Using a sketch map with your OS sheet

Guidebooks love a sketch map. It is clear, it fits a page, and it draws your eye along a single red line. An Ordnance Survey sheet looks fussy beside it: too many boundaries, too much brown, a legend you have to remember. The fuss is the point. A sketch tells the story of one walk. The OS sheet tells you about the ground the story left out, which is exactly the ground you step onto when the story goes wrong.

What a sketch is allowed to simplify

A good sketch keeps the turns that matter and drops the hedges that do not help. It may stretch a scale so that a fiddly farmyard is large enough to read, then shrink a dull mile of lane. North is sometimes sideways because the page is portrait. None of this is cheating. It is editing. Your job is to know that it is edited.

Look for a scale bar, a north arrow and a note of which OS map the artist used. If those are missing, treat the drawing as a diagram, like a set of directions you might give a friend in a pub. Useful, and not a thing to navigate a cloud by.

Laying one on top of the other

Before the day, trace the red line onto your sense of the OS sheet. Churches, named bridges, the edge of a wood and the shape of a hill should appear in both. Where the sketch shows a smooth curve and the OS shows three fields and a kink at a barn, believe the kink. That barn is where people go wrong, and the sketch may have considered it too small to draw.

This is the same homework as reading a route description, done with pictures instead of sentences. Circle the places where the two maps disagree. Those circles are your careful miles.

On the hill, which one leads

In clear weather, on a waymarked path, the sketch can lead. It is quicker to glance at, and it keeps you on the intended line without the distraction of every other path in the parish. Keep the OS sheet where you can open it without a struggle. The moment a junction is not on the sketch, or a waymark points somewhere the red line did not mention, the OS sheet takes over.

If you leave the route to shorten the day, the sketch is suddenly a map of somewhere you are no longer going. The exits live on the OS sheet. How to shorten a walk depends on that wider picture. A sketch that ends at the edge of the page cannot show you the lane half a kilometre off the line.

The mistakes the red line encourages

People walk the sketch as if it were the country. They ignore a fence the artist omitted, then climb it. They miss a path that leaves the route because it was not drawn, and that path was their bus. They also trust distances printed on the drawing when the scale has been squeezed. Check any mileage against the OS sheet or against a description that shows its working.

A phone photo of the sketch is a worse version of a worse map. It dies, it glares, and it has no surrounding ground. If you carry a sketch, carry it on paper, in a pocket that survives rain, next to the sheet that can rescue it. Waymarks still need one of those two maps behind them.

A pocket routine

Fold the OS sheet so the day's ground is outward. Slip the sketch beside it, not buried in the rucksack. At each tricky junction, spend ten seconds asking which map you are using and why. The question sounds fussy at the first stile. At the third unexpected barn it is the whole of navigation.

When you get home, note where the sketch lied by omission. Next time you will read that author's drawings with the right amount of affection and the right amount of doubt.

Frequently asked questions

Is a sketch map enough on a waymarked lowland walk?

On a short, clear, well-signed loop, many people manage with one. You will still be glad of the OS sheet if a post is missing or you need to leave the line.

Do I need the whole OS sheet if the sketch covers the route?

Yes, if you can. The value of the sheet is everything around the route: the road, the river, the path you did not mean to be on.

What scale of OS map should sit beside a sketch?

The 1:25,000 sheet shows field boundaries, which is what you want when a sketch has smoothed them away. A 1:50,000 sheet is better than nothing and coarser at the farmyard.