🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Technology & Design › GIS / Surveying / Drone Operations › QGIS and ArcGIS Pro for Surveying and Geospatial Analysis › Lesson 5
TEC-GIS45-02 · Lesson 5 · 14 clock hours · lab

Raster Data: DEMs, Orthomosaics, and Terrain Analysis

Objective. Students will load and symbolize digital elevation models and drone orthomosaics, then run slope, aspect, hillshade, and contour generation tools to support surveying and site analysis tasks.

⚠ 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.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Raster Data: DEMs, Orthomosaics, and Terrain Analysis

What Is a Raster?

A raster is a grid of cells — like a spreadsheet laid flat on the earth — where each cell stores one numeric value. In GIS and surveying work, the two raster types you will use most are digital elevation models (DEMs) and orthomosaics. Understanding how each type stores information, and how to analyze it, is a core skill for site planning, construction layout, and drone data processing.

Digital Elevation Models

A DEM records ground elevation at every cell. The USGS 3D Elevation Program (3DEP) provides publicly available DEMs for the contiguous United States at resolutions as fine as 1 meter. The value stored in each 1-meter cell is the bare-earth elevation — vegetation and structures have been filtered out. Because 3DEP data is derived from lidar surveys flown by USGS partners, its vertical accuracy is generally high, making it reliable for generating contours and calculating slopes for engineering applications.

When you load a DEM into ArcGIS Pro, the software reads the coordinate reference system (CRS) embedded in the file. Always confirm the CRS before analysis. A mismatch between the DEM's CRS and your project CRS will shift features spatially or distort measurements.

Hillshade

A hillshade is a derived raster that simulates how sunlight would illuminate the terrain from a chosen azimuth and altitude angle. It does not store real elevation — it stores a brightness value (0–255) calculated from the slope and aspect of each cell relative to the simulated light source. Hillshades reveal subtle landforms — drainage channels, berms, old road beds — that are invisible in a raw elevation grid. The standard default sun position in ArcGIS Pro is 315° azimuth (northwest) and 45° altitude. You can change these to match the actual sun angle during a drone flight if you want the hillshade to match shadows in the orthomosaic.

Slope and Aspect

Slope is the rate of elevation change between a cell and its neighbors, expressed in degrees or percent. For construction site analysis, a common five-class breakout is:

These thresholds align with typical grading and stormwater standards used in civil engineering. Use a sequential or diverging color ramp so the steepest terrain reads visually as the most intense color.

Aspect records the compass direction a slope faces (north, south, east, west). Aspect matters for solar panel siting, frost-pocket analysis, and predicting where ice will form on a construction site.

Contour Generation

A contour line connects all points of equal elevation. Contours are generated from a DEM by the Contour geoprocessing tool (ArcGIS Pro: Spatial Analyst toolbox → Surface → Contour; QGIS: Raster menu → Extraction → Contour). The key input is the contour interval — the elevation difference between successive lines. For a 1-m DEM, a 1-foot (~0.3 m) or 2-foot (~0.6 m) interval is practical. At the 2-foot interval, every other line from a 1-foot run should coincide, giving you a built-in accuracy check.

When comparing generated contours to a paper reference map, the acceptable tolerance is one contour interval. If your 2-foot contours are off by more than 2 feet at a check point, investigate the vertical datum. USGS 3DEP data is referenced to NAVD 88 (North American Vertical Datum of 1988). If the paper map uses a different datum, a datum shift will appear as a consistent offset across the entire map.

Orthomosaics

A drone orthomosaic is a photogrammetrically corrected aerial image stitched from hundreds of individual drone photos. It is stored as a GeoTIFF — a standard raster format that embeds georeferencing information directly in the file. Unlike raw aerial photos, an orthomosaic has been corrected for camera tilt and terrain displacement, so distances and areas can be measured directly from it.

Orthomosaics contain spectral (color) information that DEMs do not. Overlaying them reveals features the elevation model misses: pavement markings, vegetation species boundaries, utility vault lids, and surface drainage patterns. To overlay a color orthomosaic on a hillshade, reduce the orthomosaic's transparency to roughly 50–70% in the Layer Properties so the terrain shading shows through.

File Handling and Lab Safety

Always work from a copy of source data files. Accidental edits to a USGS DEM or an instructor-provided GeoTIFF cannot easily be undone. Store originals in a read-only folder.

Long geoprocessing runs can overheat a laptop. Ensure the machine has adequate ventilation. Do not block the vents with papers or a bag. Save your project before running any Surface tool — ArcGIS Pro can close unexpectedly on memory-intensive raster jobs.

When downloading 3DEP data from The National Map, verify the tile footprint covers your study area fully before closing the browser session. Partial tiles will produce edge artifacts in contours and hillshades at the tile boundary.

Connecting to Credentials

The Esri ArcGIS Pro certification tests your ability to perform raster analysis workflows — hillshade, slope, contour — using correct tool parameters. The NSPS CST exam covers contour interpretation and the relationship between contour interval, map scale, and terrain. FAA Part 107 pilots need to understand orthomosaic products because clients will ask about the accuracy and usability of the data their drone delivers.

Key terms10

Digital Elevation Model (DEM)A raster grid in which each cell stores a bare-earth elevation value; USGS 3DEP provides 1-meter DEMs for the US referenced to NAVD 88.
HillshadeA derived raster that assigns each cell a brightness value (0–255) based on the angle between the cell's slope-aspect and a simulated light source, used to visualize terrain relief.
SlopeThe rate of elevation change between a raster cell and its neighbors, expressed in degrees or percent grade; used to classify terrain for construction suitability.
AspectThe compass direction that a slope faces, derived from a DEM; relevant for solar analysis, frost prediction, and drainage planning.
Contour IntervalThe fixed difference in elevation between successive contour lines on a map; common values for 1-meter DEMs are 1 foot and 2 feet.
OrthomosaicA georeferenced, photogrammetrically corrected aerial image stitched from drone photos, stored as a GeoTIFF, from which true distances and areas can be measured.
GeoTIFFA standard raster file format that embeds coordinate reference system and georeferencing metadata directly inside the TIFF file, requiring no sidecar file.
NAVD 88North American Vertical Datum of 1988; the official vertical reference datum for USGS 3DEP elevation data in the contiguous United States.
3DEP (3D Elevation Program)The USGS program that collects and distributes high-resolution lidar-derived elevation data, including 1-meter DEMs, for the United States.
Coordinate Reference System (CRS)The mathematical framework — including datum, projection, and units — that ties raster or vector data to real-world locations; must match across all layers in a GIS project.

Worked examples4

1. A site's high point reads 312.6 ft and the low point reads 287.4 ft in the DEM. How many 2-foot contour lines will cross the site, and what are the first and last contour elevations?
  1. Step 1 – Find the total elevation range: 312.6 − 287.4 = 25.2 ft.
  2. Step 2 – Identify the lowest contour inside the range. Round 287.4 up to the nearest 2-foot multiple: 288 ft.
  3. Step 3 – Identify the highest contour inside the range. Round 312.6 down to the nearest 2-foot multiple: 312 ft.
  4. Step 4 – Count the contours from 288 to 312 at a 2-foot interval: (312 − 288) ÷ 2 + 1 = 24 ÷ 2 + 1 = 13 contours.
Answer: 13 contour lines will cross the site; the lowest is at 288 ft and the highest is at 312 ft.
2. A slope measurement between two points shows a rise of 9 feet over a horizontal run of 60 feet. Convert this to a slope angle in degrees and identify which of the five construction slope classes it falls into.
  1. Step 1 – Calculate percent slope: (9 ÷ 60) × 100 = 15%.
  2. Step 2 – Convert to degrees using the arctangent: angle = arctan(9 ÷ 60) = arctan(0.150) ≈ 8.5°.
  3. Step 3 – Compare to the five-class table: Flat 0–2°, Gentle 2–5°, Moderate 5–15°, Steep 15–30°, Very Steep >30°.
  4. Step 4 – 8.5° falls within 5–15°, so the class is Moderate.
Answer: The slope is approximately 8.5°, classified as Moderate (5–15°).
3. When comparing 2-foot contours generated from a 1-meter DEM against a paper reference map, you find that generated contour 300 ft passes through a point that the paper map places at 301.6 ft. Is this within the acceptable tolerance?
  1. Step 1 – State the tolerance rule: acceptable error is one contour interval, which is 2 feet for this analysis.
  2. Step 2 – Calculate the observed error: 301.6 − 300.0 = 1.6 ft.
  3. Step 3 – Compare: 1.6 ft < 2.0 ft (one contour interval).
  4. Step 4 – Conclusion: the error is within the acceptable tolerance.
Answer: Yes. A 1.6-foot discrepancy is within the one-contour-interval (2-foot) tolerance, so the output passes the accuracy check.
4. You need to classify a raster slope layer into five classes with the correct threshold values. List the minimum and maximum degree values for each class to enter into ArcGIS Pro's Reclassify tool.
  1. Step 1 – Write out the five classes and their published thresholds: Flat 0–2°, Gentle 2–5°, Moderate 5–15°, Steep 15–30°, Very Steep 30–90°.
  2. Step 2 – In Reclassify, Old Values are entered as ranges with a new integer code: 0–2 → 1, 2–5 → 2, 5–15 → 3, 15–30 → 4, 30–90 → 5.
  3. Step 3 – Set the upper bound of the Very Steep class to the maximum possible slope value in the dataset (or 90° as the absolute maximum for a degree slope raster).
  4. Step 4 – Confirm that class boundaries do not overlap; each threshold value belongs to only one class (use 'greater than or equal to lower bound and less than upper bound' logic except at the final class).
Answer: Class 1 (Flat): 0–2°; Class 2 (Gentle): 2–5°; Class 3 (Moderate): 5–15°; Class 4 (Steep): 15–30°; Class 5 (Very Steep): 30–90°. Enter these ranges in Reclassify with output values 1 through 5.

The math in this lesson1

Learners find a 25.2 ft elevation range and count 13 two-foot contours from 288 to 312 ft.
N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Lab: students load a 1-meter DEM (USGS 3DEP source) into ArcGIS Pro, generate a hillshade, classify slope into five categories relevant to construction (flat, gentle, moderate, steep, very steep), and symbolize with a meaningful color ramp. · Contour generation: students use the 'Contour' geoprocessing tool in both ArcGIS Pro and QGIS to produce 1-foot and 2-foot contours from the same DEM, then compare outputs for accuracy against a paper contour map of the same area. · Orthomosaic integration: students drag a drone-derived GeoTIFF orthomosaic into the project, adjust transparency to overlay the DEM hillshade, identify three features visible in the ortho that are not captured by elevation data alone, and write a brief interpretation paragraph.
Materials: ArcGIS Pro, QGIS, USGS 3DEP 1-m DEM (downloaded for local area), drone orthomosaic GeoTIFF (instructor-provided), reference paper contour map, lab log template
Check: Contours match reference within one contour interval, slope classification uses correct threshold values, and the interpretation paragraph identifies three distinct features with justification; pass mark 75% on a graded rubric.

Practice check8

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

NSPS CST L1-2 — Vertical Datums and Elevation
1. A student loads a USGS 3DEP 1-meter DEM and runs the Contour tool at a 1-foot interval. Which vertical datum is the elevation data referenced to?
Esri ArcGIS Pro — Raster Analysis
2. In ArcGIS Pro, which toolbox and tool path is correct for generating a hillshade from a DEM?
NSPS CST L1-2 — Terrain Analysis and Site Planning
3. A slope raster shows a value of 22° at a proposed building pad location. Using the five-class construction slope classification, this site is categorized as:
Esri ArcGIS Pro — Layer Visualization and Cartography
4. A student overlays a drone orthomosaic GeoTIFF on a DEM hillshade and reduces the orthomosaic transparency to 60%. What is the primary reason for adjusting transparency in this workflow?
NSPS CST L1-2 — Map Accuracy and Contour Interpretation
5. When comparing 2-foot contours generated from a DEM to a paper reference map, the maximum acceptable positional error in elevation is:
Esri ArcGIS Pro — Raster Data Interpretation
6. Which of the following features would most likely be visible in a drone orthomosaic but NOT captured in a bare-earth DEM?
FAA Part 107 / Esri ArcGIS Pro — Geospatial Data Quality
7. A drone pilot delivers an orthomosaic as a GeoTIFF. A GIS technician opens it and finds it displays in the wrong county. What is the most likely cause?
NSPS CST L1-2 — Contour Interval and Map Reading
8. A site has a high point of 420.0 ft and a low point of 395.0 ft. Using a 5-foot contour interval, how many contour lines will appear within this elevation range?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →