Introduction to GIS & RS: Coordinate Systems & Projections

34 question-and-answer flashcards covering definitions, historical measurements, ellipsoids, geoids, datums, coordinate systems, map projections, choosing projections, and coordinate conversions from the lecture notes.

What does a georeferencing system do?

Positions objects in 2-D or 3-D space.

Define geodesy.

The science of measuring the Earth’s shape and establishing earth datums.

What is a geodetic datum?

A reference model (ellipsoid) that relates a coordinate system to the Earth and is realized through monumented points with known coordinates.

What are coordinate systems used for?

Locating and measuring geographic features on the Earth’s surface.

What is a map projection?

The systematic transformation of coordinates from the curved Earth to a flat map.

Name the three main factors that complicate defining coordinates.

Projection distortion, the Earth’s irregular shape, and measurement imprecision.

Which early Greek measured Earth’s circumference using shadows at Syene and Alexandria?

Eratosthenes.

Eratosthenes’ estimate of Earth’s circumference differed from modern value by roughly what percentage?

Less than 4 %.

Which Greek used zenith angles to estimate Earth’s circumference and got ≈ 38 600 km?

Posidonius.

What shape did Isaac Newton propose for Earth due to rotation?

An oblate ellipsoid (flattened at the poles).

What are the two characteristic dimensions of an ellipsoid?

Semi-major axis (equatorial radius) and semi-minor axis (polar radius).

How is the flattening factor of an ellipsoid expressed for Earth?

About 1/300 shorter at the poles than at the equator.

Differentiate ellipse and ellipsoid.

An ellipse is a 2-D oval; an ellipsoid is the 3-D form created by rotating an ellipse about its minor axis.

Which reference ellipsoid underlies the WGS 84 datum?

GRS 1980 (semi-major axis 6 378 137 m, 1/f = 298.257223563).

What is the geoid?

The 3-D surface where gravity pull is constant, approximating mean sea level but undulating relative to the ellipsoid.

What is orthometric height?

Elevation measured above the geoid (mean sea level).

What is ellipsoidal height?

Height measured above the reference ellipsoid.

Define geoidal separation (geoidal height).

The difference between ellipsoidal height and orthometric height.

Name two instruments historically used to measure the geoid.

Plumb bobs and gravimeters.

What do latitude lines represent?

Parallels—east-west lines of constant latitude.

What do longitude lines represent?

Meridians—north-south lines converging at the poles.

Why is GCS not a Cartesian coordinate system?

Because longitude lines converge, so grid cells are not right-angle or equal in size everywhere.

How far apart are 1° of longitude at the equator versus 80° N?

111 km at the equator; about 19 km at 80° N.

List the two main components of a geodetic datum.

1) An ellipsoid with origin and orientation; 2) A network of accurately surveyed points.

Give three commonly used horizontal datums.

NAD27, NAD83, and WGS84.

Which horizontal datum is officially used in Namibia?

Schwarzeck datum on the Bessel 1841 ellipsoid.

Differentiate vertical and horizontal datums.

Vertical datums reference height (Z) to mean sea level or an ellipsoid; horizontal datums locate X-Y positions on Earth using an ellipsoid.

What are the four properties that projections may distort?

Shape, area, distance, and direction (scale).

Name the three geometric families of map projections.

Planar (azimuthal), conic, and cylindrical.

Which projection preserves angles and shows meridians/parallels as straight lines but exaggerates area near poles?

Mercator (normal).

For areas with greater north-south than east-west extent, which conformal cylindrical projection is preferred?

Transverse Mercator.

State the key parameters of the Transverse Mercator used for Namibia.

Datum : WGS84; Central meridian 17°E; False Easting 600 000 m; False Northing 10 000 000 m; Scale factor 1.0; Latitude of origin 0°.

What global projection divides Earth into 6-degree longitudinal strips measured in meters?

Universal Transverse Mercator (UTM).

Give two criteria to consider when choosing a projection.

Extent/latitude of area and which property of distortion should be minimized (e.g., area vs shape).

Outline the steps to convert decimal degrees (DD) to degrees-minutes-seconds (DMS).

Separate whole degrees, multiply fractional part by 60 for minutes, then multiply remaining fraction by 60 for seconds.

How many seconds are in one degree of latitude or longitude?

3 600 seconds.