Ordnance Survey Maps (Leaving Cert Geography): Revision Notes

OS Map Key Skills

Understanding Ordnance Survey (OS) maps is a fundamental skill in geography that requires mastering various techniques for reading and interpreting map information. These skills build upon your Junior Cycle Geography knowledge and include both basic and advanced techniques essential for your Leaving Certificate exam and beyond.

The core skills you need to master include measuring scale, reading grid references, determining direction, measuring distances and heights, calculating areas, and creating sketch maps. Let's explore each skill systematically.

Skill 1: Measuring scale

Scale represents the relationship between distances on a map and actual distances on the ground. Understanding scale is crucial for accurate map interpretation and measurement.

Types of scale

Maps use different scales depending on their purpose:

• Small-scale maps show large geographical areas with limited detail (e.g., world maps, country maps)
• Large-scale maps show smaller areas with greater detail (e.g., street maps, town plans)

Reading scale

When you see a scale of 1:50,000, this means:

• 1 cm on the map represents 50,000 cm (500 metres) on the ground
• 2 cm on the map equals 1 kilometre on the ground

This relationship helps you calculate real distances by measuring map distances and applying the scale ratio.

Skill 2: Reading and recording grid references

Grid references provide a systematic method for locating specific places and features on OS maps. The system uses three components working together.

The grid system

Ireland uses the National Grid, which divides the country into:

• 25 subzones measuring 100 km × 100 km, each identified by a letter (all letters except 'I')
• Each subzone contains 100 equal parts numbered 00-99
• These create the blue grid lines visible on OS maps

Grid components

1. Eastings: Vertical lines running north-south, numbered along the bottom and top edges
2. Northings: Horizontal lines running east-west, numbered along the left and right edges
3. Subzone letter: Identifies which 100 km square you're working in

The LEN method

Four-figure grid references

Four-figure references locate a complete 1 km square and are used for:

• Large features like towns or villages
• General area locations
• Islands or extensive landmarks

Six-figure grid references

Six-figure references provide precise locations within a grid square by:

• Dividing each 1 km square into 100 smaller sections
• Adding a third digit to both easting and northing coordinates
• Locating specific features like castles, churches, or individual buildings

To calculate six-figure references, imagine each grid square divided into 10 equal parts along both axes, creating a mental grid of 100 smaller squares.

Skill 3: Reading map direction

OS maps include a north arrow enabling you to determine direction accurately. The compass system divides directions into:

• 4 cardinal points: North (N), South (S), East (E), West (W)
• 8 intermediate points: Northeast (NE), Southeast (SE), Southwest (SW), Northwest (NW)
• 16 total directions when cardinal and intermediate points are subdivided further

This system allows precise description of directions between locations on the map.

Skill 4: Measuring distance

Distance measurement on maps involves two main techniques depending on whether you're measuring straight lines or curved features.

Straight-line distance

For measuring direct distances between two points:

Curved-line distance

Measuring rivers, roads, or coastlines requires a different approach:

This technique breaks curved lines into manageable straight sections for accurate measurement.

Skill 5: Measuring height

OS maps show height information through four different methods, each serving specific purposes.

Contour lines

• Standard contours appear as thin brown lines every 10 metres
• Index contours are thicker, darker lines at 50-metre intervals with height values marked
• Closely spaced contours indicate steep slopes
• Widely spaced contours show gentle gradients

Colour layers

Height is also represented through colour coding:

• Light blue: Seas and large water bodies
• Dark green: Land from 0-100 metres above sea level
• Light green: Land from 100-200 metres
• Cream: Land from 200-300 metres
• Brown: Land above 300 metres (darker brown indicates greater height)

Spot heights

Round black dots with numbers show exact elevations at specific points, providing precise height measurements for particular locations.

Triangulation stations

These appear as black triangles and usually mark the highest points in an area, providing accurate elevation data for mapping purposes.

Skill 6: Identifying slope

Contour line patterns reveal different types of slopes across the landscape:

Slope types

• Even slopes: Regularly spaced contours creating uniform gradients
• Concave slopes: Gentle at the bottom, steeper towards the top (contours widely spaced below, closer together above)
• Convex slopes: Steep at the bottom, gentler at the top (contours close together below, widely spaced above)
• Compound slopes: Combinations of steep and gentle sections with varying contour spacing

Skill 7: Drawing cross-sections (Higher Level only)

Cross-sections transform the bird's-eye view of OS maps into profile views showing landscape relief.

Construction steps

1. Prepare the transect: Place a straight-edged paper along the line connecting your start and end points
2. Mark elevations: Note the height of every contour line the paper crosses, marking these on the paper edge
3. Record features: Mark where the line crosses roads, rivers, lakes, or other significant features
4. Create axes: Draw horizontal (x-axis) and vertical (y-axis) lines on graph paper
5. Scale the y-axis: Mark height intervals (e.g., every 100 metres) up the vertical axis
6. Plot points: Transfer your marked heights from the paper strip to the graph, plotting each elevation point
7. Connect smoothly: Join the plotted points with a smooth curve (avoid straight lines between points)
8. Add labels: Mark important features like rivers, roads, or settlements
9. Calculate vertical exaggeration: Compare horizontal and vertical scales

Vertical exaggeration

Skill 8: Calculating average gradient

Average gradient measures the steepness of slopes between two points, expressed as a ratio.

The formula

$\text{Average Gradient} = \frac{\text{Vertical Interval}}{\text{Horizontal Equivalent}}$

Where:

• Vertical Interval: The height difference between two points
• Horizontal Equivalent: The actual ground distance between the same points

Practical application

Skill 9: Calculating area

Area calculations help determine the size of regions, settlements, or geographical features shown on maps.

Regular shapes

Irregular shapes

For complex boundaries like coastlines or administrative areas:

This method provides reasonably accurate area estimates for irregular geographical features.

Skill 10: Sketch mapping

Creating accurate sketch maps requires careful attention to proportion, orientation, and detail selection.

Essential steps

1. Check orientation: Rotate the map extract so the north arrow points upward
2. Draw the frame: Create a border at half-scale (if original is 4 cm wide, draw 2 cm wide)
3. Add title and north arrow: Include the map title and directional indicator
4. Insert major features first: Begin with prominent elements like coastlines, rivers, or main roads
5. Add detail progressively: Include settlements, smaller roads, and specific landmarks
6. Create a legend: Explain all symbols used in your sketch
7. Maintain proportions: Ensure features occupy appropriate relative space

Key principles

Skill 11: Identifying human processes

Maps reveal how people interact with and modify their environment through various activities and settlement patterns.

Communications

Transportation networks visible on OS maps include:

• Roads: Built away from rivers to avoid flooding, often crossing at narrow points called bridging points
• Railways: Follow gentle gradients and avoid steep terrain
• Waterways: Canals and navigable rivers for transport
• Airports: Large, flat areas with runways clearly marked
• Ports and harbours: Coastal facilities for maritime transport

Settlement patterns

Maps show different types of human habitation:

• Rural settlements: Scattered farms, small villages, isolated buildings
• Urban areas: Towns and cities with dense building patterns, clearly defined boundaries
• Linear settlements: Villages stretched along roads or rivers
• Nucleated settlements: Clustered around central points like crossroads or harbours

Economic activities

Various land uses reflect economic processes:

• Agriculture: Field patterns, farm buildings, different land use areas
• Industry: Factories, quarries, industrial estates
• Recreation: Golf courses, caravan parks, tourist facilities
• Services: Schools, hospitals, religious buildings marked with specific symbols

These human processes create the cultural landscape visible on OS maps, showing how people have adapted to and modified the physical environment over time.