Geodesy & Map Projections Flashcards

A project sits at 1,000 m ellipsoid height. Using R = 6,371 km, what is the approximate ellipsoid distance for a 1,000.000 m ground distance?

Answer: 999.843 m

Elevation factor = R/(R+h) = 6,371,000/6,372,000 = 0.999843, so 1,000 m becomes about 999.843 m.

What is the main purpose of a Low Distortion Projection (LDP)?

Answer: To keep grid and ground distances nearly equal across the project area

An LDP is scaled and positioned at the project's ground elevation so grid distances closely match ground distances.

On which projection do all great circles appear as straight lines?

Answer: Gnomonic

The gnomonic projection projects from Earth's center, so every great circle maps to a straight line.

Lines of constant bearing (rhumb lines) appear as straight lines on which projection?

Answer: Mercator

Mercator is conformal and shows meridians as parallel lines, so rhumb lines plot straight, which is why navigators use it.

Across most of the conterminous United States, what is true of the geoid height N relative to the GRS 80 ellipsoid?

Answer: It is negative, so the geoid lies below the ellipsoid

Geoid heights in the conterminous US range from roughly -8 m to -50 m, so the geoid lies below the ellipsoid.

Why do a US station's ITRF2014 coordinates change by centimeters per year while its NAD 83(2011) coordinates stay nearly constant?

Answer: NAD 83 is fixed to the North American plate, while ITRF is a global frame

NAD 83 rotates with the North American tectonic plate, so plate motion appears as time-dependent velocity in ITRF.

The US survey foot, deprecated by NIST and NGS at the end of 2022, is defined as exactly:

Answer: 1200/3937 m

The US survey foot equals 1200/3937 m (about 0.3048006 m), while the international foot is exactly 0.3048 m.