Certified Photogrammetrist Photogrammetric Product Generation 1 — Questions and Answers
Question 1: What is a 'true orthophoto' as distinct from a standard orthophoto?
- An orthophoto produced using only ground control points without GPS assistance
- An orthophoto corrected for building lean using a surface model of all above-ground features (DSM), so building walls do not appear in the image (Correct answer)
- A color-balanced orthophoto with radiometric corrections applied
- An orthophoto produced from satellite imagery rather than aerial photography
Correct answer: An orthophoto corrected for building lean using a surface model of all above-ground features (DSM), so building walls do not appear in the image
A true orthophoto uses a full DSM (including buildings and trees) for differential rectification, eliminating building lean and wall visibility that appears in standard orthophotos orthorectified using only a DTM.
Question 2: What is 'differential rectification' in orthophoto production?
- Correcting images pixel-by-pixel using a DEM to remove relief displacement across the entire image (Correct answer)
- Adjusting color differences between overlapping photographs
- Correcting GPS positions using a reference base station
- Removing systematic sensor distortions using a flat-field calibration
Correct answer: Correcting images pixel-by-pixel using a DEM to remove relief displacement across the entire image
Differential rectification corrects each pixel individually using a DEM, removing the varying relief displacement across the image to produce a geometrically accurate orthophoto.
Question 3: What does 'contour interval' mean in topographic map production?
- The horizontal distance between adjacent contour lines
- The vertical elevation difference between successive contour lines (Correct answer)
- The time interval between adjacent flight lines
- The pixel spacing in a digital elevation model
Correct answer: The vertical elevation difference between successive contour lines
Contour interval is the vertical elevation difference between consecutive contour lines on a topographic map, determining the level of vertical detail shown.
Question 4: For USGS National Map Accuracy Standards, what horizontal accuracy is required for a 1:24,000-scale map?
- ±5 meters at the 90% confidence level
- ±12.2 meters (40 feet) at the 90% confidence level (Correct answer)
- ±1 meter at the 95% confidence level
- ±50 meters at the 68% confidence level
Correct answer: ±12.2 meters (40 feet) at the 90% confidence level
USGS NMAS requires that 90% of well-defined points on a 1:24,000-scale map be within ±40 feet (±12.2 meters) of their true position.
Question 5: What is an 'orthomosaic' in photogrammetric production?
- A single seamless orthophoto created by mosaicking (stitching) multiple overlapping orthophotos together (Correct answer)
- A mosaic of ground control point sketches
- A color mosaic combining different spectral bands into a composite image
- A terrain model created by combining multiple stereomodels
Correct answer: A single seamless orthophoto created by mosaicking (stitching) multiple overlapping orthophotos together
An orthomosaic is a large, seamless orthophoto produced by mosaicking multiple individual orthophotos, providing wall-to-wall coverage of a project area.
Question 6: What is 'seam line generation' in orthomosaic production?
- The process of defining boundaries between adjacent orthophotos in a mosaic to minimize visible transitions (Correct answer)
- Drawing boundaries between map sheets
- Detecting cracks in the terrain surface from elevation data
- Separating vegetated from non-vegetated areas automatically
Correct answer: The process of defining boundaries between adjacent orthophotos in a mosaic to minimize visible transitions
Seam line generation defines the boundaries where one source orthophoto transitions to another in the mosaic, with lines typically placed in areas of low contrast (open water, fields) to minimize visible seams.
Certified Photogrammetrist Photogrammetric Product Generation 1
What is a 'true orthophoto' as distinct from a standard orthophoto?