Slope Analysis
Definition
Slope analysis calculates the rate of elevation change across terrain, expressed in degrees or percent rise, using digital elevation models (DEMs). It reveals how steepness influences runoff, erosion, construction feasibility, and visibility. Analysts compute slope with neighborhood windows and may combine it with aspect, curvature, and roughness to classify landforms. Resolution and vertical accuracy determine reliability—fine DEMs capture micro‑slopes for urban design, whereas coarser DEMs support watershed planning. Slope layers become constraints, drivers, or predictors across many GIS models.
Application
Engineers screen corridors for roads, pipelines, and trails by excluding slopes above design thresholds. Planners map landslide susceptibility with slope‑angle bands and lithology. Hydrologists couple slope with soil to estimate erosion. Solar modelers use slope and aspect to correct irradiance on roofs and hillsides. Wildfire models and military terrain analysis use slope to estimate spread and mobility.
FAQ
How does DEM resolution change the slope you measure?
Coarser cells smooth steep features and underestimate extremes; finer cells capture sharp cuts but may amplify noise. Choose a resolution aligned to the decision scale and, where needed, smooth or median‑filter to reduce artifacts.
What is the difference between percent slope and degrees, and when does it matter?
Percent slope is rise over run times 100; degrees is the arctangent of that ratio. Design manuals often specify percent for roads while geotechnical limits may use degrees; consistent units prevent misinterpretation.
How do you incorporate slope into cost distance or least‑cost path models?
Translate slope into a friction surface using empirical effort or speed curves for the mode (hiking, cycling, vehicles). Combine with land cover and barriers, then run cost distance to produce realistic travel time surfaces.
Which quality checks catch gross slope errors before they mislead?
Inspect hillshades from multiple azimuths, compare with contours, and flag impossible patterns such as steep slopes across lakes or building roofs when using bare‑earth DEMs.