Contour lines represent points of equal elevation. A 3 dimensional hill can be projected onto a 2 dimensional topographic map as shown below:
Say for example, each orange block is 20 meters high. Every increase in elevation of 20 meter corresponds to a contour line on the topographic map below. As we hike from the surrounding plains (0 meter altitude) to the innermost contour line (which corresponds to the peak of the hill) we have to climb 8 contours x 20m = 160 meters. From the left / East, the contour lines are widely spaced which means the slope is very gradual. On the right / West, the contour lines are closely spaced which indicates a steep gradient.
Time to check the topomaps for some real-world mountains
As usual you ll see a empty green area displayed
Now switch to “Terrain” view in the top-left menu and zoom out till you see contour lines appearing:
The irregular curved contour lines correspond to varying elevation of the terrain also called the relief or topography of the terrain
Every thin contour line shows a change in elevation of 20 meters. Every thick contour line corresponds to 100 meters change. The exact elevation or altitude is shown on the thick contours.
We can identify several peaks in the Parvathamalai range corresponding to the innermost contour lines/circles (highest altitude). The two adjacent peaks in the center correspond to 800m altitude. The two peaks in the West/left are 640m high. The peak in the Southeast/bottom-left is 520m high.
A hill with perfect round / circular shaped contour lines is a perfect cone – same slope / steepness on all sides of the hill
In the above topomap however we see irregular curved contours corresponding to different terrain features
U-shaped contours correspond to “Ridges” (red lines below). The more profound / narrow the U-shape, the sharper the ridges will be.
A navigator prefers to climb up a mountain following the ridges as they are usually less steep (wider spaced contours) compared to the steeper surrounding slopes (narrow spaced contours). Observe the wider spaced contours along the red ridge lines in the topomap above versus the closer spaced contours along the left/right sides/slopes of the same ridge lines.
The opposite terrain feature of a ridge is a “Valley” represented by inverted U-shaped contour lines (blue lines below)
The sharper the inverted U-s, the more profound the valleys will appear in the terrain
Similar to ridges, valleys are usually less steep (wider spaced contours) then the surrounding slopes (narrow spaced contours) which makes them (one of the) preferred ways to hike inside the mountains
As we hike from lower (plains) to higher (peaks) altitudes along the ridges we will cross U-shaped contours. As we hike from lower to higher altitudes along the valleys we will cross inverted U-shaped contours. Verify the same for all ridges and valleys in the above topomap.
A fourth and final important terrain feature is the “Saddle” (blue balloon) or the lowest point along the ridge line between two peaks (red balloon). The blue saddle in the West / Left side is at 600m altitude, lowest point along the red ridge line between both peaks at 640m altitude.
Opposite ridge lines typically connect peaks (U-shaped contours merging as O shape). Valley lines typically connect saddles (inverted U contours merging as X shape). Navigators usually hike along ridges and valleys connected through peaks and saddles.
Open the above Terrain Map over here
, zoom in and inspect the various terrain features in detail to enhance your understanding
3D Terrain View
Contour lines / Peaks / Ridges / Valleys / Saddles are fundamental terrain features to plan hiking trails in the mountains
An even better way to visualize (and understand) the above features is in the 3D views offered by Google Earth
Google Earth on your laptop or phone
Open Google Earth and search for “Parvathiamalai”
The default view will be the same as the “Satellite” view in Google Maps:
Click on the topmost Blue arrow to lower viewing angle / perspective and see a 3D view of the same terrain
Pull the “N” (North) symbol around the white circle to view the terrain from different directions:
As you change the angle and direction you get a clear 3 dimensional view of the Parvathiamalai mountain range
Now download the above terrain features over here
(exported KMZ file from map) and click on the same to view in Google Earth:
Change your direction and angle of viewing to clearly see and understand the various terrain features: peaks, ridges, valleys and saddles shown on the 3D satellite view of the Parvathiamalai mountain range.
To make things even more comprehensive now download the Google Contour map over here
and click on the same to view it in 3D in Google Earth:
In this 3D view you can clearly see how the contour lines represent the topography of the Parvathiamalai mountain range
Check / Uncheck the Terrain Features and Contour overlays in the “Places” sidebar to switch between different views above
Spent sufficient time viewing the terrain in 3D from different angles and directions while showing/hiding the terrain/contour overlays until you have a clear understanding of the contour lines, ridges, peaks, valleys and saddles which are essential to navigate the mountains.
Congrats! You have now mastered the key concepts of topographic maps and are ready for some hands-on in the next module. Stay tuned!