Thematic Maps and Thematic Mapping Techniques

Posted on April 24, 2018 by Rashid Faridi

A thematic map is a type of map specifically designed to “show a particular theme connected with a specific geographic area.

Unlike reference maps, which tell us where something is, thematic maps tell us how something is.
There are a number of visualization techniques and thematic map types that have different applications depending on the type of data that you are exploring and the type of spatial analysis that you are looking to do. The methodology and the type of map that you want to create may be different, for example, if you are exploring global shipping data or voter propensity, or environmental disaster impact.

All thematic maps use maps with coastlines, city locations and political boundaries as their base maps. The map’s specific theme is then layered onto this base map via different mapping programs and technologies like a geographic information system (GIS).

Broad Types

Quantitative 

Choropleth: Color gradient

Chorochromatic: Various Color

Choroschematic: Shown by Symbols

Isopleth:  Lines

Proportional Symbols: Size of symbols

Dot Density Maps: Show Population

Qualitative

Qualitative thematic maps are not dependent on numbers (data). For example, One can map the hospitals in Manhattan without counting them. I may only be interested in their locations to see where the medical gaps are. Then one could layer the map with population data to determine how many people are served, theoretically, by each one—and then compare that ratio with ones for the other boroughs of New York City.Following  are some examples

  • Communication and Transportation Maps
  • Ethnography Maps
  • Linguistic Maps
  • Military History Maps
  • Urban Planning Maps

History of Thematic Maps
Thematic maps did not develop as a map type until the mid-17th Century because accurate base maps were not present prior to this time. Once they became accurate enough to display coastlines, cities and other boundaries correctly, the first thematic maps were created. In 1686 for example, Edmond Halley, an astronomer from England, developed a star chart. In that same year, he published the first meteorological chart using base maps as his reference in an article he published about trade winds. In 1701, Halley also published the first chart to show lines of magnetic variation- a thematic map that later became useful in navigation.Halley’s maps were largely used for navigation and the study of the physical environment.

 In 1854, John Snow, a doctor from London created the first thematic map used for problem analysis when he mapped cholera’s spread throughout the city. He began with a base map of London’s neighborhoods that included all streets and water pump locations. He then mapped the locations where people died from cholera on that base map and was able to find that the deaths clustered around one pump and determined that the water coming from the pump was the cause of cholera.

 In addition to these maps, the first map of Paris showing population density was developed by a French engineer named Louis-Leger Vauthier. It used isolines (a line connecting points of equal value) to show population distribution throughout the city and was believed to be the first use of isolines to display a theme that did not have to do with physical geography.

Lets take a look at some popular thematic maps and mapping techniques.

 Choropleth maps
A choropleth map is a thematic map where geographic regions are colored, shaded, or patterned in relation to a value.This type of map is particularly useful when visualizing a variable and how it changes across defined regions or geopolitical areas.

For example, a choropleth map is extremely useful when looking at vote totals by political party per county in the United States, as below. In a choropleth map, color can be used to represent distinct attributes or, as in the example below, to represent weight of a value (a strong or weak party vote-share shown as light or dark colors).

Isarithmic or Isopleth
Isarithmic maps, also known as contour maps or isopleth maps depict smooth continuous phenomena such as precipitation or elevation. Each line-bounded area on this type of map represents a region with the same value. For example, on an elevation map, each elevation line indicates an area at the listed elevation. An Isarithmic map is a planimetric graphic representation of a 3-D surface. Isarithmic mapping requires 3-D thinking for surfaces that vary spatially.

Heat maps
A heat map represents the intensity of an incident’s occurrence within a dataset. A heatmap uses color to represent intensity, though unlike a choropleth map, a heatmap does not use geographical or geo-political boundaries to group data. This technique requires point geometries, as you are looking to map the frequency of an occurrence at a specific point.

Visualizing the intensity of occurrence using a heat map is a technique commonly used when tracking weather and natural phenomena, in which established borders and boundaries are less useful for understanding impact areas. In the heat map below, drought conditions across the United States are visualized based on intensity, giving us a greater understanding of past and potential impact areas.

Proportional symbol maps
A proportional symbol map can represent data tied to a specific geographical point or data that is aggregated to a point from a wider area.

In these maps, a symbol is used to represent the data at that specific or aggregate point, and then scaled by value, so that a larger symbol represents a greater value. The size of each symbol can be proportional to the value you are visualizing or you can set 3 to 5 ‘classes’ of values allowing for comparison and classification of locations.

Dot density maps
A dot density map uses a dot to represent a feature or attribute in your data.

Some dot density maps are ‘one-to-one’ in which each dot represents a single occurrence or data point, or ‘one to many’ in which each dot represents a set of aggregated data, for example one dot may represent 100 individuals with a certain attribute. Both of these types of dot density map visualize the scatter of your data, which can provide insights into where instances of an occurrence are clustered.

Animated time-series maps
More of a technique than a type, if your data has a temporal component (taking place over time), you can transform any of the above visualizations into an animated time-series map. Looking at your data over time can both improve your ability to gain insights and create a stronger and more compelling visual.

Putting your data on an appropriate time scale will allow you to make important business decisions. Mapping foot traffic over the course of a week, for example, may inform hours of operation for a retail location while mapping and animating a century’s worth of sea level measurements can paint a vivid picture on the impact of global climate change.

With many applications from social listening to resource management to demographic projection, animating your data as a time-series map unlocks a new dimension at which to view your data.

Cartograms
A dasymetric map is an alternative to a choropleth map. As with a choropleth map, data are collected by enumeration units. But instead of mapping the data so that the region appears uniform, ancillary information is used to model internal distribution of the phenomenon. For example, population density will be much lower in forested area than urbanized area, so in a common operation, land cover data (forest, water, grassland, urbanization) may be used to model the distribution of population reported by census enumeration unit such as a tract or county.

Source(s):

Carto

Wikipedia

ThoughtCo

 

Posted in Class Notes, earth, Geography Practicals/Lab and Statistical Techniques | Leave a comment

Migration :Why and How people Migrate or they Dont

Posted on April 23, 2018 by Rashid Faridi

Migration is defined as a permanent or semi-permanent change of residence with an opening intention of settling at destination region permanently or temporarily of an individual or group of people over a significant distance.

The roving instinct, it is said, is intrinsic to human nature: the need to search for food, pasture, and resources; the desire to travel and explore; but also to conquer and possess. Population movements have been the carriers of innovation from one region to another.in In this quest, he becomes a tourist and a migrant. there is a strong inter-linkage between these two.

Cause of migration can be a natural calamity, climatic change, epidem­ics, over population, better employment opportunities, desire to get rich quickly need for political freedom.

Migration can be voluntary and involuntary. People migrate for economic benefits under voluntary migration and involuntary includes social, religious and political. It can be short-term where people move for short periods and long-term where they move for good. The long-term migration is called emigration.

 The latest figures from the United Nations Population Division inform us that, as of 2010, there are 214 million international migrants in the world. If all these migrants were put in a country of their own, it would be the fifth-largest in the world.

There are some pressing questions. Why has such a large proportion of the world’s population not migrated? Is it because they do not want to, or do not have the need to? Is it because their ‘moorings’ are holding them firmly in place – their family ties, jobs, culture, familiarity and simply feeling ‘at home’? Or could it be that many millions would want to migrate, but are prevented from doing so, either by their own poverty which isolates them (they do not have a passport, and/or cannot pay for the ticket to travel) or because of the political and institutional barriers to their movement? These can be called intervening obstacles to migrate. It is one of the ironies of globalization that whilst goods, capital, knowledge, entrepreneurship, and the media are free to flow across borders, labour, that other crucial factor of production, is not. In fact, on the whole, people are less free to migrate now than they were a hundred years ago.

Hence, the otherwise attractive notion of the ‘age of migration’ needs to be qualified: migration for some, but not for others. Fine if you are white, from a wealthy country in Europe, North America or elsewhere in the developed world, or if you have money to invest or valuable skills to deploy. But if you are from a poor country in Africa, Latin America or parts of Asia: forget it. Basing his analysis on the empirical example of Cape Verde, an island country with a long tradition of emigration to various parts of the world, Jørgen Carling draws attention to the separation between Cape Verdeans’ widespread aspiration to migrate, and their current inability to do so. For them, the ‘age of migration’ has become the ‘age of involuntary immobility’ (Carling 2002).

 The UNPD’s figure of 214 million can be regarded as a ‘best estimate’ but it obscures two major statistical problems. First, the criteria for defining who is a migrant vary from country to country, the chief difference being between citizenship and birthplace or prior residence. Naturalisation converts foreign-born immigrants into citizens and thus removes them from the migration count if citizenship is the criterion of measurement. People born in the host country to immigrant parents – the  ‘second generation’ – can remain classified as non-citizens on the ius sanguinis or ‘blood’ rule and thus be counted as part of the ‘foreign’ or ‘immigrant’ population, even though they themselves have not immigrated. The second problem is the – by definition unknown – quantity of ‘undocumented’ or ‘irregular’ immigrants, often branded ‘illegal immigrants’.Migration is important because of the way it shapes and re-shapes societies, making them more diverse and complex. But it also creates sharp divisions between those who accept the need for migrants and welcome the economic and cultural contributions they make, and those who oppose them. The latter group, politically motivated, often exaggerate the numbers of migrants, employ repeated use of prejudicial terms such as ‘illegal immigrants’ and ‘bogus asylum-seekers’ and tend to scapegoat migrants for the ills of the society they seek to join – like crime, drugs and unemployment. These anti-migration discourses need to be confronted by a more objective analysis of the process of migration, starting with a recognition of the diversity of the phenomenon.

Links(s) and Source(s):

Click here

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Isopleths and Isopleth Maps

Posted on April 21, 2018 by Rashid Faridi

An Isopleths is a line drawn on a map through all points having the same value of some measurable quantity.

The following is a list of all the important isopleths encountered in operational meteorology. An isopleth is a line or curve of equal values.

Constant Pressure Surface
Most analysis and model images are shown using a pressure surface. The most common are the 1000 mb, 850 mb, 700 mb, 500 mb, and 300 mb surfaces. Every location on the image has the same pressure, however, heights will vary (thus the contouring of height contours). Below is a listing of pressure surfaces and their approximate height above zero geopotential meters.

Isallobar / Height change contours
A line of equal pressure change. They are used to forecast the propagation of low and high pressure systems. Low pressure tends to develop toward regions of the greatest pressure falls (height falls). Heights and pressures fall due to the evacuation of mass in the upper levels of the atmosphere and the chilling of air within a vertical atmospheric column.

Isobar
A line of constant pressure. Isobars are found ONLY on surface charts. They most commonly connect lines of equal pressure in the units of millibars. High pressure isobars generally occurs with isobars above 1010 mb while low pressure isobars occur with lower than 1010 millibars. Isobars “kink” along fronts and otherwise have a smooth curved trajectory. Isobars represent the pressure at zero geopotential meters. This is done to compensate for elevation changes. Isobars of close proximity represent higher wind speeds than isobars of wide spacing. When isobars are “pack together”, this represents an increase in the pressure gradient force and thus stronger winds. The pressure on an analysis chart in Colorado may be near 1028 mb on the chart when in reality the surface pressure is closer to 850 mb. This surface chart has isobars (solid lines).

Isodop
Contour of constant doppler velocity values.

Isodrosotherms
A line of equal dewpoint. They are contoured most often in the low levels of the atmosphere. Isodrosotherms can be used to locate frontal boundaries, regions of moist air or dry air advection, and mesoscale precipitation boundaries. The following image is an example of isodrosotherms (the colored lines). The highest dewpoints are often found bordering the Gulf of Mexico.

Isohyet
Contour of constant rainfall. Used to assess soil moisture, flooding potential, mesoscale wet/dry boundaries, and rainfall coverage as well as intensity. This image shows an example of isohyets.

Isohypse (height contour)
A line of equal geopotential height. Geopotential assumes the earth is perfectly flat and a perfect sphere. The geopotential height is the distance above the Earth’s surface if it was a perfect and flat sphere. Isohypse are shown on a constant pressure surface. For example, when looking at a 850 mb chart, all isohypses no matter their value are located at 850 mb. Regions of low isohypse values are correlated with low pressure (trough) while high isohypse values are correlated with high pressure (ridge). This 850 chart has height contours. The lines are the isohypses. Above 850 mb, the wind flows close to parallel to the height contours. A curving down of the height contours represents a trough while a curving up a ridge.

Isotachs
These are lines of equal wind speed. They are most often contoured in the upper levels of the atmosphere, especially at the jet stream level. They are important for locating the jet stream and jet streaks within a jet stream. This 300 mb image is an example of isotachs.

Isotherm
A line of equal temperature. Each of the analysis charts will show isotherms in either a 2,4,5 or 10 degree increment. They are most commonly used at pressure surfaces below 500 millibars and on surface charts. Isotherms are used to find regions with warm air advection and cold air advection as well as short waves, fronts, temperature gradient boundaries, and instability zones. This 850 mb model panel shows isotherms.

Streamlines
Lines of equal wind direction. They are not a pure isopleth by definition. Streamlines are used primarily in tropical regions since the pressure gradient is weak. They show areas of convergence, divergence and pressure circulation.

Thickness lines
Same as an isohypse except they represent the distance from one pressure level to a selected pressure level (usually 1000 to 500 mb thickness). They are used to forecast snow, cold air advection, and warm air advection. This image is an example of thickness lines (dashed lines). Colder air (since it is denser) will have a smaller thickness than warm air.

ISOPLETH MAPS

An isopleth map generalizes and simplifies data with a continuous distribution. It shows the data as a third dimension on a map, thus isopleth maps are more common for mapping surface elevations, amounts of precipitation, atmospheric pressure, and numerous other measurements that can be viewed statistically as a third dimension.

The third dimension is shown by a series of lines called isopleths which connect points of equal value. The isopleth interval is the difference in value between two adjacent isopleths. Note, the values of the isopleths drawn on the map are ALWAYS multiples of the interval. Isopleths never cross or divide and always form enclosed circles, however, this occurrence may not be in the mapped area.

Source(s) and Link(s):

weather.gov

Jsu.edu

 

Posted in Class Notes, earth, Geography Practicals/Lab and Statistical Techniques | 2 Comments

Rail Tourism in India and Pakistan (from the site of Salman Rashid)

Posted on April 20, 2018 by Rashid Faridi

Railways is among the more enduring legacies of the British Raj in the subcontinent. There is virtually an inexhaustible body of extremely interesting lore and history of the building of this great system of transportation discussed in a few excellent books and in the esoteric journals in the Punjab Archives. It is another story that the ignorant and asinine bureaucrats do not permit access to that great treasure trove.

Even if one has not read about the intricacies and heroism of the laying of the line from, say, Ruk (near Shikarpur) to Sibi, one can still stand on the platform of Ruk and wonder what the letters KSR and IVSR that adorn the façade in blue on white ceramic tiles mean. The lettering signifies that this little-known station was the junction of the Indus Valley State Railway coming up from Kotri and the new line to Quetta and Chaman called the Kandahar State Railway.

As railway stations go, the all-time favourite for anyone should really be the quaintly turreted, mud-plastered station of Kan Mehtarzai. Sitting at 2,224 metres above the sea, on the narrow-gauge Zhob Valley Railway (ZVR) between Bostan and Zhob, this was the highest train station in Pakistan. Once railwaymen told hair-raising tales of those lovely toy trains becoming snow-bound in winter. There were particularly hairy stories of the blizzards during the 1970s and one can imagine how anxiety making a journey on ZVR could be.

After this service was suspended in 1986, Kan Mehtarzai was forgotten. For many years, the line and other bits of infrastructure remained in place, but in 2008, all steel fittings were auctioned and removed. This is Pakistan Railways’ highest line in terms of height about sea level and surely there is some merit in revitalising it at least from Bostan as far as Kan Mehtarzai.

In Balochistan, too, there is what its builders labelled ‘The Lonely Line’. This is the line that travellers ride on their way to Iran. It was lonely because back in 1916 when they first started to lay it, they would go a hundred and fifty kilometres between villages. The sand and gravel desert that the line wended its way was littered with the bleached bones of camels and horses.

Today, as one rides the irregular service or drives on the highway parallel to the line, one passes by deserted and ruined railway stations that are already being overtaken by the desert that had once made way for them.

On the subject of the attraction of railway stations, one simply cannot neglect Attock Khurd. A little doll’s house of a building with a pitched red roof and sitting under the crenulated turrets of the magnificent Attock Bridge, it belongs to a film set. Without doubt this is the most picturesque railway station in the entire country. If Pakistan Railways has been neglectful of its heritage on the whole, it has redeemed itself by turning Attock into a tourist destination.

Station buildings are not the only railway attraction, however. There are some magnificent bridges to be seen as well. My favourite is the remains of the Princess Louise Margaret Bridge in the Chhappar Rift, north of Sibi, in Balochistan. This line remained operational only for some six decades and was dismantled in the summer of 1942. But when it ran, it was the first connection between the Sindh plains and Quetta in the hills. The line through the Bolan Pass came four decades after its launch.

Then there are the very picturesque spans of line from Attock to Daudkhel. Though they all merit a visit, the winner among them is the superlative piece of construction across the Soan River. If the Louise Margaret bridge was once the highest in Pakistan (65 metres above the riverbed), the Soan Bridge now holds the honour.

Of bridges there is no death, however. The little known Victoria Bridge across Jhelum on the line from Malakwal to Gharibwal and Khewra is a treat for the ordinary tourist as well as the students of architecture. It is no exaggeration that its handsome criss-cross of red girders forming the flat arch above and the parallel lines below make a fantastic study of shapes for the artist and photographer.

One thing that we can be eternally thankful to railway managers for is the rehabilitation of Golra station and its conversion into a railway heritage museum. Lying just outside Islamabad, it is within easy reach of all. Within its walls it preserves an array of railway memorabilia ranging from crockery to signalling equipment and clocks to ceiling fans, platform furniture and other equipment. On the platform outside sits a metre gauge carriage that was once the private saloon of the Maharana of Jodhpur.

Source:

Salman Rashids Site

 

Posted in Class Notes, earth, India, opinions, Reblogged | Leave a comment