Maps and Map Interpretation

Any aspect of the environment must be considered in its spatial context:

• where is it?
• how high is it?
• what is its climate?
• what is around it?
• what is under it?

and a host of other issues - all best resolved through maps.  You know where to find maps - in the University Library, through government and commercial outlets and on-line from the Montana MapFinder, Topozone (now a subscription service), TerraServer, Google Maps, or elsewhere - but how do you use maps?  In teams of two, attempt as many of the following as time permits.  You do not need to perform them in order, although some logically follow others (e.g., "relief" after "elevation")

NOTE:  All exercises refer to the Sedan, MT, 1:62,500 (1951) topographic quadrangle.  Please do NOT write on the maps provided!

Location:

Location is determined with reference to one or more of the many grids [nearly rectangular or square coordinate systems] within which maps are defined.

• The best-known is the (N-S) latitude-longitude (E-W) grid, which is familiar on world maps and globes.  Most US maps are defined within that system, although they may include other systems.
  • The corners of the quadrangle are labeled with their latitude and longitude.  the units are degrees (°), minutes ('), and seconds (") of arc.  What are the maximum and minimum latitude and longitude on this map? _________ _ long. to _________ _ long.; __________ _ lat. to __________ _ lat. [Note: longitude here is West of the Prime Meridian at Greenwich, UK; latitude is North of the Equator.]  What is the N-S extent of the map, in units of arc?  ________  What is the E-W extent? _________   [that's why this is called the a "15-minute quadrangle.]  Why isn't it square? _______________________________________________________________
  • The map is divided by tics along the margins and four small pluses (+) within the map into nine segments, each ______ minutes of arc on a side.  These are small enough to measure with a standard ruler. 
    • Locate the segment in which Fairy Lake is located.
    • Measure the E-W distance, in cm, between the 5-minute tics to the N of Fairy Lake ______.  That is the map-equivalent distance to 5 minutes, or 300 seconds, of arc.   How many seconds of longitude are covered by 1 cm?  _________  Is that number the same on all other maps at the same scale? _________
    • Measure the distance, in cm, by which Fairy Lake is E of the western map margin.   _______  What is the equivalent in seconds of longitude?  ________   That is the amount by which the longitude of Fairy Lake is less than 111° 00' 00"W.  What would the longitude of Fairy Lake be in ° ' " W of Greenwich? ____________
    • Measure the N-S distance, in cm, between the 5-minute tics to the W of Fairy Lake ______.  That is the map-equivalent distance to 5 minutes, or 300 seconds, of arc.   How many seconds of latitude are covered by 1 cm?  _________   Is that number the same on all other maps at the same scale? _________
    • Measure the distance, in cm, by which Fairy Lake is S of the 55' tic on western map margin.  _______  What is the equivalent in seconds of latitude?  ________   That is the amount by which the latitude of Fairy Lake is less than 45° 55' 00"N.  What would the latitude of Fairy Lake be in ° ' " N of the Equator? ____________
• The other major grid system is the Land Office, or "Township-Range" grid.  The grid consists of Townships, six miles square, defined relative to a number of N-S Principal Meridians and E-W Baselines.  Each township is labeled along the margins of the maps by their order N or S of their baseline, and by their range E or W of their prime meridian.  The townships are outlined by heavy solid or dashed red lines.
  • Townships are subdivided into sections, one mile "square".  Those are labeled on the map.  Notice the unusual order of numbering!  Sections are subdivided into quarters (NE, NW, SE, NW), which can in turn be quartered, ad infinitum.
    • What is the location of Fairy Lake in the Land Office system?  ___¼, ___¼, section ______, T____, R____.

Elevation

Elevations are determined on maps in either English units (feet) or metric units (meters).  They are shown as labeled points (BM = bench mark) rounded to the nearest foot from accurate surveys; as spot elevations (x or nothing), accurate to ± 0.3 contour interval; or as interpolations from contour lines, accurate to ± 0.5 contour interval.  Interpolation is generally to the nearest 1/2 contour interval (here, equal to what? _____).

• What is the elevation of Fairy Lake? __________ ± ____ feet.  Of Sacajawea Peak? __________ ± ____ feet.  Of the Fairy Lake road/Sedan Road junction?   __________ ± ____ feet.  Of the lowest point on the map?  __________ ± ____ feet.  [Hint: follow the major streams to the edge of the map!]

Relief

Relief is simply the arithmetic difference (highest - lowest) between two points of interest.  The uncertainty is the square root of the sum of the squared individual errors.  Note that slope (rise/run) can be calculated from relief (rise) and distance (run).

• What is the relief on this quadrangle?  __________ ± ____ feet.

Distance

Distance is measured using either the bar scale or the fractional scale.  The bar scales are located at bottom center of the map, and are given in a number of units.  The fractional scale is found above the bar scales, in the form of the ration between a distance on the map and the distance between the same points on the ground.  What is the fractional scale of this map?   ___:_____________

• Straight-line distance is simply scaled directly from the map. 
  • What is the straight-line distance (in inches) between Fairy Lake and the Fairy Lake/Sedan road junction?  _____ in
  • From the bar scale, what is that distance (in feet)?   ______ ft  [Note that you can line the zero point of your ruler up on a 1000-ft tic and interpolate between 500-ft tics at the other end.]
  • Now, multiply your measurement by 62,500 and divide by 12.  [Note: use Start, Programs, Accessories, Calculator to bring up the Windows calculator.  Use View to switch from standard to scientific as desired.] _________ ft.
• Curved-line distance is a different story.  perhaps the easiest way to measure a curvy line is to break it into a number of (nearly) straight segments, and add them together by rocking your ruler along the line as it bends.  You can do the same thing with the edge of a piece of paper, making a very light tic on map and paper at each corner to help line up the next straight.  Then - measure the length of the edge of the paper!
  • What is the road distance from the above road junction to Fairy Lake? __________ ft   [Note: it can be a real nice ski or snowmobile trip after a fresh snow!]

Direction

Direction is measured with a protractor relative to True North or Magnetic North according to the declination diagram in the lower map margin.   "Relative to" means in degrees E or W of North (or South), or as an azimuth (out of 360°, where N is 0°, E is 90°, S is 180°, and W is 270°).Why do you need more than one North, anyway? ____________________________________________________________________________________________________________

• What is the approximate direction from the same road junction to Fairy Lake in degrees from South ____ azimuth? ________ 
• What should your compass read if you were trying to find your way straight back from Fairy Lake to the road junction in a white-out? {Note:  NOT a good idea!] ____________ Consider declination as well as direction!

Contouring

Contours are lines connecting equal values.  Many data types other than elevation are shown by contoured maps - ground-water levels, geochemical characteristics, and rainfall amounts, to name a few.  How do they do that?  By following a few simple rules (some of which have rare exceptions).

• Apply the rules above to contour the data points you are given.  When you are done, compare your results to the topographic map of the Bozeman area.  Can you recognize the source of the data?

NOTE:  For a more in-depth discussion of topographic map interpretation, see the instructions for Geomorphology - ESCI 307.

REFLECTION:  

As a group, discuss what you have learned. Have you heard of these concepts before - if so, in what context? In what contexts do you expect to hear about them
again?  Have you used topographic maps before - when and where?  What have you learned here that might help your future map use?