Geodesy & Map Projections Flashcards
14 cards from real Certified Photogrammetrist practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 14 Geodesy & Map Projections flashcards as text
NAD 83 coordinates are referenced to which ellipsoid?
Answer: GRS 80
NAD 83 uses the GRS 80 ellipsoid, while NAD 27 used Clarke 1866.
A GNSS point has ellipsoid height h = 152.40 m and geoid height N = -30.10 m. What is its orthometric height H?
Answer: 182.50 m
H = h - N = 152.40 - (-30.10) = 182.50 m.
What scale factor is applied at the central meridian of every UTM zone?
Answer: 0.9996
UTM uses a central meridian scale factor of 0.9996 to balance distortion across the 6-degree zone.
Which projection is typically used for State Plane zones that extend mainly east-west?
Answer: Lambert Conformal Conic
Lambert Conformal Conic keeps scale error low along its standard parallels, which suits east-west zones.
What is the deflection of the vertical?
Answer: The angle between the plumb line and the ellipsoid normal
Deflection of the vertical is the angle between the direction of gravity (plumb line) and the ellipsoid normal at a point.
In a conformal projection grid, what does the convergence angle describe?
Answer: The angle between grid north and geodetic (true) north
Grid convergence is the angle between grid north and geodetic north, and it grows with distance from the central meridian.
NAVD 88 was held fixed at a single tide-gauge benchmark located where?
Answer: Father Point/Rimouski, Quebec
NAVD 88 was a minimally constrained adjustment fixed at the Father Point/Rimouski benchmark in Quebec.
A project site lies at longitude 105 degrees W. Which UTM zone has this longitude as its central meridian?
Answer: Zone 13
Zone 13 spans 108 to 102 degrees W, so its central meridian is 105 degrees W.
What false easting is assigned to the central meridian of every UTM zone?
Answer: 500,000 m
A 500,000 m false easting keeps all eastings within a zone positive.
What false northing is applied to UTM coordinates in the Southern Hemisphere?
Answer: 10,000,000 m
Southern Hemisphere UTM assigns the equator 10,000,000 m north so northings stay positive.
Which factor converts a measured horizontal ground distance to a State Plane grid distance?
Answer: Combined factor (grid scale factor times elevation factor)
Multiplying ground distance by the combined factor first reduces it to the ellipsoid and then applies projection scale.
Which projection keeps areas true, so it is preferred for thematic land-cover maps of the conterminous United States?
Answer: Albers Equal-Area Conic
Albers Equal-Area Conic preserves area, which is why USGS uses it for national land-cover data.
Which current NGS hybrid geoid model converts NAD 83(2011) ellipsoid heights to NAVD 88 orthometric heights?
Answer: GEOID18
GEOID18 is the NGS hybrid model fitted to NAD 83(2011) GPS-on-benchmarks to yield NAVD 88 heights.
A 7-parameter Helmert (similarity) transformation between datums consists of which parameters?
Answer: 3 translations, 3 rotations, 1 scale
The Helmert transformation uses three shifts, three axis rotations, and one uniform scale factor.
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