PS Topographic & Contour Mapping 3 — Questions and Answers
Question 1: A closed contour with hachure marks (tick marks pointing inward) on a topographic map represents:
- A hilltop
- A depression (Correct answer)
- A volcano
- A lake
Correct answer: A depression
Hachure marks on a closed contour indicate a depression, with ticks pointing downhill toward the lower interior of the closed area.
Question 2: When interpolating contour lines from a grid of spot elevations, which method assumes uniform slope between adjacent points?
- Bilinear interpolation
- Kriging
- Linear interpolation (Correct answer)
- Spline interpolation
Correct answer: Linear interpolation
Linear interpolation assumes a uniform (constant) slope between two adjacent elevation points and positions the contour proportionally between them.
Question 3: The horizontal distance between two contour lines on a map is called the:
- Contour interval
- Map scale
- Horizontal equivalent (Correct answer)
- Relief
Correct answer: Horizontal equivalent
The horizontal equivalent is the plan (map) distance between adjacent contour lines, which varies inversely with slope steepness.
Question 4: In LiDAR-based topographic mapping, what does the term 'bare-earth DEM' mean?
- A DEM created without GPS control
- A DEM representing only the ground surface after removing vegetation and structures (Correct answer)
- A DEM at 1-meter resolution or finer
- A DEM derived from satellite imagery alone
Correct answer: A DEM representing only the ground surface after removing vegetation and structures
A bare-earth DEM is produced by filtering out LiDAR returns from vegetation, buildings, and other above-ground objects, leaving only ground-level elevations.
Question 5: On a topographic map, two contour lines of different elevations may touch but never cross because:
- It would make the map too difficult to read
- A single point on Earth cannot simultaneously have two different elevations (Correct answer)
- Cartographic standards prohibit it
- Crossing contours indicate map errors only in flat terrain
Correct answer: A single point on Earth cannot simultaneously have two different elevations
Each contour line represents a unique elevation, so two different contours crossing would imply one ground point has two elevations, which is physically impossible.
Question 6: What is the primary purpose of a Digital Terrain Model (DTM) as distinguished from a Digital Elevation Model (DEM)?
- DTM includes only underwater bathymetric data
- DTM incorporates break lines and mass points to better represent terrain features (Correct answer)
- DTM is always at a finer resolution than a DEM
- DTM uses only photogrammetric data sources
Correct answer: DTM incorporates break lines and mass points to better represent terrain features
A DTM enhances elevation data by incorporating structural lines (break lines) and mass points that capture abrupt terrain changes, producing a more accurate surface representation than a bare-grid DEM.
Question 7: A surveyor notices that on a topographic map, contour lines appear to zigzag across a ridge. This pattern most likely indicates:
- A mapping error
- Multiple parallel ridges and valleys
- The ridge has a serrated or irregular crest (Correct answer)
- The map uses an incorrect contour interval
Correct answer: The ridge has a serrated or irregular crest
Zigzagging contour lines across a ridge indicate an irregular or serrated ridge crest with alternating high and low points along its length.
PS Topographic & Contour Mapping 3
A closed contour with hachure marks (tick marks pointing inward) on a topographic map represents: