A Green Trails Map(GTM) is a Topographic Recreation Map. The hallmark of a Green Trails Map is it clear, accurate and current content. Each map is extensively ground-truthed by Green Trails GPS Mapping Teams and crafted by GTM cartographers using the latest in digital mapping technology.



A Green Trails Topographic Map is distinguished by contour lines. Contours are placed on the map to represent lines of equal elevation above (or below) sea level. To visualize what a contour line represents, picture Mount Rainier (or any other topographic feature) and imagine slicing through it with a perfectly flat, horizontal piece of glass. The intersection of the mountain with the glass is a line of constant elevation on the surface of the mountain and could be put on a map as a contour line for the elevation of the slice above a reference datum.

This scale is listed on a topographic map as the contour interval. The contour interval is the vertical distance represented by consecutive contour lines on the map. In general, the smaller the scale of the map (small scale maps show a larger area of the earth�s surface) the larger the contour interval will be. For example, the traditional GTM 15� 69,500 map has a contour interval of 80 feet, while several GTM �S� maps have a contour interval of 20 feet.

Many terrain and geologic features are revealed from the orientation of contours.� U-shaped glacier valleys where the ice will occupy a valley and hollow out its sides and head. V shaped river valleys, where a river cuts down creating steep cliffs. As a glacier excavates into the mountain at the head of the valley, another glacier may be doing the same on the other side of the mountain. This results in a sharp ridge, or arete. If three glaciers surround and erode a mountain, they may form a three-sided peak, known as a horn.

In very flat areas, such as floodplains, contour intervals of one hundred, or even forty, feet may not be very useful as they will be very widely spaced. Similarly, in very steep mountainous areas the contours may be more widely spaced to avoid clustering of lines into unreadable masses.

Regardless of the contour interval, there are two types of contour lines on a GTM. Thick contour lines, called index contours, have elevations printed on them periodically over their length. Between each index contour are four intermediate contours that are thinner lines than the index contours. The elevation change between the intermediate contours is what is given in the map legend. So, if the contour interval listed in the map legend is eighty feet, each intermediate contour represents eighty feet and the elevation change between index contours is 400 feet. The contour interval used on a GTM is printed below the scale in the map legend.

Map Legend


The legend and margins of Green Trails Maps contain a myriad of useful information. Printed along the margins of a GTM are location coordinates, UTM coordinates, and latitude/longitude minute and second subdivisions.

The map legend defines many of the graphic elements found on a GTM. Summer and winter recreation trails for hikers, equestrians, mountain bikers, cross county skiers, and snowshoers are distinguished by different line styles.� Road classifications include paved, gravel, dirt, four wheel drive and key access roads to trailheads. In addition, a variety of recreation details are defined in the legend: trail mileage/trail junction elevations/gates/lookout towers/benches/viewpoints etc.

Typical S Map Legend from Forest Park - 426S:

Traditional 1:69,500 Map Legend:



GTM come at a variety of scales. The GTM 15� Series is a consistent scale of 1:69500 producing uniform 100% coverage of the Washington Cascades and Olympics as well as Oregons Northern Cascades. GTM S (Special) Maps vary in scale, typically a larger scale (more detail) than the 15� series.

The scale of the map is determined by the amount of real-world area covered. For example, a 15� GTM topographic map has a scale of 1:69500. This type of scale is known as a Ratio Scale meaning one inch(one of anything- km, cm, foot etc) on the map is equal to 69,500 inches(km, cm, foot etc)in the real world. The smaller the ratio is between distances on the map and distances in the real world, the smaller the scale of the map is said to be.

A map with a scale of 1:69,500 is a smaller scale map than a 1:24,000 scale S map, but it covers a larger real-world area. In addition to the ratio scale, a bar scale is also shown to allow measurement of distances on the map and conversion to real-world distances.

Perhaps one of the most important sources of information on a Green Trails Map is the date of revision, typically printed in the legend box. Although large scale topographic features, such as mountains, take millions of years to be formed and eroded, smaller scale features change on a much more rapid scale. For example, the path of a trail may change fairly rapidly as a result of flooding, landslides that may alter topography significantly, roads and trails are added or go out of use, etc. Because of these changes, it is important to have a the most recently updated GTM to ensure accurate and current recreation content.



Another feature found in the legend of a GTM is the north arrow and magnetic declination. The north arrow is designated by the �N� pointing to True North (the axis around which the earth rotates). Magnetic declination is the difference between True North and Magnetic North, designated by �MN� on the map (the direction the needle of a compass will point).

Magnetic North is determined by the earth�s magnetic field and is not the same as True (or geographic) North. The location of the magnetic north pole changes slowly over time, but it is currently northwest of Hudson�s Bay in northern Canada (approximately 700 km [450 mi] from the true north pole). GTM are based on the geographic north pole because it does not change over time, so North is always at the top of the map.

If you walk a straight line following the direction your compass needle indicates as North, you would find that you don�t go North on the map. How far your path varied from true north depends on where you started from; the angle between a straight north-south line and the line you walked is the magnetic declination in the area you were walking.

Magnetic declination has been measured throughout the U.S. and can be corrected for on your compass (see below).

The line of zero declination runs from magnetic north through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. If you are working west of the line of zero declination, your compass will give a reading that is east of true north. If you are working east of the line of zero declination, your compass reading will be west of true north. The exact amount that you need to adjust the declination on your compass to reconcile magnetic north to true north is given in the map legend to the left of the map scale.

The map below shows lines of equal magnetic declination throughout the U.S. and Canada.

Calculators for determining current declination can found at the National Geophysical Data Center.



A reference datum is a known and constant surface which can be used to describe the location of unknown points. On Earth, the normal reference datum is sea level.

All GTM 15 minute topographic maps in circulation use the NAD-27 (North American Datum, 1927) referencing system based on the Clarke ellipsoid of 1866. More recent GTM S maps use the NAD-83 referencing system which is based on the GRS-80 ellipsoid.

The datum used is printed on the front or back of the map. Although the reference ellipsoids used in the NAD-27 and NAD-83 are different, the changes are slight on large-scale maps.



In order to represent the surface of the earth on a flat piece of paper, the map area is projected onto the paper. There are many different types of projections, each with its own strengths and weaknesses. GTM topographic maps use either a Lambert Conformal Conic or a UTM (Universal Transverse Mercator) projection both of which are known to preserve shape. (Individual GTM map projections can be found here).


The Geographic Coordinate System, one of the most commonly known coordinate systems, uses degrees of latitude and longitude to describe a location on the earth�s surface. Lines of latitude run parallel to the equator and divide the earth into 180 equal portions from north to south (or south to north). The reference latitude is the equator and each hemisphere is divided into ninety equal portions, each representing one degree of latitude.

In the Northern Hemisphere degrees of latitude are measured from zero at the equator to ninety at the north pole. In the Southern Hemisphere degrees of latitude are measured from zero at the equator to ninety degrees at the south pole. To simplify the digitization of maps, degrees of latitude in the southern hemisphere are often assigned negative values (0 to -90�). Wherever you are on the earth�s surface, the distance between lines of latitude is the same (60 nautical miles,), so they conform to the uniform grid criterion assigned to a useful grid system

To be truly useful, a map grid must divided into small enough sections that they can be used to describe with an acceptable level of accuracy the location of a point on the map. To accomplish this, degrees are divided into minutes (') and seconds ("). There are sixty minutes in a degree, and sixty seconds in a minute (3600 seconds in a degree). At the equator, one second of latitude or longitude = 101.3 feet. For Example, the summit of Mt Rainer has a latitude of 46� 51' N and a longitude of 121� 46' W.

An alternative method of notation in the geographic coordinate system, often used for many GIS/GPS applications (Geographic Information Systems, Global Positioning System), is the decimal degree system. In the decimal degree system the major (degree) units are the same, but rather than using minutes and seconds, smaller increments are represented as a percentage (decimal) of a degree.

The decimals can be carried out to four places, resulting in a notation of DD.XXXX, DDD.XXX. When using four decimal places, the decimal degree system is accurate to within � 36.5 feet (11.12 m). For Example, the summit of Mt Rainer in decimal degrees has a latitude of 46.852947N and a longitude of -121.760424W.

The GTM 15 Minutes Series cover 15 Minutes of Latitude North/South and15 minutes of longitude East/West.Latitude longitude- tics and points are noted on all GTM.


UTM Projections are used on GTM because they preserve shape, their grids are the easiest to use with a GPS and allow precise measurements in meters to within 1 meter.

What a transverse mercator projection does, is orient the �equator� north-south (through the poles), thus providing a north-south oriented swath of little distortion. By changing slightly the orientation of the cylinder onto which the map is projected, successive swaths of relatively undistorted regions can be created.

Each of these swaths is called a UTM zone and is six degrees of longitude wide. The first zone begins at the International Date Line (180�, using the geographic coordinate system). The zones are numbered from west to east, so zone 2 begins at 174�W and extends to 168�W. The last zone (zone 60) begins at 174�E and extends to the International Date Line.

The zones are then further subdivided into an eastern and western half by drawing a line, representing a transverse mercator projection, down the middle of the zone. This line is known as the �central meridian� and is the only line within the zone that can be drawn between the poles and be perpendicular to the equator (in other words, it is the new �equator� for the projection and suffers the least amount of distortion). For this reason, vertical grid lines in the UTM system are oriented parallel to the central meridian. The central meridian is also used in setting up the origin for the grid system.

Any point can then be described by its distance east of the origin (its �easting� value). By definition the Central Meridian is assigned a false easting of 500,000 meters. Any easting value greater than 500,000 meters indicates a point east of the central meridian. Any easting value less than 500,000 meters indicates a point west of the central meridian. Distances (and locations) in the UTM system are measured in meters, and each UTM zone has its own origin for east-west measurements.

To eliminate the necessity for using negative numbers to describe a location, the east-west origin is placed 500,000 meters west of the central meridian. This is referred to as the zone�s �false origin�. The zone doesn't extend all the way to the false origin.

UTM coordinates are typically given with the zone first, then the Easting, then the Northing. So, in UTM coordinates, the summit of Mount Rainer is located in Zone 10 at E, The UTM system may seem a difficult to understand at first, once one become familiar with it, it becomes an extremely fast and efficient means of finding exact locations and approximating locations on a GTM map especially with a GPS.


The Lambert Conformal Conic projection superimposes a cone over the sphere of the Earth, with two reference parallels secant to the globe and intersecting it. This minimizes distortion from projecting a three dimensional surface to a two-dimensional surface. Distortion is least along the standard parallels, and increases further from the chosen parallels.



A compass is a navigational instrument for finding directions on the earth. It consists of a magnetized pointer free to align itself accurately with Earth's magnetic field, which is of great assistance in navigation. The cardinal points are north, south, east and west.

Pictured below are two different types of compasses. The compass at left is a Brunton compass used by geologists and others for many specialized mapping purposes. On the right is a more common type of compass used for general orienteering and some mapping purposes.

Compass Setup


The first thing you need to do with your compass before ever taking it into the field is to set its magnetic declination. If you fail to do this, any readings you get from your compass will be erroneous and you may wind up far from where you want to be-LOST

The magnetic declination is printed on the front or backside of your GTM. After finding the declination on the map, you need to transfer that information to your compass. If you are using a Brunton compass, you set the magnetic declination by turning the declination setting screw on the side of the compass until the reading on the graduated circle in the compass lines up with the index pin at the top of the compass at the proper declination.

For most other types of compasses you can set the declination by simply rotating the graduated circle on the outside of the compass unti l it lines up with the indicator marker at the top of the compass at the proper declination.

Once you have set the declination on your compass, any reading you obtain from it will be accurate. In Wahsington and NW Oregon, the magnetic declination varies from roughly 18�E to 20�E. So, after setting the declination at 19�, when you line your compass up with 0� it will be pointing to true north but it will appear to be 16� off from the �N� printed on your compass.

A word of caution here: be sure that you set your declination in the proper direction (east in the Pacific Northwest). If you set it to 19�W rather than east, you will be off by 38� in all of your measurements, rather than the 19� you would be off if you hadn�t adjusted it at all. To make sure you have set your declination properly, orient your compass so that the north end of the needle is lined up with the 0� mark on the graduated circle.

If you are located west of the line of zero declination, then the index pin or marker on your compass should be west of the 0� marker on the graduated circle (and vice-versa if you are east of the line of zero declination).

Compass GTM


The Scenario:

You are descending Mt. Olympus(NO133S) in a white-out.

The wands you placed on the way up are nowhere to be seen.

What direction should you go from the plateau on Snow Dome to avoid Panic Peak and get to ridge above Glacier Meadows Camp?

Place the edge of the compass along the route you want to take.

Make sure the big arrow points in the direction you want to go.

Rotate the bezel on the compass so that the red lines under the needle point to MAGNETIC NORTH.� (Use the declination diagram to find MN from TN.)

Holding the compass in your hand, with the big arrow pointing away from you Turn your body (not just your hand) so that the magnetic needle is in the red arrow on the bezel.� Put �Red Fred in the Shed.

You are now facing the �way to go.


Global Positioning System (GPS)

GPS is currently the only fully functional Global Navigation Satellite System. More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the location, speed and direction. A GPS receiver calculates its position by measuring the distance between itself and three or more GPS satellites. Measuring the time delay between transmission and reception of each GPS radio signal gives the distance to each satellite, since the signal travels at a known speed. The signals also carry information about the satellites' location. By determining the position of, and distance to, at least three satellites, the receiver can compute its position using trilateration.Receivers typically do not have perfectly accurate clocks and therefore track one or more additional satellites to correct the receiver's clock error.

GPS Setup


Setting up your GPS to sync with your GTM.

Check the map projection data on the backside of the map.

Set the GPS to reflect this projection.

Set the GPS to read coordinates in UTM, not Latitude and Longitude.

Check to see if the GPS has enough satellites to have a good position




The UTM grids on a Green Trails Map (1:69,500) are every 2000 meters.

Your GPS reads: 444400E, 5259000N

You are at the crossing of the red lines near a small tarn.