20', or 39o20'). 39o20' + 2'5" = 39o22'5".
Consequently, the Peter Grubb Hut lies at latitude 39o22'5", longitude 120o22' determined by the quick and dirty method. [Note that we don't give the full designation of "latitude 39o22'5" north, longitude 120o22' west." Because everything in the Northern Hemisphere is "north" latitude, and almost everything in the United States is "west" longitude, we drop the "north" and "west" designations when giving geographic coordinates in the U.S.] Adding an elevation reference (in this case "7800 feet") is helpful but not essential.
Mathematical method
The Q&D method is actually the mathematical method without long division calculations. With the mathematical method, you simply measure the distance between the next lowest geographical grid tick and your mark, and divide that distance by the distance between geographical grid ticks:
- first, measure and record the distance between geographic grid ticks on the map. E.g., the distance between longitude marks on the top or bottom edge of the Norden 7.5 map is about 150 mm.
- then, measure from the nearest geographical grid tick to the right of where your point bisects the top or bottom map border of the map.
- take that figure, in millimeters, and divide it by 150 mm, then multiply by 2.5, the number of minutes between geographical grid ticks. This will give you a figure in minutes.
- to convert your answer to minutes and seconds, simply multiply the fractional portion of your answer by 60 seconds/minute.
- then add your answer to the longitude of the nearest geographical grid tick to the right of your point (i.e., the nearest grid tick below your point).
Use the same process to determine latitude, but note that the distance between geographical grid ticks on the left and right borders of the map is approximately 190 mm rather than 150 mm. To determine the latitude and longitude of the Peter Grubb Hut, first draw lines to intersect the nearby top and left borders of the map:
- the distance from the 20' (120o20') grid tick to the mark you placed across the top border of the map is approximately 120 millimeters.
- 120 mm/150 mm x 2.5 minutes = 2 minutes.
- add the 2 minutes to the 120o20' (nearest longitude grid tick to the right) and the longitude of the Peter Grubb Hut is 120o22'.
To determine the latitude of the Peter Grubb Hut:
- measure the distance from the nearest latitude mark below your transferred mark to your mark. This distance is approximately 158 millimeters.
- 158 mm/190 mm x 2.5 minutes = 2.08. To convert the fractional portion of your answer to seconds, multiply it by 60. .08 minutes x 60 seconds/minute = 4.8 seconds. Round it to 5 seconds to produce 2'5".
- add the 2'5" to the latitude mark you measured from. 39o 20' + 2'5" = 39o22'5".
Consequently, the Peter Grubb Hut lies at latitude 39o22'5", longitude 120o22' determined by the mathematical method, the same answer we obtained using the Q&D method.
Geometrical method
(see Figure 3): If you can remember how to do long division or have a pocket calculator with
you, the mathematical method is quickest. However, if the calculations give you a headache, try this method. To determine latitude or longitude geometrically:
- draw a line from the nearest reference point below your mark at an angle approximately 30o inward from the map border and approximately the distance between that point and the next highest geographical reference point. [See line A-B on the map excerpt on page 13 of this appendix.] The exact angle is not important, nor is the length of the line, but it should be approximately the length of the distance between longitude grid ticks on your map.
- consider this line to be divided into 150 equal increments. The figure 150 represents the number of seconds between geographical grid ticks on a 7.5 minute map (2.5 minutes x 60 seconds/minute = 150 seconds). On the Norden 7.5 map the distance is approximately 150 mm - extremely convenient for our purposes. Consequently, measure 150 mm along your 30 ° line and mark that point (see point B on the excerpt).
- connect that point (B) with the next highest longitudinal mark on the border of your map (point C) by drawing a line between those two points (line B-C).
- now draw a line parallel to your last drawn line from your intersecting mark (point X) to the 30 ° line (see line D-X). Be certain that lines B-C and D-X are parallel.
- next, measure along the 30 ° line from the beginning (point A) to the point (D) where your last drawn line (D-X) intersects the 30 ° line (point D). This measurement (line A-D) is the number of seconds to be added to determine the correct longitude. Because in this case 1 mm = 1 second, you don't have to do any calculations to convert from millimeters to seconds.
To determine latitude, use the same method, but a different base measurement. The 150 mm trick won't work because the distance is approximately 190 mm. However, as we learned in the quick and dirty method, it is also approximately 7.5 inches, so each inch equals 20 seconds, and each 1/4 inch equals 5 seconds. Consequently, make all latitude measurements in inches rather than millimeters. To determine the geographic coordinates of the Peter Grubb Hut by the geometrical method, first draw lines from the hut to intersect the nearby top and left borders of the map. To determine longitude (points X and Y on the map excerpt):
- draw a line from the 20' mark on the top border (just above the red section 29 number) diagonally downward to the left at an angle of approximately 30o. Draw it all the way to the left border of the map. Then measure along this line 150 mm from where you started and make a mark (point B).
- connect this point with the upper left corner of the map (the next highest longitude grid tick). [See line B-C.]
- draw a line parallel to your last line from the Peter Grubb Hut mark on the top border (point X) to the 30o line. [See line D-X.] Be certain that lines B-C and D-X are parallel.
- measure along the 30o line from the 20' mark (point A) to the point where the second parallel line meets the 30o line (point D). It should be approximately 120 mm. [Sound familiar?]
- 120 mm x 1 second/mm = 120 seconds ÷ 60 seconds/minute = 2 minutes. Add the 2 minutes to 120o20' and you have 120o22' for Peter Grubb Hut longitude.
To determine Peter Grubb Hut latitude, employ the same process:
- draw the 30o line upward to the right from the 20' (39o20') mark (point E) on the left border of the map.
- measure off 7.5 inches on the 30o line and draw a line from that point (F) to the upper left corner of the map (point C). [See line C-F.]
- draw a line, parallel to your last drawn line, from the Peter Grubb Hut intersecting point on the left border (point Y) to the 30o line. [See line G-Y.] Be certain that lines C-F and G-Y are parallel.
- measure the distance from the 20' mark (point E) to this point on the 30o line (point G) in inches - it should be approximately 6.25 inches. [See line E-G.]
- 6.25 inches x 20 seconds/inch = 125 seconds.
- 125 seconds ÷ 60 seconds/minute = 2'5".
- add 2'5" to 39o20' and you get a latitude of 39o22'5".
So the geographical coordinates of the Peter Grubb Hut are latitude 39o22'5", longitude 120o22' as determined by the geometrical method. This is the same answer we got by using the other two methods.
PRATICE PROBLEM 3: Can you describe the location of Anderson Peak using geographic coordinates? Try all three methods to see if you can arrive at identical answers. See answer 3 on the last page of this appendix. If you didn't get the correct answer, please review the text and try again. This material may bend your brain, but it is extremely important to be able to precisely describe your position in the field.
Lat/Long Scale Method: Skewed Scale To Find Position
To determine the longitude and latitude of a point using the special longitude/latitude scale, the scale cannot be laid parallel to the map border because in this orientation the scale does not match the interval and is too long. The scale must be turned to an angle to find a solution. The following procedure describes how to turn the scale to find the latitude and longitude of a point. The 7.5 minute map has tick marks at 2ý30ý intervals along both the horizontal and vertical axis. To carryout this procedure, straight lines must be drawn across the map and down the map at these 2ý30ý intervals. See Figure 4 for an example of correct positioning of the scale to find Longitude. Note that the ruler has one side for measuring distance labeled ýUSGS 7.5ý MAP SCALEý and one side for Latitude/Longitude measuring.
Longitude
- On the longitude side of the scale align the first (right) tick mark of the scale on a 2 minute 30 second (2ý30ý) line.
- Align the last (left) tick mark of the scale on the adjacent 2ý30ý line so the scale is straddling the point to be located. The numbers on the longitude scale need to be facing right side up.
- Slide the scale up or down the map (keeping the tick marks on the 2ý30ý lines) until the skewed scale aligns with the desired point on the edge of the scale. The first and last scale tick marks must still be aligned the with the 2ý30ý lines.
- Each tick on the scale has a pair of two-digit numbers that stand for seconds (e.g. 00·30). The first number of the pair allows you to count up the scale starting at 0 seconds., and the second number of the pair allows you to count up the scale starting from 30 seconds. If the longitude line to the right that is being measured off ends in 30ý (e.g. 120o 17ý 30ý), then count on the scale starting with the 30 minute values. If the longitude line being measured from (to the right) is an even minute (e.g. 120o 20ý 00ý), then count on the scale using the numbers starting with 00 minute.
- The longitude position is the sum of the values counted on the scale plus the value of the longitude line measured from.
Latitude
- On the Latitude side of the scale align the first (bottom of scale) tick mark of the scale on a 2ý30ý line.
- Align the last (top of scale) tick mark of the scale on the adjacent 2ý30ý line straddling the desired point to be measured. The numbers on the latitude scale need to be facing right side up.
- Slide the scale across the map (keeping the tick marks on the 2ý30ý lines) until the scale aligns skewed with the desired point on the edge of the scale. The first and last scale tick marks must still be aligned with the 2ý30ý lines.
- Similar to the longitude, on the Latitude edge of the scale, each tick on the scale has a pair of two- digit numbers that stand for seconds (e.g. 00·30). Count using the pair of numbers similar to Longitude.
- The latitude position is the sum of the values counted on the scale plus the value of the longitude line measured from.
Position Shorthand
Each grid coordinate system is a method for communicating a location, and that location needs to be expressed in a language the receiving party will understand. TBSP has specific guidelines for expressing positions in the coordinate system:
- LAT-LONG: All lat-long coordinates should be expressed with the DATUM first, like "NAD27 datum, 37 degrees 43.44 minutes north, 118 degrees 30.2 minutes west". When written, a single hash ' represents minutes and a double hash " represents seconds.
- UTM: UTM six digit shorthand is read "right->up", meaning the x or East coordinate is read first, and the y coordinate or North is read second.
- Township and Range: Township/range has no shorthand, and is read from most specific-to-least.
- All coordinate systems: Regardless of the coordinate system, it's always good to specify a couple known landmarks to prevent confusion and to allow the receiving party to double-check the position they are given.
GLOBAL POSITIONING SYSTEM (GPS) USES
GPS units can be of great value in locating your current position and for route-finding. However they have their advantages and disadvantages, and can only supplement, not replace map and compass skills. You have to understand how it works, including the difference between the multiple coordinate systems and how to translate from one to the other. By convention, TBSP Patrol maps use the NAD 27 CONUS system, however most modern GPS units will default to the newer WGS 84 standard. This alone can result in a position error of several hundred meters.
- ADVANTAGES
- Works in bad visibility when you cannot make a compass sighting
- Can be faster to get the coordinates of your location than a map and compass
- Can convert easily between coordinate systems (i.e. Lat/Long to UTM)
- DISADVANTAGES
- Batteries have a limited life (and life decreases with lower temperatures ý as little as 4 hours)
- Does not locate you on a map or help route-finding
- Hard to get a sense of map scale on a 2 inch screen
- Satellites may not be very visible (esp. in canyons), or few satellites may give low accuracy
- May not have a compass
- NECESSARY SKILLS
If one is going to carry a GPS unit, to use it effectively one should master the following skills:
- Basic operation (navigate through screens, changes scales, start/stop navigating, etc)
- Set up the system, including:
- Choose datum. In California, should use North American Datum (NAD27)
- Choose coordinate system and units (for example, if using Lat/Long, the units should be degrees/minutes/second instead of degrees/minutes/tenths of minutes)
- Set up to conserve battery power
- Marking waypoints
- Navigating to waypoints
- Entering waypoints in the field
- Changing coordinate systems (for example, from Lat/Long to UTM)
- Backtracking
PRATICE PROBLEM ANSWERS
- "333488 on the 7.5 minute Norden Quadrangle."
- "Southwest 1/4, Southeast 1/4, Section 3, Township 16 North, Range 15 East" or, using the alternative method: "Section 3, Township 16 North, Range 15 East, at a point 2350 feet west of the eastern section boundary and 750 feet north of the southern section boundary, at about 8700 feet elevation."
- "Latitude 39°15'33", longitude 120°17'46" at about 8700 feet elevation." [Remember, we always give latitude first, and don't have to express "latitude" and "longitude".]
If you missed any of these, reread the pertinent sections and try again. Tahoe Backcountry Ski Patrol Manual