Relief representation

Relief is important to many map users and is represented, in one form or another, on most topographic and some thematic maps. As a continuous 3D surface, it presents challenges to cartographers seeking to represent it in a graphical format, particularly in the form of a 2D map. Obviously with developing technology, 3 dimensional digital models are now possible. Here, however, we will focus on relief representation in 2 dimensions (i.e. on a flat map).

Even on very early maps, relief formed a key part of the composition and was commonly represented by drawings of hills and mountains. Although this did indicate the presence of hilly or mountainous terrain, it did not show very accurately the spatial extend of the hills, their height, how slope varied across them, how valleys dissected them or what other features existed within the hills in much detail. Maps now commonly use one of, or a combination of, spot heights, contours, hachures, hypsometric tints and hill shades to represent relief. Each one has its pros and cons and which technique to use will likely depend on the scale of your map and the user requirements.

Spot heights

Elevation can be measured and defined at a single point and expressed numerically as a height above or below a datum (for most people, it is best understood as a height above or below sea level). Spot heights can provide an accurate measure of elevation at a single point and are often represented by a small point symbol labelled with its elevation. Spot heights are best placed at critical points in the terrain such as the top of peaks, the bottom of depressions, on passes/saddles, at major path or road junctions and close to points of interest. Just using spot heights to represent relief however, can present a challenge to the user in visualising the terrain and how it changes between two defined points. Spot heights are therefore best used in combination with another form of relief representation.

Spot heights can be particularly useful on nautical or aeronautical charts where the user may need to know the absolute height (or depth) of certain features as this will likely influence whether they can safely pass over an area or not. For example, knowing the exact height of a mountain peak or the minimum depth of a water body.

Contours

Contours are a form of isoline; lines of constant height above or below a datum (often defined as sea level). They are commonly used on maps and are generally drawn in a neutral colour so that they are visible and usable but do not overpower other features on the map or visually influence the overall look and colour of the map. Generally, contour lines have equal vertical intervals and thus the spacing between contours indicates slope. Closer contours indicate a steeper slope whilst spread out contours represent shallower slopes. Read more about how to read contour lines here.

How contour lines show a pair of small hills

How contour lines show a pair of small hills Steep slope - contours are close together

The vertical interval of contours will depend on the scale of the map but also the vertical variation within the area of interest. Areas with large vertical variation will likely require larger contour intervals (e.g. 50 or 100m intervals) than a flatter area which may need smaller vertical intervals (e.g. 5 or 10m intervals) in order to show variations in the terrain. Take care not to set your contour interval so small that they start to clutter the map and make it difficult to read and interpret.

Contours generally have an index contour, which occurs every 4/5 contours and is usually represented by a slightly thicker line. For example, for a 10m contour interval, the index contour might be every 50m (every 5th contour). This allows for better terrain visualisation and easy contour counting and tracing by the user. Contours, particularly index contours, should be labelled to aid user understanding. Labelling is best achieved by breaking the contour and placing the contour value in line with the contour, with the bottom of the number facing downhill. Contour numbers can either be placed in a ladder format or spread across the map such that if the height of a contour is to be determined by a user, a numbered contour is never that far away.

One of the key issues with contours is that on very steep slopes, contours can coalesce. Contours should never touch, so in these areas, contour lines are often broken to maintain legibility and may be replaced by steep slope or cliff symbols/ rock drawings if appropriate. The contour lines at the top and bottom of the slope and index contours should be retained where possible to do so.

Despite their common use across maps, many users find difficulty visualising the 3-dimensional nature of terrain that the contours are representing. Contours are sometimes used in combination with other methods of relief representation such as spot heights, hypsometric tints or hill shading to aid user understanding and visualisation of terrain.

Hypsometric tints

An alternative way of representing relief is via hypsometric tints/ layer colours which essentially classifies the terrain into elevation bands (area symbols). Typically, the intervals between the layers are greater than would be used for contours on a traditional topographic map. Hypsometric tints are often used where representation of relief is important to the overall purpose of the map when relief needs to be shown over large areas. As such, hypsometric tints are commonly used on small scale maps.

Various approaches exist in terms of the representation of elevation using hypsometric tints. Stepped layers can be used where a small number of layers are used with medium contrast between the layers. Alternatively, continuous/semi-continuous layers can be designed which use a large number of layers and the colours may blend from layer to layer. Both approaches are valid.

Care should be taken when choosing your colour ramp as the colours will have a major effect on your map design and, due to the way in which colour is perceived by the human eye, the colours can also have a big influence on the meaning of your map. Colours should be sequential (see our section colour) and can be of a single hue (dark to light) or of multiple hues. Common colour sequences using multiple hues start with greens and yellows in lower areas up to oranges and browns and maybe even purples and whites in higher areas.

A popular colour ramp approach when using a single hue is a β€˜higher the lighter’ approach – this is due to the eye visually perceiving lighter colours as being closer to the user, and therefore perceived as being higher. If using this method, care must be taken that the lower elevations aren’t so dark that other features, which are often located in the low points in the landscape, are not visible.

Other considerations in terms of hypsometric tint colours include considering the characteristics of the landscape you are mapping. For example, greens are often used for lower elevations, but this may not be appropriate for arid environments where a colour ramp using yellows, oranges and browns may be more appropriate. Similarly blues could be used for water environments.

Although hypsometric tints may help users to better visualise the terrain in 3 dimensions than contours, it is often the case that hypsometric tints are combined with hill shading to really help give an impression the terrain in 3 dimensions.

Hill shading

Hill shading is a technique used by many to really bring terrain to life and can really help users to visualise terrain in 3 dimensions on a flat map. Hill shading is based on the amount of light falling on a surface from a defined light source. This technique essentially illuminates surfaces that are facing the light source and making those which face away from the light source appear to be in shadow. The light source is normally positioned to the north-west.

Although hill shades represent slopes well, they do not show elevation very effectively. As such, hill shades are often combined with contours or hypsometric tints to better represent slope and elevation.

Hachures

Hachures are seen on many older maps, including the first Ordnance Survey Map of 1801, and are a pattern of lines which run parallel to the direction of the steepest slope. The length of the lines varies dependent on the length of the slope they are representing and steeper slopes are often depicted using thicker lines, or more densely spaced lines. The main downside of hachures is that they can make the map image quite heavy if there are lots of them and they can also clutter the map and overprint other features.

Digital

More recently, digital methods of representing relief have become common. These include the creation of digital terrain or digital surface models or triangulated irregular networks, which model the terrain in 3 dimensions. These 3D models are often created using a combination of aerial photography and/or LIDAR data.

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