Map and Fieldwork Skills » Interpreting topographic maps

What you'll learn this session

Study time: 30 minutes

How to read and interpret topographic maps
Understanding contour lines and relief features
Calculating distances, areas and gradients
Identifying natural and human features on maps
Using grid references and scale to locate places
Practical applications of map skills in fieldwork

Introduction to Topographic Maps

Topographic maps are detailed, accurate representations of the Earth's surface showing both natural and human-made features. They're like a bird's-eye view of the landscape, with special symbols and lines that help us understand what the land looks like in three dimensions.

Key Definitions:

Topographic map: A detailed map showing both natural features (hills, valleys, rivers) and human features (roads, buildings, boundaries) of an area.
Contour lines: Lines that join points of equal height above sea level, showing the shape of the land.
Relief: The shape and height of the land surface.
Scale: The ratio between distance on the map and actual distance on the ground.
Grid reference: A set of numbers used to pinpoint an exact location on a map.

🗺 Map Essentials

Every topographic map includes these essential elements:

Title: Tells you what area the map covers
Scale: Shows the relationship between map distance and real-world distance
Legend/Key: Explains what the symbols on the map mean
North arrow: Shows which way is north
Grid lines: Help you find specific locations
Date: When the map was created or updated

📏 Map Scale Types

Map scales can be shown in three different ways:

Ratio scale: 1:25,000 means 1cm on the map equals 25,000cm (250m) in real life
Linear scale: A line marked with distances
Statement scale: Written as "1cm to 250m" or similar

Common scales for topographic maps include 1:25,000, 1:50,000 and 1:100,000.

Understanding Contour Lines

Contour lines are the key to visualising the three-dimensional landscape on a flat map. By learning to read these lines, you can identify mountains, valleys, slopes and other landforms.

📏 Contour Line Basics

Contour lines join points of equal height above sea level
The contour interval is the vertical distance between contour lines (often 5m, 10m, or 20m)
Index contours are thicker lines, usually every fifth contour, labelled with their height
Contour lines never cross each other
Closely spaced contours indicate steep slopes
Widely spaced contours indicate gentle slopes

🏔 Landforms from Contours

Hills/Mountains: Concentric circles with the highest elevation in the centre
Valleys: V-shaped contour lines pointing upstream
Ridges: V-shaped contour lines pointing downhill
Spurs: Projections of land jutting out from higher ground
Saddles/Passes: Dip between two areas of higher ground
Cliffs: Very closely spaced or merging contour lines

Grid References and Location

Grid references are a vital skill for precisely locating features on a map. The UK Ordnance Survey and many other mapping systems use a grid system to divide maps into squares.

📌 Four-Figure Grid References

A four-figure grid reference identifies a 1km square on the map:

Read the eastings (horizontal lines) first - "Along the corridor"
Then read the northings (vertical lines) - "Up the stairs"
The reference gives the bottom-left corner of the square
Example: 3246 means 32 east, 46 north

Remember the phrase: "Along the corridor, then up the stairs"

🔍 Six-Figure Grid References

A six-figure grid reference pinpoints a location to within 100m:

Find the four-figure grid reference for the square
Estimate tenths along from the easting grid line (3.4)
Estimate tenths up from the northing grid line (4.7)
Example: 324467 means 32.4 east, 46.7 north

This gives you a much more precise location within the grid square.

Measuring Distance, Direction and Gradient

Topographic maps allow us to calculate real-world measurements from the map itself.

📏 Distance

To measure distance:

Use a ruler to measure the distance on the map in cm
Multiply by the scale factor
Example: 4cm on a 1:25,000 map = 4 × 25,000 = 100,000cm = 1km

For curved lines, use a piece of string or a map measurer.

📍 Direction

Direction can be expressed as:

Compass bearings (0-360°)
Cardinal directions (N, S, E, W)
Intermediate directions (NE, SW, etc.)

To find a bearing, use a protractor aligned with grid north.

📈 Gradient

Gradient shows how steep a slope is:

Find the height difference (vertical distance)
Measure the horizontal distance
Divide height difference by horizontal distance
Multiply by 100 for percentage

Example: 20m height over 100m distance = 20 ÷ 100 = 0.2 = 20%

Interpreting Map Symbols and Features

Topographic maps use standardised symbols to represent different features. Learning these symbols is essential for map reading.

🏠 Human Features

Settlements: Towns, villages, individual buildings
Transport: Roads (different colours for different types), railways, paths, tracks
Boundaries: National, county, district, parish
Land use: Farmland, parks, recreational areas
Industry: Factories, mines, quarries
Infrastructure: Power lines, pipelines, telecommunications

🌲 Natural Features

Water: Rivers, streams, lakes, marshes, coastlines
Vegetation: Woodland, forests, orchards, scrubland
Relief: Mountains, hills, valleys, cliffs
Geology: Rock types, outcrops
Coastal features: Beaches, cliffs, mudflats

Case Study Focus: Ordnance Survey Maps

The UK's Ordnance Survey (OS) maps are considered some of the best topographic maps in the world. They come in two main scales for walkers and geographers:

OS Explorer Maps (1:25,000): Show great detail with each grid square representing 1km². These are ideal for walking and detailed fieldwork.
OS Landranger Maps (1:50,000): Cover a larger area with less detail, good for planning and overview studies.

OS maps use a specific colour scheme: blue for water features, brown for contour lines, green for vegetation, black for most human-made features and red for important roads and urban areas.

Practical Applications in Fieldwork

Map interpretation skills are essential for geographical fieldwork. Here's how to apply these skills in practical situations:

🔭 Planning Fieldwork

Use maps to identify suitable study sites
Plan routes and access points
Estimate journey times using distances and terrain
Identify potential hazards like steep slopes or river crossings
Select sampling locations based on different landforms or land uses
Create base maps for field sketches and data collection

📄 Recording and Analysis

Accurately record locations using grid references
Compare field observations with map features
Create cross-section profiles of landscapes
Calculate areas affected by geographical processes
Identify relationships between relief and other geographical features
Annotate maps with field observations and measurements

Top Tips for Map Interpretation

Start with orientation: Identify where north is and rotate the map to match the direction you're facing.
Look for landmarks: Identify obvious features like hills, rivers, or major roads to locate yourself.
Visualise in 3D: Try to picture the landscape that the contour lines represent.
Use your thumb: Keep track of your current location by placing your thumb on it.
Practise estimation: Get good at estimating distances on the map without always measuring.
Check the date: Remember that older maps may not show recent developments.
Look for patterns: Settlement patterns, drainage patterns and land use patterns can tell you a lot about an area.