Reviewing a topographic map is usually the starting point
for the planning of any back country trip. A topographic map is your road map to the
outdoors. It provides you information at
a scale that is meaningful and detailed.
For years, the US Geologic Survey (USGS) has
been the principal publisher of accurate maps.
Within the last decade we have seen many innovations in mapping products
that include new mapping companies and publishers, software, maps for the GPS, and
“Apps” for the smart phone.
Still, the USGS map remains the standard for back country
navigation (visit the USGS’s site at www.topomaps.usgs.gov.) I’d
also recommend looking at June Fleming’s “Staying Found” or Bjorn Kjellstrom’s
“Be Expert With Map & Compass.” Once
you develop a map foundation you will easily shift to many of the other
products on the market today.
Many publications, videos, and web sites will give you a
complete rundown on the features, symbols and components to a map. The key features that you should be aware of
are:
·
Contour
Lines These are the thin brown lines that snake across the map. Contour
lines connect equal points of elevation
such that every point on a specific line will be at that elevation above sea
level. Visually, the contour lines give
you a mental three dimensional view of the terrain. These lines provide shape and a sense of
texture. Contour lines provide a view of
slope and pitch, depressions, ridge lines and level ground; the highs and lows
of the earth’s surface. There are two
primary types of lines, index and intermediate lines. Index lines stand out as they are a touch
wider, a darker shade of brown and indicate the elevation with numbers such as
4500; the elevation is in feet. Between
the index lines are the thin intermediate line that are spaced uniformly and
further define the elevation, slope and contour. The distance intervals between the
intermediate lines are specified at the bottom of the map adjacent to the scale
data.
·
Scale
Consider scale as your view of the map; it is like your “overhead zoom”
setting. To cut to the chase, a 7.5
minute map or quadrangle has a scale that is referred to as 1:24,000; where one
inch is equal to 2000 feet. It is your
best source of information of the back country.
At this scale, the map has much more validity and provides more usable
information for your backcountry planning.
You can view important landmarks, streams and geographic features. To complete the navigation picture I always refer
a second map, such as a map of the national forest (e.g., the Deschutes National Forest.) Commonly, such a map will be “zoomed” way out
and have a scale of 1:100,000 or 1:250,000.
Imagine that such a map would be made up of many 7.5 minute quadrangles.
·
North Features on a map such as trails, roads, mountain
peaks and streams are all laid out in relation to true North; the North Pole. The north-south borders of the map and the
small declination diagram are your best references for true North. Other grid lines (such as the red Township,
Section and Range lines) may not be aligned to true north at all. Be careful of these lines should you need to
triangulate your position on a map.
·
Declination
This is the angular difference between true North and Magnetic North. The red needle on your magnetic compass
points to Magnetic North. The accuracy
of the information found in the Declination Diagram is dependent on the age of
the map. To get the latest declination
for any area visit www.magnetic-declination.com.
Personally I use a magnetic compass
that I can adjust for declination; it just makes my navigation easier. When adjusted, my compass provides bearing
information in degrees true as does my map and my adjusted GPS.
·
Coordinates
Latitude and Longitude (Lat/Long) are the familiar coordinate system to most
outdoorsmen and women. Coordinate data
is found at the top and bottom corners of each map. Lat/Long coordinate increments are also found
every 2’ (minutes) and 30” (seconds) on the sides of the Map. A scaling device is necessary to pull
complete coordinates off a map; this is a pain.
In the 1940’s a coordinate system know
as Universal Transverse Mercator (UTM) was developed. To keep a very long story short, your 7.5
minute map has a new grid laid over it, the grid dimensions are 1000 meters by
1000 meters. For more complete
information on UTM grid visit the USGS’s web site UTM
or Letham’s “GPS Made Easy” (which is probably at your local library.)
Simplicity is the essence of
UTM. Scouts, hunters and hikers have
joined Search and Rescue (SAR) teams around the country in using this
system.
Your GPS receiver can easily be
switched to UTM from the set-up menu.
·
Bar
Scales Notice the bar scales at
the bottom of the 7.5 minute map. The
scales provide measuring data in miles, feet and meters. On the far left side of the meter scale, the
scale is broken down into units of 100 meters, this applies directly to UTM.
Notice on the scale bar (feet) that
1 inch equals 2000 feet.
·
Map
Datum Information about map datum is found in the lower left corner of
a 7.5 minute map. I have found that the
simplest definition from GPS maker Garmin is:
“A math model which depicts a part of the surface of the earth. Latitude and
longitude lines on a paper map are referenced to a specific map datum. The map
datum selected on a GPS receiver needs to match the datum listed on the
corresponding paper map in order for position readings to match.”
The bottom line: most 7.5 minute maps are made to the
North American datum of 1927 (NAD27 or NAD27 CONUS on your GPS). New GPS receivers are set to datum
WGS84. The difference between the datum
could be over 100 meters/yards. The
solution: When pulling points off a map shift your GPS’s datum to match the
map.
If precision is not an issue for your outing don’t worry
about datum.
I find that the Silva "Ranger" style is my pick. It is accurate and keeps my navigation simple. Importantly, this model can be adjusted mechanically for declination, has a decent sighting system and is simple to use. Prices for this model will range from about $38 - $50. Suunto makes a similar model too that is just fine.
