Chapter 3 Graphical Representation Of Data

Thematic Maps

While graphs and diagrams effectively compare data and show internal variations, they often lack a regional perspective. Thematic maps are specifically designed to show the spatial distribution patterns or variations of a particular theme or phenomenon over a geographical area. These maps are also known as distribution maps.

Requirements For Making A Thematic Map

To create a thematic map, you generally need:

• Statistical data for the chosen theme, corresponding to specific administrative units (state, district, etc.) or point locations.
• An outline map of the study area showing the administrative boundaries or the relevant physical features.
• A physical map (e.g., physiographic, relief, drainage) of the region can be helpful for accurately depicting distributions influenced by physical factors (e.g., placing dots in population distribution maps considering mountains, deserts).

Rules For Making Thematic Maps

Thematic map creation requires careful planning and execution:

1. The map should clearly display essential components: Name of the area, Title of the subject, Source of data and year, Legend explaining symbols/shading/colours, and Scale.
2. Select a suitable thematic mapping method based on the nature of the data (e.g., dot map for distribution, choropleth for density/rates, isopleth for continuous data).

Classification Of Thematic Maps Based On Method Of Construction

Thematic maps are broadly classified into quantitative and non-quantitative types.

• Quantitative Maps (Statistical Maps): Show variations in data values. They use visual techniques (dots, shading, lines) where the quantity or intensity corresponds to the data value (e.g., map showing rainfall categories: >200cm, 100-200cm).
• Non-quantitative Maps (Qualitative Maps): Depict non-measurable characteristics or categories in distribution (e.g., map showing areas of high vs. low rainfall, different types of vegetation).

We will focus on the methods for constructing quantitative thematic maps:

Dot Maps

Dot maps are used to show the distribution of phenomena where individual units can be represented by dots of a specific value (e.g., population, cattle, crop production). Dots of the same size are placed within administrative units or over point locations on the map according to a chosen scale. The total number of dots within an area represents the total quantity of the phenomenon in that area. The visual density of dots highlights the distribution patterns.

Requirements:

• Administrative map with boundaries.
• Statistical data for the chosen theme for each administrative unit.
• A suitable scale determining the value represented by a single dot.
• Physiographic map to guide accurate placement of dots, especially in areas where distribution is influenced by terrain (e.g., avoiding placing dots in mountainous regions if representing population).

Precautions: Use thin lines for boundaries. All dots must be of uniform size.

Answer:

Construction Steps:

1. Obtain an outline map of India showing state boundaries.
2. Select the size and value of a dot (here, 1 dot = 100,000 persons).
3. For each state, calculate the number of dots needed by dividing the state's population by the value of one dot (e.g., for Maharashtra: 96,752,247 / 100,000 = 967.52, rounded to 968 dots). Table 3.9 part 2 shows the calculated number of dots for each state.
4. Place the calculated number of dots within the boundaries of each state. Dots should be placed where people actually live, considering physiographic features. Fewer dots should be placed in mountainous areas, deserts, and snow-covered regions (referencing a physiographic map) than in plains or populated areas.
5. Include a legend showing the value of a dot and other standard map design elements (title, source, year).

Following these steps results in the following dot map of India's population:

Choropleth Map

Choropleth maps are used to represent data that are related to administrative units (states, districts, blocks). They are suitable for showing variations in density, rates, or percentages across these units (e.g., population density, literacy rates, sex ratio, percentage of area under a crop). The administrative units are shaded or coloured with different patterns or hues, where the intensity of the shading/colour corresponds to the data value within that unit. Higher data values are typically represented by darker shades or more intense colours.

Requirements:

• An outline map showing the boundaries of administrative units.
• Appropriate statistical data (density, rate, percentage) for each administrative unit.

Steps for Construction:

1. Arrange the statistical data for all administrative units in ascending or descending order.
2. Group the data into a suitable number of categories, typically 5 (e.g., very high, high, medium, low, very low concentration).
3. Determine the class intervals for these categories. A common method is to divide the range of the data (maximum value minus minimum value) by the number of categories (e.g., Range/5). Adjust the intervals to convenient round numbers if necessary.
4. Assign different patterns, shades, or colours to each category. The patterns or shades should be arranged in a logical sequence (e.g., from light to dark) corresponding to the order of categories (e.g., lighter shades for lower values, darker for higher values).
5. On the map, shade or colour each administrative unit according to the category its data value falls into.
6. Add a legend clearly showing the categories and their corresponding shades/patterns. Include all other standard map design elements (title, source, year, name of area).

Answer:

Construction Steps:

1. Obtain an outline map of India showing state boundaries.
2. Arrange the literacy rate data for all states/UTs in ascending order (as provided in Table 3.10 part 2).
3. Identify the range: Minimum literacy rate = 47.0% (Bihar), Maximum literacy rate = 90.9% (Kerala). Range = $90.9 - 47.0 = 43.9$.
4. Decide on the number of categories (here, 5). Calculate the approximate interval width: Range / 5 = $43.9 / 5 = 8.78$. Round this to a convenient number, like 9.0.
5. Determine the class intervals and assign categories (e.g., Very Low, Low, Medium, High, Very High):
   • 47.0 - 55.9: Very Low (approx. 47-56)
   • 56.0 - 64.9: Low (approx. 56-65)
   • 65.0 - 73.9: Medium (approx. 65-74)
   • 74.0 - 82.9: High (approx. 74-83)
   • 83.0 - 91.9: Very High (approx. 83-92)
6. Assign shading patterns or colours to each category, ranging from light for the 'Very Low' category to dark for the 'Very High' category.
7. On the map, shade each state/UT according to the category its literacy rate falls into.
8. Add a legend showing the categories and their corresponding shades/patterns. Include title, source, year, and name of area.

Following these steps results in the following Choropleth map:

Isopleth Map

Isopleth maps are used to represent the spatial variations of continuous data that are not tied to administrative boundaries, such as temperature, pressure, rainfall, or elevation. These maps depict variation by drawing lines (isopleths) that connect places of equal value for the specific phenomenon. The word "Isopleth" comes from "iso" (equal) and "pleth" (lines).

Common examples of isopleths include Isotherms (equal temperature), Isobars (equal pressure), Isohyets (equal rainfall), Contours (equal height), Isobaths (equal depth), etc.

Requirements:

• A base map showing the locations where data was collected (point locations).
• Appropriate data values (temperature, rainfall, etc.) recorded at these point locations.
• Drawing instruments, especially a French Curve for drawing smooth lines.

Rules for Drawing:

• Select an equal interval between the values of consecutive isopleths (e.g., lines for 10°C, 20°C, 30°C). Intervals of 5, 10, or 20 are often considered ideal.
• Write the value of the isopleth along the line, either by the side or by breaking the line.

Interpolation

Drawing isopleths often requires interpolation, which is the process of estimating the intermediate values between known data points. Specifically, it involves locating the exact point on a map between two known data points where a particular isopleth value would pass.

Method of Interpolation:

1. Determine the minimum and maximum data values on the map.
2. Calculate the range: Range = maximum value - minimum value.
3. Based on the range, determine the suitable interval for drawing isopleths (e.g., every 5 units, 10 units).
4. To find the exact point where an isopleth passes between two known points, assume a linear relationship between the two points. Measure the distance between the two points on the map. Calculate the difference in data values between the two points. Then, for the desired isopleth value, determine its "interval" or difference from one of the known points. Use a formula to find the distance from that known point where the isopleth should be drawn.
5. For example, if Point A is 28°C and Point B is 33°C, and the distance between them is 1 cm (10 mm). To draw the 30°C Isotherm:
   • Difference between values = $33 - 28 = 5$.
   • Desired isopleth value = 30°C.
   • Interval from Point A (28°C) to 30°C = $30 - 28 = 2$.
   • Use proportionality: If 5 units difference spans 10 mm, then 2 units difference spans $ (2/5) \times 10 \text{ mm} = 4 \text{ mm} $.
6. So, the 30°C isotherm passes 4 mm away from Point A (28°C) along the line towards Point B. Alternatively, the interval from Point B (33°C) to 30°C is $33 - 30 = 3$. Distance from B is $ (3/5) \times 10 \text{ mm} = 6 \text{ mm} $. The point is 4 mm from A and 6 mm from B, summing to the total distance of 10 mm.
7. After locating points for the same isopleth value based on interpolation between multiple pairs of points, connect these points with a smooth curve using a French Curve.
8. Start by drawing the isopleth with the minimum value first, then proceed to higher values at the selected intervals.