National Map Reading Week Q&A

We asked on Facebook and through our Customer Services for some of the more interesting questions about map reading and navigation - here's the results.

​Will the accuracy of the maps ever be increased from 2.5m?

The accuracy is dependant on the scale used for the original survey. The 2.5m accuracy is correct to places that we survey in 1:2500 scale. See here for more information on expected accuracy.

Surveying at higher resolutions is much slower. This means that we survey places where accuracy is vital, such as cities, at higher resolutions than areas where it is less important, such as a remote footpaths. The detail we are able to survey at will change over time - we are constantly working on new technology, and advances such as aerial drones may make it easier to capture more detail over a wider area.

Even at 1:25 000 scale, 2.5m on the ground is pretty small - only 0.1mm on the map - so it reaches the limits of human ability to discern it. More accurate mapping is more useful for civil engineering and constructions projects, so for these were do create data with better accuracy.

If you are following maps using your GPS, please be aware that consumer grade GPS devices are generally quoted to +/-15m or so for accuracy, so the usefulness of more accurate mapping is limited unless you also have access to more accurate surveying equipment.

How many extra steps should you allow for pacing when you are walking up an incline?

I like this one - unleash the maths!

If I know the distance on the flat is 100m as measured from the map, but I am also gaining 30m in height, we are walking further. Pythagoras gave us a neat way to do this:

horizontal_distance2 + height2 = ground_distance2

1002 + 302 = 10900 | √10900 = 104.4m

So for our walk uphill we only cover an extra 4.4m, compared to walking on the flat. The steeper the slope the more extra distance you will cover (up to a 45 degree slope - after this the distances reduces but you are less walking and more climbing).

But the question was steps, not distance. This is much trickier. Walking up a slope shortens your stride and it gets worse the steeper the slope is. A sustained slope (at least for me!) tends to make my steps get shorter and shorter over time. Lastly, most slopes are not smooth, so you end up having to judge foot placement more, which alters your stride length...

My best advice would be to test it on some known slopes and distances and get an estimation for your personal steps. You could even work out an approximate extra percent for shallow, medium and steep slopes.

An alternative is use Naismith's rule and use time, rather than steps, to judge distance. His classic rule is 10 minutes extra per 100m in height gained. You can adjust this based on your personal speeds.

How do you use a clinometer or slope ruler on a sighting compass?

Some compasses are equipped with a clinometer to measure tilt or vertical angle. It's used to judge a slope gradient, which can be compared to a map to identify a specific slope as part of navigation, or to estimate avalanche risk. It can be also be used to work out the height of a distant object.

There are a number of different types of clinometer (you can even make one with a simple protractor), but here are the instructions from Suunto (who make the OS compasses)

The scale for declination correction on the back of the capsule also functions as a scale for the clinometer.

1. Turn the capsule so that the bearing index is at 270° (due West) and tilt compass on its side with the declination scale downwards.

2. If you have a clear view across the slope, align the bottom edge of the compass to the slope with the back of the compass is facing you. Read off the small scale inside the capsule straight down to get the angle.

3. Alternately, if you have a clear view to the top or bottom of the slope, sight the top or the bottom - see the image for the two different orientations.

4. Read from the clinometer needle to get approximate inclination reading.

If you have a compass with a built in clinometer, the best bet is to check the instructions that came with it, as different brands work in slightly different ways.

Images copyright Suunto

What’s the strange spider symbol on the OS Explorer map 266?

"I'm emailing about something which may seem rather trivial; However, I've come across a map symbol in my local area OS map (Wirral & Chester / Caer). Being interested in the geography and history of my area, I'm keen to know what this symbol is.

The symbol is not in the key of the map, and i have never come across it before. "

The mysterious map spider (circled)

The mysterious map spider (circled)

This took a bit of digging. On the OS Landranger map for the area the “dead spider” does indeed not appear in the legend.

After checking with the cartography team, and they have advised me that the symbol in question is a combination of two mapping features. It depicts steep slopes (the 'legs') leading down into a very small pond (the 'body').

However, this particular feature was removed from the latest editions of the map - most likely because it is no longer visible on the ground. Or maybe it was just a bug!

How far above the earth's surface would you need to be in order to see the earth directly beneath at a 1:50,000 scale?

An interesting question, and possibly useful for anyone planning on building their own jet pack. I had to refer to a couple of our top map nerds for this one.

The answer depends on the medium you are using to view the 50,000 map – human eye, or camera – but the principle is still the same. We need to know the focal length of the viewing lens and the resolution of the ‘photo plane’ - for both a camera and a human eye.

We are unclear about what “would appear as a 1 centimetre square” actually means. If you think about testing it, you could cut out an exact 1cm square from a 1:50 000 map, and float gently to a height at which the features on the ground below exactly fit the map surrounding that one centimetre square.

But then you think about it again, and this depends on how close that one centimetre square is to your eyes! If you take that distance to be the average distance (45cm) you would normally hold the map when reading it, then there is still some leeway, because we all have slightly different reading distances.

Calculating the view angle

Calculating the view angle

Best answer:

Using a typical reading distance of 45cm from the human eye to the paper (on which your 1cm square is cut out), using the principle of congruent triangles, the height above the ground works out as 22,500 m (22.5 km).

This is well above any commercial (and most likely almost all military) aircraft, so any astronauts able to demonstrate this with an OS Landranger map can post their pictures in the comments! OS Explorer maps, with their larger scale would be lower at 11.250m, which is much more achievable.

I've bought two Explorer maps one uses ‘OL’ and the other just a number - why?

Simple answer on his one: the prefix OL stands for 'Outdoor Leisure' which is used to show the maps that cover the National Parks.

There are 62 of the 'OL' maps - they can be easily spotted as the top corner is yellow. The remaining 341 Explorer maps have a grey corner

Please use the comments below to ask any more questions about maps, map reading or navigation and I'll do my best to answer them!

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