A GIS analyst calculates the area of forest polygons using geographic coordinates (latitude/longitude in WGS 84) and gets results in square degrees. What is wrong with this approach?
AWGS 84 is an outdated coordinate system
BGeographic coordinates are angular units on a curved surface; area calculations require data projected to a planar coordinate system with linear units (meters), ideally using an equal-area projection to avoid systematic area distortion
CThe analyst should have used feet instead of degrees
DForest polygons cannot be represented in geographic coordinates
Latitude and longitude are angles, not distances. One degree of longitude covers ~111 km at the equator but ~0 km at the poles. Area in square degrees is physically meaningless and will be systematically wrong -- overestimating area near the equator and underestimating it at high latitudes. Projecting to a local equal-area projection converts angular coordinates to meters and preserves area relationships.
Question 2 True / False
The Universal Transverse Mercator (UTM) system divides Earth into 60 zones, each 6 degrees of longitude wide, because a single Transverse Mercator projection cannot accurately cover the entire globe.
TTrue
FFalse
Answer: True
The Transverse Mercator projection is accurate near its central meridian but distortion increases with distance. By limiting each zone to 6 degrees of longitude, UTM keeps maximum scale distortion below 0.04% -- adequate for large-scale mapping and engineering. Using a single zone for the entire globe would produce extreme distortion far from the central meridian. This zone system is why UTM coordinates always include a zone number.
Question 3 Short Answer
Explain the difference between a datum and a map projection, and why both are needed to define a coordinate reference system.
Think about your answer, then reveal below.
Model answer: A datum defines the size, shape, and orientation of the reference ellipsoid (mathematical model of Earth's shape) and its relationship to Earth's actual surface. WGS 84 and NAD 83 are datums with slightly different ellipsoid parameters and alignment. A map projection is the mathematical transformation that converts 3D ellipsoidal coordinates (latitude/longitude on a specific datum) to 2D planar coordinates (x, y in meters). Both are needed because the projection operates on the ellipsoid defined by the datum -- using the wrong datum shifts features by meters to hundreds of meters, while using the wrong projection distorts shapes, areas, or distances.
Datum = which Earth model; Projection = how to flatten it. Together they form a complete Coordinate Reference System (CRS) like UTM Zone 15N on WGS 84 (EPSG:32615).