EAS 112 Lab Topographic Maps

I. Introduction to maps

Map: representation (usually on a flat surface) of a surface; can show various features of interest, such as roads, boundaries, mountains, rivers, continents, etc.

Problems: projecton of 3-D features onto a 2-D surface can cause (1) distortions, and (2) loss of information.

Types of maps:

§ Planimetric maps: 2-D representation of a part of the Earth’s surface, typically show roads, land boundaries, and some natural features (e.g., lakes and rivers). Often used as the base for special purpose maps. Example: road map.

§ Topographic maps: show the topography (i.e., 3-D configuration of the Earth’s surface–hills, valleys, etc.) with contours; most also show cultural features (those of human design), e.g., land boundaries, buildings, and roads.

§ Geologic maps: show the distribution of different rock units exposed at the Earth’s surface, cultural features, and often topography also.

§ Land-use maps: combine elements of topography and geology to indicate the locations of such features as stream floodplains, potential natural hazards, or good agricultural soil.

II. Map coordinates and land subdivision

§ Latitude-Longitude System (Overhead 1)

Ø Lines of latitude/parallels: form east-west circles around a globe, never intersect with each other. Geographic poles: Latitudes 90° N and 90° S.

Ø Lines of longitude/meridians: form (north-south) circles on a globe that pass through, and intersect at, the geographic poles. Prime meridian: 0° longitude, pass through the Royal Observatory at Greenwich, England.

Ø Quadrangles: rectangular-shaped areas on maps, bounded by latitudes and longitudes, both of which are usually separated by 1°, ½° (= 30'), ¼° (= 15'), or 1/8° (7.5').

Ø Convention to locate points on a map: Latitude is given first, longitude second, e.g., latitude 43°35'N, longitude 132°15'W, or 43°35'N/132°15'W.

§ Universal Transverse Mercator (UTM) system (Overhead 2)

Ø Used throughout the world, based on a grid of 60 north-south zones, each 6° wide.

Ø Zones numbered from west to east, beginning at the International Date Line (longitude 180° E/W); zone number is indicated in the small print in the lower left corner of a USGS quadrangle map. Example: zone 15: longitude 96°–90° W.

Ø Origin of each zone: at the intersection of the equator and the central meridian of that zone.

Ø False origin (0 m): located 500,000 m west of the true origin

§ U.S. Public Land Survey System (also called the Township-Range System) (Overhead 3)

Ø In the western two-thirds of the United States, land is divided by this system.

Ø Townships: 6- x6-mile squares. Further subdivided into 1- x1-mile squares called sections.

Ø Starting point for subdivision: the intersection of selected latitude and longitude lines. Starting latitude – base line, starting longitude – principal meridian.

Ø Tiers: lines drawn 6 miles apart and parallel to the baseline form east-west rows

Ø Ranges: north-south lines parallel to the principal meridian and 6 miles apart form north-south columns.

Ø Tiers and ranges are numbered by reference to the baseline and principal meridian. Tiers is always written first; range second, e.g., T3S, R4E.

Ø Longitude converge toward the poles => impossible to maintain squares that are 6 miles on a side => correction made at every fourth tier line (labeled correction line).

Ø Points are located according to the smallest subdivision required. Locations are always written from the smallest unit to the largest, and tier is written before range.

III. Elements of Maps

§ Scale:

Ø Ratio, or fractional scale: 1:24,000 (or 1/24,000), means that one unit (e.g., an inch) on the map equals 24,000 of the same units on the ground. Will change when maps are enlarged or reduced, i.e., the number will not be correct.

Ø Graphic scale: usually consists of a line or bar subdivided into divisions corresponding to a mile or kilometer, or fractions of a mile or kilometer. Will stay in true proportions when maps are enlarged or reduced.

Ø Verbal scale: used to discuss a map but is rarely written on it, e.g., “one inch on the map represents, or is proportional to, one mile on the ground” or, simply, “one inch to one mile.”

§ Magnetic Declination:

Ø True geographic north: the north refers to on a map

Ø Magnetic north pole: the north pointed to by compass, will change over time, or even reverse

Ø Magnetic declination: angular distance between true north and magnetic north

§ Symbols (Overhead 4)

IV. Topographic maps

Shows the size, shape, and distribution of landscape features; that is, the topography, or the configuration of the land surface. Planimetric map, to which a third dimension, elevation, has been added. Elevation is shown by contour lines.

§ Contour line: (Overhead 5)

§ Characteristics of contour lines: (Overhead 6)

§ Contour interval: difference in elevation between consecutive contours

§ Index contour: every fifth contour, starting from the sea level.

§ Depression contour: closes contours with hachures (short lines perpendicular to the contour) directed inward

§ Heigh and relief

Ø Height: difference in elevation between the top and the bottom (e.g., of a hill)

Ø Relief: difference between the highest and lowest elevations in a given area

§ Bench marks and spot elevation

Ø Bench mark: a point whose elevation and location have been precisely determined, designated on a map as B.M.

§ Making a topographic map (Overhead 7)

§ Topographic profiles (Overhead 8)