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Map-makers are updating the 127-year-old formula used to estimate walking times to reflect modern ramblers.

Currently, a route's time is calculated using a formula created by Scottish mountaineer William Naismith in 1892.

But Ordnance Survey (OS) will soon be using data provided by walkers through an app to provide more accurate times.

Produce manager Tim Newman should also reduce the number of people needing rescuing after being caught out by nightfall.

Naismith's rule says walkers should allow one hour for every three miles (5km) and an hour for every 2,000ft (600m) of ascent.

The OS app, which is used by more than one million people, will be updated with an activity tracker in August through which users can record their walks.

This data will then be used by OS to predict more personal time estimates as well as working out general averages for routes.

Mr Newman said: "Naismith's formula has worked as a rule of thumb but it's based on what a fairly fit Victorian mountaineer who wanted to walk long distances could do.

"It is probably not applicable to everyone because everyone has different levels of experience.

"We are trying to make sure people pick something suitable for their level and are properly prepared.

"It will also give us an understanding of the British public and how they get outdoors."