Two Types of Topographic Maps

October 05, 2017

A topographic map is a highly accurate map that shows that can show natural terrain and or man-made objects like buildings and roads. Topographic maps differ from most types of maps because they show elevation, but they have all the other elements commonly associated with maps including a legend, scale, and north arrow. Because topographic maps are usually associated with elevation data they are sometimes referred to as terrain maps, elevation maps, altitude maps, contour maps. Elevation data for these types of maps can be collected via a variety of different devices. Large-scale data collection is usually done via satellite or from an airborne platform such as an airplane or drone. But elevation data collection could easily be collected with handheld devices such as sports and fitness GPS devices and smartphones.

Paper topographic maps have been in use for many years and have been the mainstay of navigation for people looking to navigate in the outdoors, urban planners and those who with a need to understand the details of a landscape.

How Topographic Maps Show Elevation
The are many different ways topographic maps display elevation. But I am going to focus on contour lines and elevation data presented as relief maps.

Contour Lines
Contour maps display changes in elevation as contour lines. Each contour line on a map joins points that have an equal height. In theory, if you follow a single contour line, you walk at the same elevation all the way around until you return to your starting point. Contour lines follow specific requirements, including:

Each point on a contour line has the same elevation.
Contour lines do not cross one another.

In order to correctly read and understand contour lines, they are usually labeled with the elevation values or color coded in such a way that the height data can be determined with help of the map key.

Explanation of Contour Interludes

When you look at a section of contour lines on a map, you'll see they appear to be spaced at uneven intervals, but there is a logical explanation.

They are spaced at intervals that change as the elevation changes. You need to know the contour intervals to interpret changes in elevation at a glance on a map. To figure out the contour interval:

Locate two contour lines on the map that are labeled with their heights and that have one or more unlabeled contours between them.
Subtract the smaller elevation number printed on one contour line from the larger number on the other unlabeled contour.
Divide the result by the number of unlabeled lines between them to arrive at the contour interval.

For example, if you have two contour lines labeled 30 and 40 feet with a single unlabeled contour line between them, the contour interval is 5 feet. The elevation at any point on the unlabeled contour is 35 feet. The contour interval value remains constant for all the contours on the map.

There are several rules to be aware when interpreting topographic maps with contour lines:

The rule of Vs: sharp-pointed vees shapes made by contour lines usually represent stream valleys, where the drainage channel passes through the point of the vee, with the vee pointing upstream. This is a consequence of erosion.

The rule of Os: closed loops are frequently uphill on the inside and downhill on the outside, and the innermost loop is often highest area or the lowest area.

density of contours: closely spaced contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging suggests a cliff. By calculating the number of contours that cross a segment of a stream, the stream gradient can be approximated.
When you look at a contour map, you are looking straight down at a representation of the Earth, so it is difficult to identify changes in elevation. Contour maps use contour lines to indicate height and maybe, more importantly, changes in elevation across the terrain.

Relief maps
A relief map is a type of topographic map that doesn't use contour lines.
Elevation data is continuous data. This means that it does not merely exist in a particular area. Like temperature, elevation does not have a discrete boundary or an edge where it exists on one side but not on the other.

Because of this Some cartographers choose to create maps that show elevation data for the entire map and not just for discrete sections. A common way of doing this is by representing elevation data as a raster or image that covers the entire map. In the case of a relief map, the elevation data is colored to show changes in elevation. A typical way of color-coded elevation data is to display higher elevation values as lighter colors and lower elevations values as darker colores. The color contrast creates a shadow effect and gives the topographic map a realistic look. The hope is of course that the end user will easily be able to distinguish between mountains and valleys just by looking at the map. A globe with raised mountain ranges is also a type of relief map.

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