Map reading skills: Making sense of contour lines
We’ve made it to week three in our six-week blog series on map reading skills. Map reading is an essential skill for any explorer or outdoor enthusiast. We’ve teamed up with Steve Backshall to record a series of videos to remind you of the basics and help you feel confident with your map.
So far we’ve covered which OS map you need and understanding map symbols. Today we’ll be telling you how to make sense of contour lines and over the few weeks we’ll also cover:
- How to read a grid reference, both four-figure and six-figure versions
- Knowing your compass and how to take a compass bearing
- Understanding magnetic north
We’ll share Steve’s video with you, give a summary in the blog and point you in the right direction of further resources and details. Hear what Steve has to say about contour lines:
As Steve said, understanding contours is a key navigation skill as you’ll be able to interpret the landscape of an area just be looking at a map. Contours tell you whether the ground is flat, hilly, undulating, or steep. You can tell whether a route will be a gentle stroll or a hard uphill slog, so you can plan your route more easily.
Contours are shown on Ordnance Survey maps as thin orange or brown lines with numbers on them that show you the height above sea level of any point on the line. The contour lines join points of equal height together. Contour lines very close together indicate a steep slope and contours further apart show a gentle slope.
Contour height
The vertical interval is the height (elevation) between each contour and they appear 5 or 10 metres apart with thicker lines every 50 metres. The numbers on contour lines are always displayed in ascending height, so if the numbers increase it denotes an uphill slope, and if they decrease it’s a downhill slope.
Contour patterns
The relationship between higher and lower contours and the distance between them can give you valuable clues about what the real surface of the ground is like:
Smaller circles show a summit or basin, but the inside of a contour circle is normally higher ground.
Flat areas like river valleys and the sea have very few or no contours.
Contours are only ever on top of one another if it’s a vertical cave or cliff.
A ‘V’ or ‘U’ shape pointing downhill denotes the spur of a hill.
Contours descending in number on either side of a line show a ridge.
Contours bunched together on either side of lower, more evenly spaced contours show a valley or col between two areas of high ground.
Small downward lines inside a contour circle are known as ‘hachure marks’ and signify ground that is sunken beneath sea level.
Route elevation
Using contour lines you can calculate the elevation of your route, or height of travel. This is useful because you can add the uphill distance to the length of your route to get a more accurate estimate of the total distance you’ll go.
Contour lines give you a mental picture of the shape of the ground. Of course, the earth’s surface is rarely, if ever, uniform, so the patterns of contours are often mixed together, which may at first appear confusing.
The best way to get to know about contours is to take a map with you when you’re walking, running or cycling and see if you can spot different landforms and what their contour patterns are. You can match the contour lines on Ordnance Survey maps to what you can see on the ground and practice recognising some of the classic contour shapes.
Happy exploring! I’ll be back next week to talk about how to read a grid reference.
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Hi Gemma
A useful blog, but just need to point out that contours are not always 10m apart. On the 1:25000 scale map, this is just in upland areas. In lowland areas they are 5m apart. Here in the White Peak, we are right on the edge of both.
Hi Jane
Yes, good point. We were focusing on hills in the diagram and in very hilly areas the 5 m intervals would be too close and unreadable and would also obscure other map detail, so when this is the case the 10 m intervals are used instead. I’ve just updated to include the 5 m reference though.
Thanks, Gemma
Hi Gemma,
Nice article, but to follow up from the last comment the major contours are every 5th contour so on lowland maps they are spaced by 25m and not 50m . Also it is useful to note that the top of the contour height numbers is on the uphill side so given a single major contour line you can tell the direction of slope
On the ordnance survey maps, are the red contour lines measured from sea level as the zero point? If so how does it work with the Thames river in the tidal parts of the river – is the 5 meter line measured from the high tide, low tide, or midrange tide as zero ? Thank you.
Hi Anne
For consistency the contour lines are all measured from a single mean sea level, which on OS maps is taken from Newlyn in Cornwall. This was decided to be used as the reference point for the height data on Ordnance Survey maps, called the datum. We have a blog on Newlyn too with some more information: http://www.ordnancesurvey.co.uk/blog/2011/08/how-do-you-measure-sea-level/
Thanks, Gemma
This is very helpful for my Multtivariable Calculus course. Thank you.
Contour lines are set above what datum? please.
i.e.
Chart datum
Low Water
High Water
Mean Sea Level
etc
Hi Tim
We use Newlyn as our reference datum for mean sea level. There’s more information about it in this blog: https://www.ordnancesurvey.co.uk/blog/2011/08/how-do-you-measure-sea-level/
Thanks, Gemma
thank u for this.
Hi Gemma
Before OS maps went metric what was the lowest imperial measurement for contours on most maps? I am looking at a map of 1954 and the lowest contour shown is 50ft (15m). When I compare that map to a present day OS the lowest contour is 10m (32ft). If there had been a gradient on the land at 32ft back in 1954 would it have been shown on that map?
I hope you can help?
Many thanks
Hi Nick
Thanks for getting in touch. In terms of gradient, that would only be displayed for roads. For contours, we’ve had a check in JB Harley’s ‘Ordnance Survey Maps’ and historically, vertical contour intervals have varied, starting at 25ft and upwards to 100ft. This also varied by area of the country historically, once contouring was adopted around 1839.
I hope that helps.
Thanks, Gemma
hi Gemma I would like to know when I have been told to draw a cross section map and how to label it.
Hi Gemma , I just wanted to ask why are some contour lines thicker Than the others ? Please reply I need it for homework
Hi Caroline
No problem at all. The contour spacing on the 1:25,000 OS Explorer maps and 1:50,000 OS Landranger maps are usually at 10 metre intervals. Contours are given thicker lines at regular intervals, usually every 50 metres, to help users interpret the change in height. So rather than count every contour you know that when you go from one think line to another, you are seeing an increase of 50 metres. It also helps with the visual impact of the map. Good luck with the homework.
Thanks, Gemma
I’m 11 and I have just moved to secondary school, we have a big geography test on map contours and grid references! This really helped my understanding. Thankyou!
-Daisy
You’re welcome Daisy!
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Hi Gemma,
Thanks so much for this explanation. I am now starting to learn a little bit more better now:)
Hi,Gemma
thanks I am in year 4 and my test is on this and the information was good
Hi Gemma,
The Environment Agency’s interactive flood map shows potential flood zones with the help of OS maps. Those maps include contour lines but they don’t mention what measurement is used for height elevation – are they now always in metres? I’m not referring to how far apart they are but the numbers shown in the lines, ie 50/55/60 etc. May sound a daft question but I want to make doubly sure it’s not feet or yards (it used to be when I learnt map-reading)!
Thanks,
David
Hi David
Thanks for getting in touch. We use metres to mark the heights on our contours.
Many thanks
Gemma
Hi,
On maps what do the numbers in black mean? For example SK 00139 63887 which is near the Roaches, in the Peak District shows 505. This isn’t a contour so what is it for?
Thanks for your help
Hi Joe
It looks like you’re asking about the numbers showing ground height around the trig pillar.
Many thanks
Gemma
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Hi Gemma,
Are your blogs available as downloadable documents? I am a scout leader and the detail on your blogs would be a brilliant teaching tool if we could download them?
Thank you
Nick
Hi Nick
Thank you. The blogs themselves aren’t downloadable, but we have other map reading guides and summaries which are available for download. Take a look here: https://www.ordnancesurvey.co.uk/resources/map-reading/index.html
Many thanks
Gemma
Hi Gemma,
Hoping you could help as I can’t find an answer online. How are contour lines represented for areas below sea level? I’m an Army Cadet instructor, and one of the Cadets stumped me with this one last night. Really hoping to find an answer for her.
Many thanks in advance.
Hi Dan
No problem, small downward lines inside a contour circle are known as ‘hachure marks’ and signify ground that is sunken beneath sea level.
Many thanks
Gemma