Geographical Skills: Ordnance Survey Maps, Grid References, and Scale
Welcome to your study guide on Ordnance Survey (OS) maps! Whether you are planning a countryside hike, exploring a new town, or doing fieldwork in Geography, map skills are essential tools. Don't worry if map reading seems tricky at first—once you learn a few simple rules and tricks, you will be able to pinpoint any location and measure distances with ease!
---Part 1: The National Grid – Eastings and Northings
Great Britain is covered by a giant mapping grid created by the Ordnance Survey. The country is divided into huge grid squares measuring \(100\text{ km} \times 100\text{ km}\). Each of these huge squares is given a unique two-letter prefix (such as SU or TQ). Inside these squares are smaller grid lines that form a network of \(1\text{ km} \times 1\text{ km}\) squares.
To read any OS map, you need to know the two types of numbered grid lines:
1. Eastings (Vertical Lines):
• These are the lines that run vertically (from top to bottom).
• They are numbered from west to east.
• Their numbers increase as you move East (to the right across the map).
2. Northings (Horizontal Lines):
• These are the lines that run horizontally (from side to side).
• They are numbered from south to north.
• Their numbers increase as you move North (upwards on the map).
Memory Trick: Always go "along the corridor (Eastings), then up the stairs (Northings)"! You must always read the horizontal distance across first, before going up.
---Part 2: Finding Your Way with 4-Figure Grid References
A 4-figure grid reference is used to identify an entire \(1\text{ km} \times 1\text{ km}\) square on a map. Think of it like giving a general postcode to an area.
Step-by-Step: How to Find a 4-Figure Grid Reference
Step 1: Find the bottom-left corner of the grid square you want to identify.
Step 2: Follow the vertical line on the left side of that square down to read the two-digit Easting number.
Step 3: Follow the horizontal line at the bottom of that square across to read the two-digit Northing number.
Step 4: Write the numbers together: Easting first, then Northing (e.g., \(45\) and \(67\) becomes 4567).
Example: If a campsite sits inside a square where the bottom-left Easting line is \(32\) and the bottom-left Northing line is \(74\), the 4-figure grid reference is 3274.
Key Takeaway: A 4-figure grid reference gives you the whole \(1\text{ km}\) box. Always look at the bottom-left corner of the square!
---Part 3: Pinpoint Accuracy with 6-Figure Grid References
What if you need to find an exact point, such as a phone box, a church, or a specific bend in a river? A whole \(1\text{ km}\) square is too large. That is where a 6-figure grid reference comes in! It pinpoints a location to the nearest \(100\text{ metres}\).
Understanding the Digits
A 6-figure grid reference is made of two halves: 3 digits for the Easting and 3 digits for the Northing.
• The first two digits are the grid line number.
• The third digit represents how many tenths (\(\frac{1}{10}\)) across the square the feature is located.
• The fourth and fifth digits are the grid line number.
• The sixth digit represents how many tenths (\(\frac{1}{10}\)) up the square the feature is located.
Example Breakdown: Look at the reference 123 456
• 123 (Eastings): Go to line \(12\), then estimate \(3\) tenths (\(\frac{3}{10}\)) of the way across to the right.
• 456 (Northings): Go to line \(45\), then estimate \(6\) tenths (\(\frac{6}{10}\)) of the way up.
Step-by-Step Guide:
Step 1: Find the 4-figure square first (e.g., Easting \(12\), Northing \(45\)).
Step 2: Imagine dividing the square into \(10\) equal slices from left to right. Estimate how many tenths across your symbol is (e.g., \(3\) tenths \(\rightarrow\) 123).
Step 3: Imagine dividing the square into \(10\) equal slices from bottom to top. Estimate how many tenths up your symbol is (e.g., \(6\) tenths \(\rightarrow\) 456).
Step 4: Put them together: 123 456 (or with the national prefix: SU 123 456).
Part 4: Map Scale – Understanding Real-World Size
Scale is the ratio between the distance on a map and the actual distance on the ground. Because we cannot carry around a full-sized map of the UK, everything must be shrunk down accurately!
The Three Ways Scale is Shown:
1. Representative Fraction (Ratio): Written as numbers with a colon, such as \(1:25,000\) or \(1:50,000\).
2. Statement Scale: Written in plain words, such as "Two centimetres to one kilometre".
3. Linear Scale: A visual bar line printed on the map showing distances marked out from \(0\text{ km}\) to \(5\text{ km}\) or more.
Two Common Ordnance Survey Map Scales:
1. OS Explorer Series (\(1:25,000\)):
• \(1\text{ unit on the map} = 25,000\text{ units in real life}\).
• Standard measurement: \(4\text{ cm}\) on the map = \(1\text{ km}\) on the ground.
• Ideal for walking, cycling, and fieldwork because it shows lots of local details like field boundaries and footpaths.
2. OS Landranger Series (\(1:50,000\)):
• \(1\text{ unit on the map} = 50,000\text{ units in real life}\).
• Standard measurement: \(2\text{ cm}\) on the map = \(1\text{ km}\) on the ground.
• Ideal for planning longer journeys or driving across wider regions.
Large Scale vs Small Scale (A Common Mix-Up!):
• Large Scale Maps (e.g., \(1:25,000\)): Show a smaller area of land, but with more detail (objects appear larger).
• Small Scale Maps (e.g., \(1:250,000\)): Show a larger area of land, but with less detail (objects appear smaller).
Part 5: Measuring Distance on a Map
1. Straight-Line Distance ("As the Crow Flies")
Step 1: Place the straight edge of a ruler between the two points on your map and measure the distance in centimetres (\(\text{cm}\)).
Step 2: Convert your measurement using the map scale:
• On a \(1:50,000\) map: divide the number of centimetres by \(2\) to get kilometres (since \(2\text{ cm} = 1\text{ km}\)).
• On a \(1:25,000\) map: divide the number of centimetres by \(4\) to get kilometres (since \(4\text{ cm} = 1\text{ km}\)).
Step 3: Alternatively, place your ruler against the printed linear scale bar to read the distance directly.
2. Curved-Line Distance (Following a Road, Path, or River)
Natural features like rivers and winding country lanes are rarely straight! Here are two easy methods to measure them:
Method A – The String Method:
Lay a piece of string carefully along the winding route on the map, shaping it to match every curve. Make a small mark on the string where the route ends. Straighten the string out against the map's linear scale bar to read the actual distance.
Method B – The Paper Strip Method:
Take a straight strip of scrap paper. Place the edge at your starting point. Pivot the paper along the curved line, making small pencil marks at each bend. Once you reach the end, place the marked strip against the linear scale bar to find the total distance.
Part 6: Common Pitfalls & How to Avoid Them
Mistake 1: Reading Northings first instead of Eastings.
Fix: Always remember: "Along the corridor, then up the stairs." Always write down the Easting (horizontal movement) before the Northing (vertical movement).
Mistake 2: Forgetting that the 3rd and 6th digits are tenths.
Fix: In a 6-figure grid reference, the 3rd and 6th digits are estimates of tenths (\(0\) to \(9\)) within the square, not grid line numbers!
Mistake 3: Leaving off the two-letter prefix.
Fix: A complete National Grid reference starts with two letters (e.g., SU 456 789). Without those letters, your grid reference could refer to several different places across Great Britain.
Mistake 4: Confusing Large Scale and Small Scale.
Fix: Remember: A Large Scale map zooms in close to show lots of detail in a smaller area.
Quick Revision Checklist
• Eastings: Vertical grid lines numbered West to East (increase towards the right).
• Northings: Horizontal grid lines numbered South to North (increase towards the top).
• 4-Figure Reference: Identifies a whole \(1\text{ km}\) square using the bottom-left corner.
• 6-Figure Reference: Pinpoints a precise location to within \(100\text{ m}\) by estimating tenths.
• \(1:25,000\) Scale: \(4\text{ cm} = 1\text{ km}\) on the ground (Explorer series).
• \(1:50,000\) Scale: \(2\text{ cm} = 1\text{ km}\) on the ground (Landranger series).
• Curved Routes: Measured using string or a paper strip against the linear scale bar.