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TEC-GIS45-08 · Lesson 9 · 13 clock hours · lab

Topographic and Contour Mapping

Objective. Students generate contour lines from a digital elevation model, apply cartographic contour standards, and produce a topographic map suitable for a surveying project deliverable.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by an industry-certified professional or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Topographic and Contour Mapping

A topographic map communicates the three-dimensional shape of the earth's surface on a two-dimensional sheet. Contour lines are its primary tool: each line connects points of equal elevation, and the pattern of lines reveals hills, valleys, ridges, and depressions. Surveyors and GIS technicians must be able to generate accurate contour lines from elevation data, apply recognized symbology standards, and deliver a finished map that a project engineer or field crew can trust.

The Digital Elevation Model (DEM)

Modern contour maps start with a DEM — a raster grid where every cell stores an elevation value. In this program you will use either a 1-meter DEM collected by a drone equipped with photogrammetry or LiDAR capability, or a 1-meter 3DEP (3D Elevation Program) DEM downloaded from the USGS National Map. Cell size determines the smallest terrain feature the DEM can represent, so a 1-meter DEM supports contour intervals as fine as 1 to 2 feet without introducing false detail.

Before running any geoprocessing, inspect the DEM in ArcGIS Pro: check the coordinate system (a projected system with linear units of feet or meters is required), confirm there are no NoData voids in the project area, and verify that elevation values are reasonable for the site. A DEM with voids or incorrect projection will produce contour lines that are wrong in ways that are hard to spot later.

Generating Contour Lines

In ArcGIS Pro, the Contour tool (Spatial Analyst toolbox > Surface) converts a DEM raster into a polyline feature class. You specify:

A common workflow runs the tool twice: once at the fine interval to generate all contours, then a second time at five times the interval to create index contours, which are later symbolized differently. Alternatively, the tool's output includes a ContourType or index field you can use to filter.

USGS Cartographic Standards for Contour Symbology

The USGS has published topographic map standards that are widely adopted across surveying and engineering. Apply these conventions:

Contour labels must read uphill (the top of the text faces the higher elevation). In ArcGIS Pro, the Maplex Label Engine can enforce this automatically when contour label placement is set to follow the line with an offset.

Smoothing Contour Lines

DEM-derived contours often appear angular because they follow cell edges. The Smooth Line tool (Cartography toolbox > Generalization) removes this staircase effect using a PAEK (Polynomial Approximation with Exponential Kernel) algorithm. Choose a smoothing tolerance appropriate to your scale: too small a tolerance leaves jagged lines; too large a tolerance displaces contours from their true position, which is a surveying accuracy problem. For a 1-meter DEM mapped at 1:2,400 (1 inch = 200 feet), a tolerance of 10–15 feet is a reasonable starting point. Always run a spot-check after smoothing — compare smoothed contour elevations at known benchmark locations to confirm the lines have not migrated off their correct elevation.

Spot Elevations

Contour lines alone do not give exact elevation at a specific point. Spot elevations — labeled points showing a precise elevation — are placed at summits (highest point of a hill), saddles (low point between two peaks), and benchmark locations. In ArcGIS Pro, create a point feature class with an elevation attribute populated from the DEM (using the Extract Values to Points tool or by entering field-verified values). Label each point with its elevation value, styled in black with a dot or triangle symbol per USGS convention. Benchmarks established by NGS or local control surveys should show the published elevation, not a DEM-derived estimate.

Topographic Validity Rules

A contour map is only useful if it is topographically correct. Apply these rules before submitting any deliverable:

1. Contours never cross. Two lines at different elevations cannot occupy the same point.

2. V-shapes in drainages point uphill. Where a contour crosses a stream or valley, it bends upstream (uphill) forming a V. This is the single most reliable field check.

3. Closed contours indicate hills or depressions — hills have no hachures, depressions do.

4. Contour spacing reflects slope. Closely spaced lines mean steep terrain; widely spaced lines mean gentle terrain. An abrupt jump in spacing that does not match the terrain is a processing error.

Safety and Data Integrity

Working with spatial data is an office task, but data integrity is a safety issue when maps guide field operations. A contour map used for site grading, drainage design, or drone flight planning must be accurate. Always document the DEM source, date of collection, horizontal and vertical datum, and contour interval in the map layout's metadata or notes block. If drone-collected data was used, confirm the ground control points (GCPs) were surveyed with a calibrated instrument and that the vertical accuracy of the DEM meets the project specification before contouring.

Producing the Final Layout

The finished layout must include: title, north arrow, scale bar, coordinate system and datum, contour interval note (e.g., "Contour Interval: 2 feet, Index: 10 feet"), legend showing index and intermediate contour symbols, depression contour symbol, spot elevation symbol, data source and date, and preparer name. Export at a resolution of at least 300 dpi for print or PDF delivery.

Key terms10

Digital Elevation Model (DEM)A raster grid dataset in which each cell stores a single elevation value representing the bare-earth surface; the source data for contour generation.
Contour intervalThe constant vertical distance between successive contour lines on a topographic map, chosen to match map scale and terrain relief.
Index contourEvery fifth contour line, drawn heavier and labeled with its elevation; aids rapid elevation reading in the field.
Intermediate contourThe four unlabeled contour lines between successive index contours, drawn with a thinner line weight.
Depression contourA closed contour representing a topographic low (sink or pit) that is distinguished from a hilltop by hachure tick marks pointing downhill into the depression.
HachuresShort tick marks drawn perpendicular to and pointing inward on a depression contour, indicating that the enclosed area is lower than the surrounding terrain.
Spot elevationA precisely labeled point on a map showing the exact elevation at a specific location such as a summit, saddle, or benchmark.
Smooth Line (PAEK)An ArcGIS Pro geoprocessing tool that applies a Polynomial Approximation with Exponential Kernel algorithm to remove angular cell-edge artifacts from DEM-derived contour lines.
V-shape ruleThe cartographic principle that where a contour line crosses a valley or stream channel, it bends upstream (uphill), forming a V shape; used to verify topographic correctness.
3DEP (3D Elevation Program)A USGS national program that provides publicly available, high-resolution bare-earth DEMs at resolutions including 1 meter across much of the United States.

Worked examples4

1. A project DEM has a vertical range of 240 feet. The final map will be plotted at 1:2,400. Select an appropriate contour interval and state what the index contour interval will be.
  1. Step 1 — Apply the general rule: a readable contour map at a given scale should show roughly 4 to 6 index contour lines across a typical terrain cross-section. Divide the total relief by a target of 5 index intervals: 240 ft ÷ 5 = 48 ft per index interval.
  2. Step 2 — Round to a standard interval. 48 ft is not standard; the nearest standard index intervals are 50 ft (index) and 10 ft (basic). For a 240-ft range at 1:2,400 (a large scale, detailed map), a 2-ft basic interval with 10-ft index is more appropriate because the map covers a small area in detail.
  3. Step 3 — Check: 240 ft ÷ 2 ft = 120 intermediate+index lines total; 240 ft ÷ 10 ft = 24 index lines. That is manageable at 1:2,400 for a project-area map.
  4. Step 4 — Decision: use a 2-foot contour interval (intermediate) and 10-foot index contour interval.
Answer: Contour interval = 2 feet; index contour interval = 10 feet (every fifth contour).
2. After running the Contour tool at a 2-foot interval with a base contour of 0, a technician needs to identify which contour lines are index contours. The output feature class has a field called 'Contour' storing elevation values. Write the definition query expression that selects only the 10-foot index contours.
  1. Step 1 — Index contours occur at every multiple of the index interval (10 feet). Multiples of 10 that could appear in the dataset include 0, 10, 20, 30, … and also negative values if terrain goes below the base contour.
  2. Step 2 — In ArcGIS Pro SQL syntax, use the modulo operator to test divisibility: Contour MOD 10 = 0. This returns TRUE for any elevation evenly divisible by 10.
  3. Step 3 — Write the definition query: "Contour" % 10 = 0 (ArcGIS Pro uses % as the modulo operator in definition queries for most geodatabase formats).
  4. Step 4 — Apply this query to the contour layer, then assign the heavy brown symbology and enable labeling for this filtered set.
Answer: Definition query: "Contour" % 10 = 0 — this selects all contour lines whose elevation is a multiple of 10, which are the index contours.
3. A contour crosses a stream at elevation 348 feet. The next contour upslope (on the valley wall) is at 350 feet, and the contour on the same valley wall on the other side of the stream is also at 350 feet. Is the 348-foot contour correctly drawn with its V-shape pointing upstream? Explain using the V-shape rule.
  1. Step 1 — Recall the V-shape rule: contours bend upstream (uphill) when they cross a drainage. The tip of the V points in the direction of increasing elevation along the stream.
  2. Step 2 — At the stream crossing, the contour at 348 ft is lower than both valley walls at 350 ft. The stream itself flows downhill from higher to lower elevation, so water flows away from the V tip.
  3. Step 3 — If the V tip of the 348-ft contour points upstream (toward higher stream elevation), then as you move up the stream, you encounter the 348-ft crossing before reaching the 350-ft walls — this is topographically consistent.
  4. Step 4 — Confirm: the V must point toward the source (uphill) of the drainage. If the drawn V tip points downstream (downhill), the contour is incorrectly drawn and must be corrected.
Answer: The 348-foot contour is correctly drawn only if the tip of its V-shape points upstream (toward the headwaters). A V pointing downstream would violate the V-shape rule and indicate a topographic error in the contour data.
4. A benchmark in the project area has a published elevation of 412.83 feet. After running Extract Values to Points on the 1-meter DEM, the extracted elevation for that point is 411.6 feet. Calculate the vertical error and state whether this exceeds a typical 1-foot vertical accuracy specification.
  1. Step 1 — Vertical error = |DEM value − published value| = |411.6 − 412.83| = 1.23 feet.
  2. Step 2 — The specification allows a maximum vertical error of 1.0 foot.
  3. Step 3 — 1.23 feet > 1.0 foot, so the DEM exceeds the allowable vertical error at this check point.
  4. Step 4 — Action: report the discrepancy to the project manager. Do not deliver contour maps derived from this DEM until the DEM accuracy issue is investigated — possible causes include incorrect GCP elevations, insufficient GCP coverage, or a georeferencing error in the drone processing workflow.
Answer: Vertical error = 1.23 feet, which exceeds the 1.0-foot specification. The DEM must be investigated and corrected before contouring proceeds.

The math in this lesson1

Learners divide 240 ft of relief by 5 to choose a 50 ft standard index contour interval.
N-Q.3
Brush up in the math halls at newSchoolWorks.com:
In class: Students run the Contour tool on a project DEM to generate index and intermediate contours at an instructor-specified interval; verify that contour values are topographically consistent (V-shapes point uphill in drainages). · Students apply standard USPS/USGS-style symbology: brown index contours labeled at every fifth line, thinner intermediate contours unlabeled, depression contours with hachures; smooth contour lines using the Smooth Line tool at an appropriate tolerance. · Students add spot elevations at summits, saddles, and benchmark locations using a point feature class with elevation attribute labels; produce a finished layout with 2-foot and 10-foot contour intervals shown in legend.
Materials: ArcGIS Pro, 1-meter DEM (from drone or USGS 3DEP source), USGS topographic map symbol standards reference, Contour and Smooth Line geoprocessing tools
Check: Contour map checked for correct interval, proper labeling convention, and topographic validity (no crossing contours, correct uphill rule); spot elevations present. Pass = zero topographic errors and legend correct.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Esri ArcGIS Pro — Cartography and Geoprocessing
1. A technician runs the Contour tool on a 1-meter DEM and notices the output lines have a staircase (angular, stepped) appearance. Which tool should be applied next to correct this?
NSPS CST L1-2 — Cartography and Map Reading
2. On a standard USGS-style topographic map, which contour line is labeled with its elevation value?
NSPS CST L1-2 — Cartography and Map Reading
3. A closed contour with hachure tick marks pointing inward most likely represents which terrain feature?
NSPS CST L1-2 — Cartography and Map Reading
4. Where a contour line crosses a valley and stream channel, which direction does the V-shape point?
NSPS CST L1-2 — Cartography and Map Reading
5. A project map must show elevations at specific known points — the summit, saddle, and benchmark — in addition to contour lines. Which map element provides this information?
Esri ArcGIS Pro — Geoprocessing and Data Management
6. In ArcGIS Pro, a technician wants to select only the 10-foot index contours from a contour feature class where the 'Contour' field stores elevation in feet and the basic interval is 2 feet. Which definition query correctly isolates index contours?
Esri ArcGIS Pro — Coordinate Systems and Projections
7. Before generating contours from a project DEM, a technician discovers the raster is in a geographic coordinate system (decimal degrees) rather than a projected coordinate system with linear units. Why is this a problem for contour generation?
NSPS CST L1-2 — Survey Deliverables and Documentation
8. A finished topographic map layout is missing a contour interval note. According to cartographic standards, why is this omission significant for a surveying project deliverable?
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