Lab 9: Topographic Maps and Aerial
Photos
Objectives:
- Longitude and latitude
- North v. Magnetic North
- Declination and Bearing
- Elements of a topographic map: contour lines, scales, relief, gradient, etc
- Topographic profiles
Introduction:
Topographic map: a two-dimensional representation of a three-dimensional land surface
Contour lines: representation of elevation
Planimetric map: two-dimensional only; highway map, road map
Topographic Maps:
Quadrangles: a section of the Earth’s surface bound by lines of latitude at the top and
bottom, and lines of longitude on the left and right
Latitude (east-west) and longitude (north-south) are measured in degrees
Latitude: 0o at equator to 90o at the poles (North Pole = 90o N)
Longitude:
0o at Prime Meridian (North-pole to South-pole through
Degrees can be subdivided into minutes (’) or seconds (’’)
Most common size: 15 minute or 7 ½ minute
Declination:
True north: the north pole, same as grid north (GN) v. Magnetic north pole (MN):
700 km away,
north of
Declination: the difference (in degrees) between N and MN
Shown
on map:
Magnetic pole migrates: Lewis and Clark
Map symbols
and revisions: Figure 9.2 map symbols
Photorevision: looking at aerial photographs to discover changes
Contour lines:
A line that connects all points on the map that has the same elevation with
respect to sea level
Figure 9.5 – example
topo map, Figure 9.6 - rules for contour lines
Index contours: contours with printed elevation
Contour interval: increments of elevation change between contours
Estimate elevation between lines
Hachure marks indicate closed depression, if no hachure marks it’s a hill
Benchmark: permanent marker used as reference on the map
Relief and Gradient:
Relief: difference in elevation between two points on a map
Local relief: adjacent hills and valleys, Total relief: highest and lowest on a map
Gradient: steepness of a slope (feet per mile or meters per kilometers) - %
Scales of Maps and Models: Figure 9.10
Ratio scale: proportion by which you will reduce the real object to the model size:
1:4 – one inch equals 4 inches, 1:30 – one foot equals 30 feet
Fractional scale: 1/24,000 (instead of 1:24,000)
Understanding scales: 1:24000 à 1 inch=2000 ft, 1:63,360 à 1 inch=1 mile
Bar scale: scale showing relationship between map scale and real size
Principal meridians: north-south lines
Base lines: east-west lines
Square: 6 miles on each size
Townships: numbered relative to the base line (1,2,3 N or S)
Ranges: numbered relative to the principal meridian (1,2,3 E or W)
Sections: subdivision of township, 1 m2 each, 36 per township, numbered from top right
Global Positioning
System (GPS):
Used to triangulate position, relative to the north pole and Prime Meridian
Triangulation: identifying your position relative to three or more known points
Compass Bearings:
Bearing: compass direction along a line from one point to another
Quadrant: expressed in degrees east or west of north or south
Azimuth: N
0o, E 90o, S 180o, W 270o
Remember magnetic north v. true north
Figure 9.12 – How to read a bearing
Topographic Profiles
and Vertical Exaggeration:
Topographic profile: cross section that shows the elevation and slopes along a line
Step 1: identify a line of section
Step 2: mark all major intersections along the transect
Step 3: draw the profile with vertical scale and horizontal lines
Step 4: transfer points from transect line to profile
Step 5: connect the dots
Vertical exaggeration: using a different vertical scale than horizontal
Calculate: divide vertical fractional scale by the horizontal fractional scale
Aerial Photographs:
Vertical v. oblique
Center point or principle point: center of a photograph, usually they overlap
Scale: not uniform, based on distance from camera lens
Sterogram: pair of overlapping photos that can be viewed with a stereoscope
Can see areas of high relief (resistant rocks) and low areas (less competent rocks)