Introduction: How to Read a Topographical Map
When you venture into the wilderness, plan the means for a safe return. Unless you sincerely intend to get back to nature for an extended period of time then you may need some map reading and navigation skills in order to return to the usual comforts of your everyday life.
for more information on how to navigate using a topo map:
Step 1: What Do You Need
A map or map set of the area and know how to read it. What you want is a topographical map which shows elevation and preferably uses the UTM coordinate system.
A wealth of map resources exist online but a map printed from and online source may not include all the necessary information.
A good source for paper maps is the US Geological Survey (USGS) http://topomaps.usgs.gov/
Step 2: Map Name
This uniquely identifies the map typically named for some prominent geographical feature in the area
Step 3: Scale
A typical scale you may find, smaller numbers are more “zoomed in” and have more detail:
1:250000 – each map unit = 250,000 units on the ground
(i.e. 1 in = 250,000 in = about 4 mi)
Step 4: Date
Indicates when it was revised and field checked (usually 1950s). This is important if roads, trails or fences are identified, they may have changed since the revision date or they may no longer exist.
Step 5: Coordinate System
angular coordinate system
The earth can be divided into lines of latitude that run parallel to the equator (0 º) and lines of longitude that run N and S and intersect at the poles. (International Date line is 180 º)
Coordinates are expressed as Degrees:Minutes:Seconds of Longitude East or West of prime meridian ( 0º thru Greenwich, England) and Latitude North or South of the Equator.
1 degree = 60 minutes 1 minute = 60 seconds
Spherical trigonometry required for calculations using this system. It is still favored by pilots and sailors navigating over greater distances.
UTM – Universal Transverse Mercator – Square Grid coordinate system
The Earth is divided into 60 (numbered 1 – 60) zones each 6º of longitude wide, starting at the international date line, and extends from 80º S to 84º N latitude.
In polar regions the Universal Polar Stereographic system is used.
Each zone is divided into horizontal bands each of 8º of latitude wide are lettered from C to X starting at 80º S. Letters I and O are not used.
The advantage of this system is that coordinates are expressed in meters East and meters North within the zone. Distances on the map are obvious when looking at the grid. The central meridian of the zone is set at 500,000 meters. A map having a useful scale will typically be composed of a very small area within the zone.
Typical map coordinates:
10 S 0559741 559741m E.
4282182 4282182m N.
This is read as Zone 10 S Easting = 559,741 meters (East) Northing = 4,282,182 meters (North)
For locations North of the equator the equator is assigned 0 meters North
For locations South of the equator the equator is assigned 10000 meters North
Step 6: Contour Interval – CI
This is the vertical distance or elevation between the thin brown contour lines. If CI = 50 then elevation increases (or decreases) by 50 ft between each line. Each 5th darker colored line is marked with actual elevation above sea level.
Contour lines that are close together means sharp elevation, or rapids/waterfall on a river.
Step 7: Declination
This is the angular distance between True north and magnetic north or grid north and magnetic north.
True North – direction of the lines of longitude
The vertical lines that are the right and left margins of a map are almost true north(this is Grid North). Closer to poles these lines become less parallel to longitude lines. Use neat lines(smaller lines between the gridlines) to find true North.
Magnetic North – direction a compass points.
The magnetic north pole is several hundred miles south of the real pole. It moves constantly but slightly. Map may have and approximate mean declination, a date and annual change. You only need to adjust if it is a very old map.
Declination East – subtract from your map direction (compass least) or add to compass bearing
Declination West – add to your map direction (compass best) or subtract from compass bearing
Step 8: Map Datum
This describes the model used to match features on the ground to coordinates on the map. There are known points or positions surveyors use for a datum. When using a GPS it needs to be set to the same datum as the map you are using. examples:
NAD27 - North American Datum of 1927 WGS84 - World Geodetic System 1984