Conveying the Message of Land Elevation with Maps

June 21, 2018

Conveying the Message of Land Elevation with Maps

 

Example of a topographic map

Topographic maps are most distinguishable for their elevation and physical geographic elements even though they show both natural and man-made features.  These maps are essential tools for understanding the lay of the land without having to go and survey the area yourself.  Many people use topographic maps for a wide variety of purposes; from disaster relief initiatives to construction activities to sporting events and adventure seekers.  Topographic maps are not alone in showing the continuous change in height of the Earth’s surface.  Elevation can be shown on any type of map and two established techniques of doing so are using contour lines and hill shading.       

 

Contour Lines

Contour lines are most commonly used by large producers of topographic maps like the Ordnance Survey, Natural Resources Canada, and the USGS (United States Geological Survey) but hill shading is another way of giving the reader an impression of the physical geography of an area.  Using the two together can be visually impressive providing it’s subtle compared to the overlaying information and the contours can be clearly read.

topographic map with contour lines showing elevationContour lines example by Ordnance Survey

 

topographic map - an example of shaded relief

Hill shading example by Inspirit Cartographics

Contour lines are lines that connect areas of equal elevation of the ground above sea level.  The closer the lines are together the steeper the slope.  Tightly packed lines indicate a dramatic drop in the landscape.  By reading contour lines a map user can identify mountains, valleys, canyons, and plains.  River flow patterns can be discerned and even man-made features can be understood such as railway lines which generally adhere to following contours to avoid too harsh of a drop or too steep of a climb.  In the example below, you can see a flat landscape with a sudden and steep cliff just in front of the “Lewes Golf Course” indicated by closely stacked contour lines on the map and a photo taken from the river's edge showing the actual cliff that those lines represent.

Image of contour lines and real world topographic relief

This map shows an area in Lewes East Sussex as being flat with a sudden and steep cliff, next to a photograph of the area in real life as it looks facing towards the cliffs standing by the river with the major road hidden behind the houses.

Map by Inspirit Cartographics and photo by Jennifer Johnston.

Contour lines are quantitative sequential values with strict specifications.  Each line is given a value that measures the ground above mean sea level (MSL).  They are typically thin black or brown lines that are neatly placed in the background of the map.  Contour lines have the following characteristics (Rabenhorst and MacDermott, 1989):

  • Contour lines point upstream in valleys
  • Contour lines point down ridge along ridges
  • Adjacent contour lines should always be sequential or equivalent
  • Contours never split into two
  • Contour lines never cross each other (that would mean that a point has two different elevation values which are incorrect)
  • Contour lines never spiral
  • Contour lines should never stop in the middle of the map

 Contour lines are suited for large-scale maps from city scale to regional.  They are helpful for purposes that require a clear and detailed understanding of the lands gradient.   They are a better choice for disaster relief initiatives, construction activities, and outdoor sporting activities than hill shading.

Arrow showing the increase in topographic scale

Large-scale maps are “zoomed in” showing a larger amount of detail of a smaller area like a city. Small-scale maps are “zoomed out” showing a smaller amount of detail of a larger area like a country.

Hill Shading 

Hill shading also known as shaded relief is an excellent technique for giving a tactile quality to maps.  It plays on the readers intuitive understanding of how light reflects on the Earth from an angled source above.  The reader understands the darker areas are shadows or depressions and lighter areas are facing the light source or are points of higher elevation.  Mountain ranges look rough and plateaus look smooth, ridges are shaded and peaks are highlighted. 

Unlike contour lines, shaded relief does not provide the map reader with any numeric values of elevation and is therefore not useful for any purpose that requires precision or detail with regard to the terrain.  Hill shading is best used to give a general idea of a large area.  Hill shading is good for regional to world small scaled maps to help characterize a region and its physical geography.   At larger scales hill shading becomes abstracted.  Similarly contour lines become more difficult to read as the scale decreases, for example you would not see contours on a world map but you will see hill shading.

Both techniques are created in ways that are reflective of their virtues and scales.  Contour lines are made from spot heights (known points of exact elevation values) and joining areas of like elevations using TINs (Triangular Irregular Networks).  This resonates with the quantitative nature of contour lines having numeric values and the detailed depiction of a small area’s elevation.

Detailed image displaying contour lines

Source: https://www.e-education.psu.edu/natureofgeoinfo/c7_p6.html

Shaded relief is created from DEMs (digital elevation models) which are images taken by satellites where each pixel is given a value.  The image is then altered by computer software to achieve the desired effect.  You can imagine a photo taken from outer space of the Earth’s surface to give a zoomed out more generalized representation suggesting its fit for maps with large geographic extents and generalized land features.

   Digital elevation model before and after computer enhancement

 DEM image of the northeast region of Guantanamo in Cuba before and after computer alteration.

Both contour lines and shaded relief can be used as a representation of the continuous ascents and descents of the ground.  They can be used together, but more often they are used separately.  Contours are better at giving detailed elevation information at larger scales and hill shading is better at showing a summary of that information as relief on smaller scaled maps.  Both effects can be done by hand but today they are almost entirely done with the help of computer software.  Both are set in the background of maps but are the foremost in telling the story of topography.

 

 

References

DiBiase, David. PennState College of Earth and Mineral Sciences (2009).  www.e-education.psu.edu/natureofgeoinfo/c7_p6.html.  “Contouring by Hand”.

Jenny, Bernhard.  Timofey, Samsonov (2015).  www.researchgate.net/publication/288345283_Small-scale_and_multi-scale_relief_mapping.  “Small-scale and multi-scale relief mapping”.

Natural Resources Canada (2014).  www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/earthsciences/pdf/topo101/pdf/mapping_basics_e.pdf .  “TOPOGRPAHIC MAPS: The basics”.

 


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