Raster Data: DEMs, Orthomosaics, and Terrain Analysis

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:

• Flat: 0–2°
• Gentle: 2–5°
• Moderate: 5–15°
• Steep: 15–30°
• Very steep: >30°

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.