ASBOG Geology Exam

Map scale is the ratio between a distance on the map and the corresponding distance on the ground. A representative fraction (RF) such as 1:24,000 is unitless: 1 inch on the map equals 24,000 inches (2,000 feet) on the ground, and 1 cm equals 240 m. A verbal scale states the relationship in words ("1 inch equals 1 mile" = 1:63,360). A graphic (bar) scale remains correct when a map is enlarged or reduced, because it changes size with the map.

A large-scale map has a small denominator (1:24,000) and shows a small area in detail; a small-scale map has a large denominator (1:250,000) and shows a large area with little detail. Area scales with the square of the linear scale: halving the denominator quadruples the map area of a given ground area.

Latitude measures angular distance north or south of the equator (0–90°); longitude measures angular distance east or west of the Prime Meridian (0–180°). One degree of latitude is about 111 km (69 mi) everywhere; one degree of longitude is about 111 km × cos(latitude), shrinking to zero at the poles. One minute of latitude is about one nautical mile (1.85 km).

The Universal Transverse Mercator (UTM) system divides the Earth between 80°S and 84°N into 60 zones, each 6° of longitude wide. Positions are given in meters as an easting (with the zone's central meridian assigned a false easting of 500,000 m, so eastings are always positive) and a northing (meters north of the equator in the northern hemisphere). Because UTM is a Cartesian grid, distances and areas can be computed with simple geometry. State Plane Coordinate Systems use Lambert conformal conic projections for states that are wide east–west and transverse Mercator projections for states that are long north–south, giving smaller distortion within each zone; coordinates are usually in U.S. survey feet or meters.

A datum defines the reference ellipsoid and its position relative to the Earth. Horizontal datums used in the United States include NAD27 (Clarke 1866 ellipsoid), NAD83 (GRS80 ellipsoid), and WGS84 (used by GPS). Coordinates of the same point can differ by tens to more than 100 meters between NAD27 and NAD83, so mixing datums in a GIS is a classic source of error. Vertical datums (NGVD29, NAVD88) define the zero for elevations; well elevations and water levels must be surveyed to the same vertical datum before heads are contoured.

The Public Land Survey System (PLSS) covers most of the United States outside the original colonies and Texas. Surveys start from an intersection of a principal meridian and a baseline. Townships (6 miles on a side, numbered north or south of the baseline) and ranges (numbered east or west of the meridian) form a grid. Each township contains 36 sections of 1 square mile (640 acres), numbered from the northeast corner in a back-and-forth pattern.

Global navigation satellite systems (GNSS, including GPS) locate a receiver by trilateration from at least four satellites. A handheld receiver is typically accurate to a few meters; differential GPS and real-time kinematic (RTK) surveys use a base station or correction network to reach centimeter-level accuracy. Vertical accuracy is roughly half as good as horizontal accuracy, which is why well-casing elevations for groundwater contouring are normally established by a licensed land surveyor or RTK survey rather than a handheld unit.