Geomorphological Map -Small scale (Mapping the terrain)

By Syfujjaman

Geomorphology is the study of the forms of earth’s surface. Thus it is often essential to represent various attributes of the configuration of the earth’s surface.

A number of techniques are now available to represent the geomorphological attributes and it depends on the nature of the attribute and data, need of representation, scale of the base map etc.

1. Depiction of relief:

Representation of relief is a very old one. It involves the representation of the three dimensions upon a plane surface. A number of methods are used to represent relief.

1. Spot height: When heights are marked on the map by point symbol along with the numeric value indicating the height, they are called spot heights.

• Generally measured from sea level taken as datum.
• Can be expressed as dot, Triangulation points, or Bench mark.

Advantage and Disadvantage: It provides more precise and definite information but does not represent the pattern of relief.

2. Contour line: This is the isoline (line of equal value- isohypses) which carries uniform elevation value throughout the map.

• Each line has a fixed value and is drawn on the map through the points which have the same elevation.
• A number of contours are drawn with some fixed interval, called contour interval, to represent the relief of the entire region under consideration.
• Contour interval varies from map to map. It depends on the range of elevation (difference between highest and lowest point in the map) and roughness of the terrain. In small scale maps variable intervals can be used occasionally. But in large scale maps fixed intervals are always suggested. (Note: SOI toposheets).
• No contours cut or cross each other. Spacing of contours is crucial as it determines the change of elevation per unit horizontal distance (Gradient or Slope). Closer the contours steeper the slope.
• Some contours are drawn with spot heights and others are interpolated.
• Major/ significant contours are highlighted.
• To represent the cliff (where several contours superimposed at one point) strip or superimposed shading is done.

Advantage and Disadvantage:

• It provides detailed information about topography of the region, pattern of relief, type, amount and direction of slope, ruggedness, ridges, valleys, hills, col, saddle, plateau, etc.
• Contours are easily converted into 3 Dimensional Diagrams to visualise the Surface configuration.
• Sometimes contours are shaded (plastic shading) with some fixed range for better visualisation of the elevation zones and visualise the 3rd dimension.
• It is very complex and time consuming to draw for a large region with many spot heights. Computer technology helps much in this regard.
• Minor difference in the elevation or the change of relief within the contour interval is not reflected.
• In most cases interpolation is used to draw most of the contours based on some limited number of elevation points. So true representation is not done.
• With changing contour intervals the pattern of the contours may change.
• Simple orthographic contour representation is not able to visualise the 3rd dimension of relief.

3. Tanaka Kitiro’s Method of Contouring: It is another way of representation of surface relief by inclined contours (derived from joining the intersections of the orthographic contours with the horizontal straight lines).

In this method the contours visualise the slopes, thus more visual effects of the 3D surface is achieved.

4. Layer shading and tinting: To represent relatively high and low areas, areas lying in between successive contours are shaded or tinted by variation of the same colour.

It is very useful in representing topographic features.

5. Hachures: These are disconnected lines drawn down the slope in the direction of steepest gradient. Conventionally they are drawn closely where the slope is steeper.

Several methods of hachurings are there.  

Advantage and Disadvantage: It gives visual representation of the topography. It enables minor differences in relief.

The main advantages of this method are lack of absolute information.

Plastic shading/ Hill shading: While contour lines can represent the Earth's surface, they do not elevate the three-dimensional form of the Earth's surface. Therefore, the intermediate areas between contour lines are represented by adding colour, depicting the difference in elevation and slope more effectively. In this case, the coastal areas are given a lighter colour. Colours are used in such a way that when light is cast from above or from one side, the depiction matches what is observed.

Just as illuminated and shaded areas contribute to elevating the Earth's surface in three dimensions, contour lines on a topographic map are elevated using similar color techniques.

Terrain Mapping/ Landscape Colour Map: Often, it is necessary to depict the primary landforms of the Earth's surface. When showcasing major landforms such as mountains, plateaus, plains, etc., separate colours are used for different regions. Areas like snowy regions or river basins are given distinct colours as needed.

Typically, this is done on small-scale maps.

Physiographic maps (pictorial relief/ Block diagram): This is essentially a three-dimensional representation of the Earth's surface in geometric form. The development of this method, primarily undertaken by American geographers, involves drawing three-dimensional maps or block diagrams of the Earth's surface using various symbols and shading techniques.

These representations are visually interpreted, and contour lines are used to accentuate specific methods of representation. Regardless of the method used, they provide a stereoscopic perspective. It is based on isometric projection, where a clear geometric figure (cube, pyramid, cone, etc.) is used to represent the three-dimensional form of the Earth's surface, maintaining the same scale in both horizontal and vertical directions. In perspective projection, the vertical and horizontal scales are generally not the same. Typically, the way we perceive natural landscapes is represented in block diagrams in a manner that reflects what we observe. As a result, nearby objects appear larger, while distant objects appear proportionally smaller.

Panoramic block diagrams are usually drawn horizontally, somewhat like a cross-section, especially when representing geological formations, providing a clear depiction of the relationship between the Earth's surface and its geological structure. In this method, although it is possible to represent the topography of a small area with accuracy, it becomes impractical for larger regions.

B. Morphometric Analysis: The quantitative analysis of the shape of the Earth's surface is an important aspect in the field of geomorphology. Geographers measure and analyze various characteristics of the Earth's surface, such as relief, slope, linear aspects of drainage basins, and areal aspects of basins, and map them through analysis. This process is known as morphometric analysis.

1. Profiles: The external form of the Earth's surface, such as where it is elevated or depressed, where the slope is gentle or steep, the nature of the slope, and the shape of the landforms, is represented by drawing a line across the Earth's surface, known as a profile. This profile beautifully illustrates how the shape of the Earth's surface will be along a certain line.

Various types of profiles are drawn to represent different aspects of the Earth's surface. For example:

• Serial profile: In this case, several profiles are drawn at regular intervals. These profiles help in understanding changes in the Earth's surface. Multiple profiles are drawn for various purposes, such as superimposed profiles, where all profiles are drawn on top of each other in a single frame, allowing the use of different colors or line symbols. This type of profile helps easily identify the average elevation of that area, which is useful for indicating planation surfaces.
• Projected profile: In this case, serial profiles are drawn in such a way that they are superimposed according to their elevations (adding from lower to higher elevations) within the same frame, creating a perspective view of the Earth's surface at that location. It is as if we are seeing a range of distant hills from a flat area, where one rocky ridge rises behind the other.
• Composite profile: This is a type of composite profile where all profiles are added only at their highest points, indicating the summit levels of that area.

• River Profile: Apart from depicting the morphometric profile of the Earth's surface, profiles are also drawn along the length of a river, i.e., from the source to the mouth, which is known as a river profile. These profiles are drawn longitudinally along the course of the river (long profile), and profiles are sketched at different locations along the river.

From such longitudinal cross-sections, the variations in the landform along the river's course, such as changes in slope, the location of waterfalls, the position of confluences, and the elevation of river terraces, can be understood. Additionally, by examining cross-sections across the river's width, the shape of the river channel and various features of the river's floodplain under different flow conditions can be identified. This includes the V-shape, I-shape, and U-shape characteristics of river channels.

2. Slope and Gradient Analysis: Slope is a crucial factor that indicates the characteristics of any landform. Therefore, measuring and analysing the slope of the Earth's surface, along with mapping the regional variations in slope, are important tasks. Various geographic terrains have undergone measurement, analysis, and mapping of their slopes.

• Average slope: Wentworth introduced a special method that can measure the average slope of a small area on the Earth's surface. Using this method, the average slope values for several areas can be obtained, facilitating easy mapping in the Isopleth method.

Typically, in a map, grid-based calculations are used to determine the average slope for each grid, and then the overall slope values are determined. While this method provides information about the slope values, it does not indicate the orientation or direction of the slope.

• Raisz and Henry's Slope analysis:
• Robinson's Slope Analysis:
• Strahler's Slope Analysis: 

3. Relative Relief (Smith): Typically, in a map, grid-based calculations are used to determine the highest and lowest elevations for each grid, and then the ratio of the maximum elevation to the minimum elevation is determined. The relative relief is an important indicator in expressing the topographical variations of the terrain.

4. Dissection Index (Dov Nir): Typically, in a map, grid-based calculations are used to determine the relative elevation and the ratio of the relative elevation to the maximum elevation for each grid. In this case, the value ranges from zero to one.

5. Ruggedness Number: Generally, grid-based calculations are performed on a map to determine the relative elevation and the product of the elevation and river density, with the result multiplied by a constant factor (K). The ruggedness number serves as an important indicator in expressing the terrain's topographical characteristics.

6. Drainage Density: Typically, grid-based calculations are carried out on a map, and for each grid, the length of rivers is calculated per unit area. This helps in understanding the variation in river density concerning the topography of the terrain.

7. Drainage Frequency: Typically, grid-based calculations are performed on a map, and for each grid, the number of rivers is calculated per unit area. This helps in understanding the variation in river density concerning the topography of the terrain.

***Morphometric Analysis will be discussed in detail in the Topographical Map Section.***