Field walkthrough — Topographic survey with a total station
The hook
A topographic survey captures both position and elevation of natural and man-made features on a site — building corners, road centerlines, drainage features, trees, plus enough surface points to generate contours. The deliverable is the foundation for civil design.
Memorize these
Concepts that show up on the exam
Contour
A line connecting points of equal elevation. Closed loops; never cross each other; closer spacing = steeper terrain.
Contour interval
Vertical distance between adjacent contours. Picked for the scale and purpose: 0.5-2 ft for site grading, 5-10 ft for road corridors, 20-50 ft for regional.
Index contour
Every 5th (or 4th) contour, drawn heavier and labeled with the elevation. Helps the reader find a value quickly.
Spot elevation
A single point with a labeled elevation, used for high points, low points, building corners — anywhere a contour wouldn't communicate the precise value.
Breakline
A line along a feature where slope changes sharply (top of a curb, edge of a swale, ridgeline). Forces the surface model to honor this discontinuity.
Grid sampling
Methodically taking shots on a regular grid (e.g., every 50 ft). Simple but inefficient — wastes shots on flat areas, undersamples terrain breaks.
Feature-based sampling
Take shots strategically: spot elevations on highs/lows, breaklines along edges, denser near features that matter to design. The modern approach.
TIN vs. grid DEM
Two ways to represent the surface: Triangulated Irregular Network (irregular triangles between observations) or grid DEM (regular array of elevation values). TIN preserves breaklines; grid is simpler to process.
Don't fall for these
What trips people up
Topo without breaklines
Without breaklines, contours will draw a smooth surface across a curb edge or ditch, hiding the very feature the surveyor went out to measure. Always identify and code breaklines in the field.
Wrong contour interval for the scale
2-ft contours on a site that drops 200 ft create unreadable spaghetti; 20-ft contours on a flat parking lot show nothing. Pick interval to give 5-15 contours across the area of interest.
Mixing horizontal and vertical accuracy
Topo deliverables are usually graded by NMAS (National Map Accuracy Standards) or ASPRS standards. Horizontal accuracy and vertical accuracy are SEPARATE specifications — meet both.
Forgetting to capture utilities
Above-ground utilities (poles, hydrants, manholes) MUST be captured. Sub-surface utilities require Subsurface Utility Engineering (SUE) Quality Levels A-D. ALTA Table A item 11 covers these.
Test yourself
How well did it stick?
A quick 5-question check on Topographic Surveys. See where you stand and what to review.