Overview
This lecture introduces topographic maps, focusing on contour lines, elevation, slope (gradient), and how to interpret and calculate features like stream flow and land depressions.
Types of Maps and Features
- Paper maps are still useful for seeing large land areas with natural features, unlike digital maps focused on roads and businesses.
- Topographic maps show natural land features such as hills, slopes, and elevation changes using contour lines.
Contour Lines and Contour Intervals
- Contour lines (isolines) connect points of equal elevation on a map.
- Contour intervals indicate the elevation difference between contour lines and vary by map.
- Every fifth contour line is usually darker and labeled for easier reading.
- Always check the map legend to determine the contour interval.
- Contour lines never cross or end abruptly; they form closed loops or run off the map.
Benchmarks and Elevation
- A benchmark (BM) marks the exact elevation at a specific, permanent location, often shown as “BM 324”.
- Benchmarks are marked physically in the ground, often in solid rock.
Gradient (Slope)
- Gradient refers to the slope of the land: high gradient means steep, low gradient means gradual.
- The steeper the slope, the closer together the contour lines; more gradual slopes have lines farther apart.
- Gradient is calculated using the formula: gradient = change in field value ÷ distance (field value is elevation for topographic maps).
Map Profiles and Drawing
- A profile is a side view of the landscape, showing elevation change along a line.
- Topographic (map) view shows isolines, while profile view shows elevation changes from the side.
- Drawing profiles involves transferring map contour data to a side-view diagram.
Stream Flow and Contour Rules
- Where contour lines cross a stream, they form a "V" that points upstream (toward higher elevation).
- Stream flow direction is opposite the V point; streams flow downhill.
- On most maps, north is up, east is right, south is down, and west is left.
Depressions (Bowls) on Maps
- Depressions (areas lower than surrounding land) are shown using closed contour lines with short inward-pointing lines (hachures).
- These indicate the land goes down instead of up, often representing bowls or craters.
Calculating Gradient: Examples
- Use the formula: gradient = (elevation difference) ÷ (distance), with units often in ft/mile.
- Practice: mark two points, measure elevation and distance, calculate gradient to quantify steepness.
Key Terms & Definitions
- Topographic Map — a map showing elevation and landform shape using contour lines.
- Contour Line (Isoline) — a line connecting equal elevation points.
- Contour Interval — elevation difference between adjacent contour lines.
- Benchmark (BM) — a known reference point of exact elevation marked on the ground and map.
- Gradient — numerical measure of slope: change in elevation per unit distance.
- Profile — a side view or cross-section of a landscape showing elevation change.
- Depression — a landform that is lower than the surrounding area, shown with hachured contour lines.
Action Items / Next Steps
- Complete mapping practice exercises, including drawing profiles and calculating gradients.
- Fill in guided notes packet (#30) as directed.
- Review contour line rules and practice identifying stream flow and depressions on contour maps.