Understanding Fields
Contents
In Earth science, a field is defined as a region of space where every point has a measurable value of a certain quantity. These quantities can include temperature, air pressure, elevation, pollution levels, and many other scientific variables. Field maps are used to visualize how that quantity changes across a given area.
For example, imagine a map showing the concentration of gasoline in soil. Each point on that map displays a specific concentration reading, and together, the points form a complete picture of how gasoline is distributed across the land.
What Are Isolines?
Isolines are lines drawn on field maps to connect points that have equal values. These lines help to simplify and visualize complex data patterns. If a map shows values for a certain property like temperature, elevation, or pollution levels, drawing isolines will make it easier to interpret where values increase, decrease, or remain constant.
In a sample map showing gasoline concentrations, a 40-unit isoline would connect all locations where the gasoline concentration is exactly 40. These isolines help to highlight the distribution and intensity of values throughout the field.
When all isolines are drawn at regular intervals—say, every 10 units—they provide a comprehensive view of changes over space. Areas where isolines are closer together show steep gradients, while areas where they are farther apart show gentle gradients.
Topographic Maps and Contour Lines
Topographic maps are a specialized form of field map that depict elevation using contour lines. These lines are isolines that connect points of equal elevation, helping us understand the shape and features of the Earth’s surface, including hills, valleys, and slopes.
On a topographic map, contour lines are typically drawn at a regular vertical interval (e.g., every 20 meters). This interval is consistent throughout the map and allows us to visualize the “relief” or variation in land elevation.
Rules for Interpreting Topographic Maps
1. Close Lines = Steep Slope
When contour lines are packed closely together, it indicates a steep gradient. This is because the elevation is changing rapidly over a short horizontal distance. On the other hand, widely spaced lines indicate gentle slopes or flat terrain.
2. Contour Lines Bend Upstream
When contour lines cross a stream or river, they form a V-shape that points upstream (against the flow of water). This rule helps identify the direction in which water is flowing, which can be critical for understanding drainage patterns.
3. Determining the Highest Point on a Hill
To find the highest possible elevation of a hill, identify the highest labeled contour line and add just under one full interval to that number. If the highest line is 220 meters and the interval is 20 meters, the highest point must be just below 240 meters—so the maximum is 239 meters.
4. Identifying Depressions
Depressions are areas where the land dips below the surrounding terrain. These are marked on topographic maps with hachure marks—short lines pointing inward on a contour. The first hachured line has the same elevation as the previous contour, and the elevation decreases with each inner hachured line.
Topographic Profiles
A topographic profile is a side-view, or cross-sectional, representation of a landscape along a specific line. It helps visualize what the terrain would look like from the side, instead of from above.
How to Identify a Profile
If given a topographic map and asked to choose from a set of profiles (side views), begin by checking the elevations at each point (e.g., Point X and Point Y). Compare these elevations to the choices and use the process of elimination to determine which profile is accurate.
How to Draw a Profile
To create a profile from a topographic map:
- Start with a grid: Draw a baseline that represents the distance between two points (e.g., Point P to Point Q).
- Transfer elevation data: Mark the elevations where contour lines cross the line between P and Q.
- Connect the dots: Draw a smooth curve connecting all the elevation points.
- Analyze: The resulting graph will show hills, valleys, and slopes, providing a visual interpretation of the land’s elevation.
Timeline of Key Topographic Map Concepts
- 1800s – Early geologists begin using contour lines to represent elevation on maps.
- 1890s – U.S. Geological Survey (USGS) produces detailed topographic maps for national use.
- 1900s–1950s – Topographic maps become standard tools for geology, planning, and environmental studies.
- 1980s – Digital topographic maps begin to emerge through satellite imagery and remote sensing.
- 2000s–Present – Geographic Information Systems (GIS) and GPS revolutionize the way topographic data is collected, analyzed, and displayed.
Frequently Asked Questions
What is a topographic map used for?
Topographic maps are used to show the elevation and shape of the land surface using contour lines. They’re important for planning hikes, construction, geological surveys, and flood zone assessments.
How can you tell if a slope is steep on a topographic map?
A slope is steep when contour lines are very close together, indicating that elevation changes rapidly over a short distance. Widely spaced lines indicate a gentle slope.
What do hachure marks on a contour line mean?
Hachure marks indicate a depression, or an area of lower elevation such as a crater or sinkhole. They point inward and help distinguish dips in the land from hills.
How do contour lines help determine river flow direction?
Contour lines bend upstream (form a “V” shape) when crossing a river, so the tip of the “V” points uphill. The river flows in the opposite direction of the contour bend.
What is a topographic profile?
A topographic profile is a side view of the land’s elevation along a specific line on a map. It shows how hills, valleys, and slopes appear from a cross-sectional perspective.