How to interpret topographic features
Being able to interpret how topographic features fit together in the landscape gives the user a much deeper understanding of the lay of the land and how to identify where they are and how to move through it.
On topographic maps, contours represent the shape of the land. Contour lines fit together in many different ways, and they form shapes which can be recognised by the user.
Features of the landscape that are useful to know are:
Elevation and slope Understanding how elevation and slope are depicted on topographic maps
Elevation and slope are the two elements that determine how landforms physically appear and connect.
The ‘contour interval’ – the elevation between contours – is the vertical distance between adjacent contour lines. On 1:25,000 maps usually used by bushwalkers, contours are either 10 or 20 m apart.
For measuring between contour lines see here.
The rate of rise or fall of a terrain feature is known as its slope. The speed at which a bushwalking group can move is affected by the slope of the ground or terrain features. This slope can be determined from the map by studying the contour lines—the closer the contour lines, the steeper the slope; the farther apart the contour lines, the gentler the slope. Totally flat ground has no contour lines.
Four types of slopes that concern bushwalkers are gentle, steep, concave, and convex.
- Gentle: Contour lines showing a uniform, gentle slope will be evenly spaced and wide apart. Easy walking.
- Steep: Contour lines showing a uniform, steep slope on a map will be evenly spaced, but close together. Very challenging, or impossible walking (i.e. contour lines may be so close that they create an impassable cliff line).
- Concave: Contour lines showing a concave slope on a map will be closely spaced at the top of the terrain feature and widely spaced at the bottom. Bushwalkers going up the slope will find the terrain increasingly steep and challenging.
- Convex: Contour lines showing a convex slope on a map will be widely spaced at the top and closely spaced at the bottom. Bushwalkers going down the slope cannot observe most of the slope or the terrain at the bottom, so extra care must be taken when route finding.
Common terrain features Understanding how common terrain features are depicted on topographic maps
All terrain features are derived from a complex landmass known as a ridgeline, not to be confused with a ridge.
The US Army states that “A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides from which a total of 10 natural or constructed terrain features are classified”.Army, U. S. “Map Reading and Land Navigation.” Washington, DC, fm: 3-25. By comparison, a ridge is a sloping line of high ground.
Major terrain features include hills, saddles, gullies, ridges, and depressions, and they each have characteristic contour lines that make it easy to pick them out in the landscape.
- Hills, peaks, knolls, mountains: A hill, peak, knoll or mountain is an area of high ground. From a hilltop, the ground slopes down in all directions. A hill is shown on a map by contour lines forming concentric circles. The inside of the smallest closed circle is the hilltop.
- Hill = an area of high ground; generally, a smaller and rounder than a mountain, and less steep.
- Knoll = small, rounded natural hill.
- Mountain = a very tall hill, generally with a minimum size of 600m, but varies around the world.
- Peak = a mountain with a pointed top.
- Munro = a Scottish mountain taller than 3,000 feet (914 m).
- Saddle: A saddle is a dip or low point between two areas of higher ground. A saddle is not necessarily the lower ground between two hilltops; it may be simply a dip or break along a level ridge crest. When standing in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. A saddle typically looks like an hourglass.
- Gully: a gully is a stretched-out groove in the land, usually formed by a watercourse, and has high ground on three sides. Depending on its size and location water sometimes flows through it, from high to low. Contour lines forming a gully are either U-shaped or V-shaped. To determine the direction water is flowing, look at the contour lines. The closed end of the contour line (U or V) always points upstream or toward high ground. A valley is a large gully, often very flat, wide and open with a large watercourse running through it.
- Ridge: a ridge is a sloping line of high ground. When standing on the centerline of a ridge, there is usually low ground in three directions and high ground in one direction with varying degrees of slope. When crossing a ridge at right angles, there is a steep climb to the crest and then a steep descent to the base. When moving along the path of the ridge, depending on the geographic location, there may be either an almost unnoticeable slope or a very visible incline. Contour lines forming a ridge tend to be U-shaped or V-shaped. The closed end of the contour line points away from high ground.
On a map, a ridge is depicted as two contour lines (often of the same contour) running side by side at the same elevation for some distance. When the lines diverge, the ridge is either flattening out to a high plateau or continues to rise with additional contour lines. When the lines converge, the ridge is falling in elevation, creating a spur.
Closed contour loops represent hills or bumps along the ridgeline.
- Spur: A spur is a short, continuous sloping line of higher ground, normally jutting out from the side of a ridge. A spur is often formed by two roughly parallel streams cutting draws down the side of a ridge. The ground will slope down in three directions and up in one. Contour lines on a map depict a spur with the U or V pointing away from high ground.
- Depression: A depression is a low point in the ground or a sinkhole. It could be described as an area of low ground surrounded by higher ground in all directions, or simply a hole in the ground. Usually, only depressions that are equal to or greater than the contour interval will be shown. On maps, depressions are represented by closed contour lines that have tick marks pointing toward low ground.
- Cliff: A cliff is a vertical or near vertical feature; it is an abrupt change of the land. When a slope is so steep that the contour lines converge into one “carrying” contour of contours, this last contour line sometimes has tick marks pointing toward low ground (image below). Cliffs are also shown by contour lines very close together and, in some instances, touching each other.
Topographic maps cannot always be used to identify cliffs, however, particularly on those with 20m contour intervals, and hence some steep areas require careful negotiation.
Tips, tricks and common mistakes Some tips, tricks and common mistakes to avoid when reading topographic maps
The real art of map reading comes with interpreting how individual landscape features fit together in the terrain: saddles connect ridges to knolls to cliffs; gullies form into rivers and valleys. Interpreting how contour lines fit together helps understand the lay of the land and be able to navigate through it.
The big picture
This image describes a landscape by contours. In words:
Running east to west across the complex landmass is a ridgeline. A ridgeline is a line of high ground, usually with changes in elevation along its top and low ground on all sides. The changes in elevation are the three hilltops and two saddles along the ridgeline. From the top of each hill, there is lower ground in all directions. The saddles have lower ground in two directions and high ground in the opposite two directions. The contour lines of each saddle form half an hourglass shape. Because of the difference in size of the higher ground on the two opposite sides of a saddle, a full hourglass shape of a saddle may not be apparent.
There are four prominent ridges. A ridge is on each end of the ridgeline, and two ridges extend south from the ridgeline. All of the ridges have lower ground in three directions and higher ground in one direction. The closed ends of the U’s formed by the contour lines point away from higher ground.
To the south lies a valley; the valley slopes downward from east to west. Note that the U of the contour line points to the east, indicating higher ground in that direction and lower ground to the west. Another look at the valley shows high ground to the north and south of the valley.
Just east of the valley is a depression. Looking from the bottom of the depression, there is higher ground in all directions.
Several spurs extend south from the ridgeline. They, like ridges, have lower ground in three directions and higher ground in one direction. Their contour line U’s point away from higher ground. Between the ridges and spurs are draws. They, like valleys, have higher ground in three directions and lower ground in one direction. Their contour line U’s and V’s point toward the higher ground.
Two contour lines on the north side of the centre hill are touching or almost touching. They have ticks indicating a vertical or nearly vertical slope or a cliff.
The road cutting through the eastern ridge depicts cuts and fills. The breaks in the contour lines indicate cuts, and the ticks pointing away from the road bed on each side of the road show fills.
Here are some tips and tricks to identify between standard features.
- Spur vs. gully: Contour lines on a map depict a spur with the U or V pointing away from the high ground; for a gully, the closed end of the contour line (U or V) always points upstream or toward high ground.
- Knoll vs. depression: for knolls, contour lines form concentric circles, and there is lower ground all around, whereas depressions have closed contour lines with tick marks pointing toward the low ground.
- Saddle vs. ridge: When standing in a saddle, there is high ground in two opposite directions and lower ground in the other two directions. When standing on the centerline of a ridge, there is usually low ground in three directions and high ground in one direction with varying degrees of slope. Be careful not to confuse ‘ridge’ with ‘ridgeline’ here: a ridgeline is a line of high ground, which can rise and fall through saddle features.
Map reading takes practice. One of the easiest ways to do this is to become aware of the shape of the surrounding land at all times, even when driving and walking through an urban area. Most navigation and map reading is about matching up the form of the land with that on the map. Practice recognising and naming key features (knoll, hill, spur, ridge, cliff, valley, etc.). Take maps on all bushwalks and follow the route on the map, even if it’s well signposted. Look at the map regularly and match it with the surrounding landscape.
Subtle features Recognising subtle features on topographic maps
There are subtleties to map reading that take time to develop. Some common things to watch out for include:
- Implied knoll: A bump or small hill that is too small to generate it’s own closed loop contour. Occurs in places where two ridge lines diverge and converge or on the top of a hill where the contour lines are furthest apart.
- Implied saddle: The opposite of an implied knoll. An implied saddle is a saddle that is not formed enough to have two parallel contours cross the ridgeline and connect.
- Minor gully: A dent in the side of a slope or ridge often too high for a proper water course to form.
- Minor spur or outcrop: a bump on the side of a ridge or slope that’s too localised or flat to form a proper spur.
References [ + ]
|1.||↑||Army, U. S. “Map Reading and Land Navigation.” Washington, DC, fm: 3-25.|