• They are a fundamental means of communication ("A picture is worth 1,000 words.")
• They are the most efficient means of portraying parts of the earth’s surface
• Maps can record, present, and analyze data

Physical (including surface relief (topographic) and ocean bottom bathymetry); Political, Geologic and etc.

The Earth is a sphere (or more correctly a spheroid), and a globe is the best representation or model of the Earth's surface. A map, on the other hand, must represent as accurately as possible the 3-dimensional Earth on a 2-dimensional (flat) surface. In producing a map it is important to ensure a known relationship between true locations on the Earth and the corresponding points on the map. Therefore, the construction of any map must begin with a map projection . . . and there are dozens to choose from.

The process of systematically transforming positions on the Earth's spherical surface to a flat map while maintaining spatial relationships, is called map projection. This projection process is accomplished by the use of geometry and, more commonly, by mathematical formulas. In geometric terms, the Earth as a spheroid (i.e., a slightly flattened sphere), is considered an undevelopable shape, because, no matter how the Earth is divided up, it cannot be unrolled or unfolded to lie flat. Some of the simplest projections are made onto geometric shapes that can be flattened without stretching their surfaces. These shapes or forms are considered to be developable. Examples of shapes that reflect these properties are cones, cylinders, and planes.

Map scale: Most maps use two types of scales:

Latitude (or parallel) is defined as distance, measured in degrees, north and south of the equator.

• run east to west, but they measure distance north and /or south of the equator

• are always parallel to each other at all points and equidistant. However, the circumference of each successive parallel of latitude, from the equator to the poles, gets smaller.
• are measured from 0 (at the equator) to 90º at the north or south poles.

• run north and south, but they measure distance east and west of the prime meridian
• represent "Great Circles" because they are all the same size.
• unlike parallels of latitude, where all of the lines are parallel, meridians of longitude converge as you move away from the equator towards the poles. Consequently, the distance between consecutive lines of longitude decreases from 69.17 miles (111.11 km) apart at the equator to 0 miles at the poles.

Contours: Differences in relief (differences in altitude) on a topo map are shown by contour lines (lines of equal elevation). For example, all points on a 100-foot contour line are 100 feet above sea level. The elevation change between contour lines is given by the contour interval, which can be from 1 foot to 100 feet. Each map will have only one contour interval, listed on the map. Ordinary contour lines always enclose higher ground; where a contour encloses a depression, the line will have tick marks on the inside. Figure 1 shows how a three-dimensional surface can be represented on a topo map.

When looking at a topo map, one is viewing the landscape from directly above. It can be difficult to visualize how it would look from the ground. Constructing a topographic cross-section allows one to obtain a side view of the landscape along a line. Such lines are often named A–A' ("A to A-prime"), B–B', etc. Think of a cross-section as the topography you would encounter as you walked from point A to point A'.

A profile is unexaggerated if the horizontal and vertical scales are the same. To emphasize the differences in relief of a comparatively flat area, the vertical scale may be expanded relative to the horizontal scale, producing a cross-section with vertical exaggeration. Vertical exaggeration is given by:

E = H / V, where E = vertical exaggeration

H = denominator of the horizontal scale fraction

V = denominator of the vertical scale fraction

The following is an excellent procedure to construct topographic cross-sections:

"Geologic maps are uniquely suited to solving problems involving Earth resources, hazards, and environments. In particular, digital geologic maps are interactive electronic documents that put the Nation's earth science issues into geospatial frameworks. They capture the size, the shape, the depth, and the physical and chemical contexts of earth materials and they blend data display with the results of interpretive research. These are actually four-dimensional data systems, and it is the fourth dimension of time, that is crucial to assessing natural hazards and environmental or socio-economic risk. To read a geologic map is to understand not only where earth resources and characteristics are located, but also how and when these earth features formed. Were they produced last year in a fiery hurricane of volcanic ash, a hundred years ago in a major flood, or ten million years ago from a fertile sea or estuary? Should we plan

for another earthquake, eruption, or flood in the same area next year, or not in our lifetimes. Is the water beneath the surface part of a regional aquifer or is it controlled by a local fracture zone? How does subsurface distribution of porous and impermeable rock affect the flow of water, the potential for contamination, and the volume available for use? Geologic maps and data bases address all of these questions."

Paleozoic: Pz Permian P

Pennsylvanian PP

Mississippian M

Precambrian: pC

When reading geologic maps, keep in mind that they tend to be large-scale compilations of detailed information gleaned from various individuals' field work over many years. A geologist's interpretations may be from a field area of less than a square mile, or up to 20 square miles or more. However, even the most comprehensive, detailed field mapping consists of only partial sampling of the rocks (except, perhaps, with remote sensing). In addition, remapping and/or reinterpretations are commonplace. Therefore, a geologic map consists of the best interpretations and inferences at a given point in time.

• Colored patches represent areas underlain by certain geologic units, each color-coded to the age of the unit. The symbol for each colored blob is coded to the name and age of the rock unit. A short description of the rock unit is given in the legend for the map.

• Yellow colors typically are reserved for surface geologic units such as soils, young river deposits, and glacial deposits. On this map, for example, the yellow colors delineate two kinds of deposits along a meandering river.

• Other colors represent older rock units deposited millions of years ago and since faulted and distorted by earthquakes, and eroded to the modern landscape.

• Special line symbols indicate faults. One is shown by the straight heavy line cutting across geologic units on the left side of the map. These and the thinner lines for other contacts between geologic units are solid, dashed, or dotted depending on how completely the contacts are exposed.

• Symbols give the strike and dip of layers in rock, two measurements that describe their orientation in space.