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Trail talk: measuring gradients with a slope card

There cannot be many of us who have not noticed the squiggly contour lines on a map. Almost any hiker is aware that the closer these are spaced, the steeper the angle. The more widely spaced, the more gentle the slope. There is a rather nifty little aid in determing slope angles when planning a route.

Four slope cards. Two from Saven raspeberry and one each from Mountain Trails and Silva
Four slope cards. Two from Shaven Raspberry and one each from Mountain Trails and Silva

Gradient, or steepness of a slope, makes itself very apparent to the hiker. Unless on Open Access land and just about anywhere in Scotland, it is not often that we are able to reduce the steepness by traversing a slope diagonally. A path usually goes where a path goes. However we are able to gain an idea of how challenging a slope will be in advance (or even retrospectively) by using various aids. One of the simplest of these has to be a ‘slope card‘.

Look at the key of a ‘paper’ map and it will include detail on how height and natural features are drawn on the map. Included in this will be contour lines, both main index and intermediate, and specifically, the vertical interval. These are not always the same and can also vary across a series, for example, the 1: 25 000 maps from Ordnance Survey have different contour intervals depending on whether they are showing upland areas or not. Knowing map scale and contour interval a slope card can be selected. Makers such as Shaven Raspberry produce cards dedicated to specific map scales, this aids in making them simpler in appearance and easier to use.

Usually made of plastic and roughly the size of a credit card, these give an indication of slope angle and frequently carry additional information. Of critical use to hill walkers in winter and shoulder seasons are the angles at which risk of avalanche is greatest. A visual indication (pictograph) of slope might be included on the card, as with the one produced by Mountain Trails. Slope angles of between 25° and 55° deemed as being at most risk from avalanche.

There are various other hints with contours. Where the height of a contour is shown, the top of the number points uphill for example. Look at the North York Moors map sample to see how this is shown. On Ordnance Survey Landranger maps, if the slope is particularly steep, some or all intermediate contours may be missed out. If just one contour is missing, a rough guestimate of a 1 in 3 slope angle can be deduced.

Detail from Ordnance Survey 1:25 000 OL26 map of the North York Moors. This shows how the Rudland Rigg  track gradually climbs up the contours heading north. Contours are shown at 10m intervals and the thicker contours at 50m intervals
Detail from Ordnance Survey 1:25 000 OL26 map North York Moors. This shows how the Rudland Rigg track gradually climbs up the contours heading north. Intermediate contours (thin lines) are shown at 10m intervals and the thicker index contours at 50m intervals. Each fifth contour is an index contour. The blue lined overlain grid indicates 1km squares. So rising around 65m per 1000m. A gentle ascent.
Silva slope card
Silva slope card, European version

Silva now supply a slope card with some of their compasses. Different versions are produced for different World markets with due regard to prevalent map scales in use.

There is a lot of information on this card and the Silva slope card could be regarded as a little over fussy. But break it down and spend some time playing with it and it is simple to use.

Silva slope card produced for the U.S. market
Silva slope card produced for the U.S. market

Note that the Silva slope card, in common with some others, also indicates one set of intermediate contour lines between index contours of a specific slope angle.

If using a slope card, it is essential to cheack that the right card is used for both the scale of map and the contour intervals used on that map. This sheet is the South West Coast Path 3 1:40 000 map from Harvey. The Key shows that Index contours (thick lines) are drawn at 75m intervals, while contours beterrn these are at 15m intervals. Shaven Raspberry sell a card, shown here, specificlly for this scale and contour interval, and can also be used on other 15m contour interval maps
If using a slope card, it is essential to check that the right card is used for both the scale of map and the contour intervals used on that map. Though intermediate contours can sometimes be extrapolated. This sheet is the South West Coast Path 3 1:40 000 map from Harvey. The map key shows that Index contours (thick lines) are drawn at 75m intervals, while Intermediate contours between these are at 15m intervals. Shaven Raspberry produce a card, shown here, specifically for this scale and contour interval, that can also be used on other 15m contour interval maps
Walking from the minor road up to the Trig Point on Hergest Ridge will entail walking up and down the 7 degree hill
Using the simple and clearly marked slope card from Shaven Raspberry shows us that following the bridleway from the spur track, off the minor road, up to the Trig Point on Hergest Ridge will entail walking up and down a 7° hill, i.e. rising 7m in every 100m. Detail showing 10m interval contours on 1:50 000 scale Ordnance Survey 148 Landranger Presteigne & Hay-on-Wye
Detail from Harvey North Downs Way 1:40 000 scale map. Showing part of key. Main index contours (thick line) are at 50m intervals while intermediate contours (thin lines) are every 10m
Detail from Harvey North Downs Way map. This is drawn to a 40 000 scale and grid lines are at 1km intervals. The part of the key included shows that main index contours (thick line) are at 50m intervals while intermediate contours (thin lines) are every 10m. It can be seen how the route is hugging the top of a ridgeline with barely any rise further to the north of it
It can be seen how crossing at a gentle diagonal decreases the effort required. On the North Downs Way ridge, above Thurnham
It can be seen how crossing at a gentle diagonal decreases the effort required. On the North Downs Way ridge, above Thurnham
The slope cards from Shaven Raspberry are uncluttered and simple to use. The wavering path here is mving across a 14 degree slope
The slope cards from Shaven Raspberry are uncluttered and simple to use. The footpath here is wavering across a 14° slope

The sheet used in these two images is OL 26 Explorer Map, this map of the North York Moors and other Explorer series maps are to a scale of 1:25 000 and amongst the finest maps ever produced for hiking. It can clearly be seen that if terrain and features permit, how the steepness of a slope can be made more manageable by tackling it diagonally as on the section of the North Downs Way shown above.

Using the same card, it can be seen that dropping off the Cleveland Way down to the woodland across this Open Access land will entail traversing a steeper initial section of slope that moves into a 20 degree drop
Using the same card, it can be seen how dropping off the Cleveland Way National Trail down to the woodland across this Open Access land will entail traversing a steeper initial section of slope that moves into a less onerous but still quite steep 20° drop. This is a concave slope across rough grassland
This is a busy part of the map and the see-through slope card from Mountain Trails is helpful in locating properly while still seeing what is happening on the map below the card. Climbing from the minor road in the west a footpath ascends diagonally across the steep slope to join the Shropshire Way above. From a steeper part of the path, it moves through a 1in5, or 20 degree slope, shown here, before slowly decreasing in steepness. O.S.Explorer 217, Long Mynd & Wenlock Edge

Detail from O.S.Explorer 217, Long Mynd & Wenlock Edge

This is a busy part of a 1:25 000 scale map and the see-through slope card from Mountain Trails is helpful in locating properly on the sheet while still seeing what is happening on the map beneath the card.

Climbing from the minor road in the west a bridleway ascends diagonally eastward across the steep convex slope to join the Shropshire Way above. From the steeper part of the path, it swings round and moves into a more direct ascent. The card is oriented to the path itself and not the contours, thereby showing the actual slope of the path, a 1-in-5, or 20° slope.

Following Offas Dyke Path along Hergest Ridge, beautifully contoured hills offer a variety of slope angle both up and down
Following Offas Dyke Path along Hergest Ridge, beautifully contoured hills offer a variety of paths either pronounced or gentle slope angles up and down

Three Points of the Compass does not actually carry a slope card on trail. They are for occasional use when route planning or to answer simple curiousity, perhaps following a hike.

There is one item I might have with me on a longer multi-day walk that can be used if I wish. My Suunto MC-2 Global mirror compass, beside being one of the best compasses available to the navigator, also has three slope scales printed on the side of its mirror housing, or ‘slope ruler’ as Suunto call it.

A degree of extrapolation is required when using these as scales and contour intervals are limited to 1:24 000 & 40ft contour intervals, 1:25 000 & 20m contour intervals and 1:25 000 & 50m contour intervals. These are fairly limited in range of slope angle shown, ranging from 25° to 45°, being primarily of use for those hillwalkers particularly aware of the risk of slab avalanche.

The compass also has a dedicated clinometer needle which is of more use with clear ‘side-on’ sight of a slope angle. However use of this needle falls outside the scope of this glance at slope cards.

Measuring slope angles with Suunto MC-2 clinometer
Measuring slope angles with Suunto MC-2 clinometer
Slope scales on side of mirror housing on Suunto MC-2G compass
Slope scales on side of mirror housing on Suunto MC-2G compass

There is another aspect to hiking up or down a gradient that has to be taken into account, the distance walked also changes. Covering a horizontal 100m on a steep 27° ascent will mean 112m has actually been covered. Combined with the extra exertion that a slope entails, these additional metres can add up over a day. Some slope cards also give an indication of this, usually down the centre of the card. Shaven Raspberry got a bit carried away with indicating tenths of a metre on one of their slope cards. While this is more accurate, the simpler full metres on their other slope card are easier to read and use.

Extra distance covered relating to change in height, detail from Silva Slope Card
Extra linear distance covered is related to changes in elevation, detail from Silva Slope Card
Unlike with transparent slope cards, solif cards usually include an aide memoir on how to use them on their reverse. Shown here are the two scale cards from Shaven Raspberry with the one provided by Silva with many of their compasses
While not possible with transparent slope cards, some cards include an aide-memoire on their use on their reverse. Shown here are the instructions on the Silva slope card and the two cards from Shaven Raspberry

Experience will be enough for most hikers, backpackers and navigators when looking at either map or the hill in front. Visual clues will be sufficient for many. Slope cards are not essential to planning or enjoying most hikes, though they can be an important tool for those walking in winter. Beside that, they are simply a cheap little tool to be aware of and use if it suits you to do so. As winter draws in, why not spend a half an hour or so at home practicing their use prior to getting out and actually puffing up the slopes.

6 replies »

    • Hi Richard, I would presume that purchasing one of the Silva compasses that comes with a slope card, sold on the US market, would provide one

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