You can use a choropleth maps when your data are 1 attached to enumeration units e. For example, number of people is a count and not appropriate for choropleth maps; number of people per square mile is a ratio and is a continuous statistical surface even if it drops to zero over uninhabited places, every location has a data value and, thus, is appropriate for choropleth maps. Choropleth maps are extremely popular, probably the most common thematic map in use today.
By comparison, tax rates are very closely tied to enumeration units, do change abruptly, and make perfect sense as a choropleth map. The less the thing you are mapping is tied to enumeration units, the less sense a choropleth map makes. Not sure you should use a choropleth map? Below is a 5-class choropleth map that uses a sequential color scheme from light to dark attached to an equal-interval classification scheme.
Note that the appearance of the choropleth colors will appear to change depending on what other colors are used on the map, such as blue water or black city labels.
The colors of the enumeration unit borders county and state lines here also have a very large impact on the look of the map, so experiment with both fill and stroke color combinations. You may even decide to not draw those enumeration unit borders no stroke, just fill.
Note: it may be harder for your audience to locate places on the map without those borders. For a more complete discussion of color in thematic mapping, have a look ColorBrewer.
If you want to be safe, make a map with 3—7 data classes. The more classes you use, the less data generalization which is goodbut this comes at the expense of legibility and the associated risk of map reading errors since more colors are harder to see and print reliably which is bad.
The key question is how much generalization do you want? There is no ideal number of classes for a map, so experiment. Not sure how many classes to use? Have a look at the distribution of your data in a histogram see examples below : Are there obvious clusters within your data?
Are there large gaps in your data range that suggest nice compact data classes? If so, pick that number of classes and place those class breaks around those clusters. Just as there is no single correct number of classes, there is no single best way to classify you data into ranges.
Above all else the goal of data classification is to put places with similar rates in the same class, and separate places with very different rates into different classes.
The form of this histogram suggests that 3 or 4 data classes seem most appropriate. Outliers in that case will likely produce empty classes, wasting perfectly good classes with no observations in them. The problem with quantiles is that you can end up with classes that have very different numerical ranges e.
Quantiles can also separate locations with very similar rates and group together places that have very different rates, which is very undesirable, so use the histogram to see if this is happening. CAUTION: In the hotel room example above, the quantile produced a questionable class break by lumping a portion of the third cluster back into class 2, despite it being much closer numerically to the other observations in class.
One drawback of this approach is each dataset generates a unique classification solution, and if you need to make comparison across maps, such as in an atlas or a series e. Does one of the class breaks need to be the mean? Is this map part of a series that needs the same classes across all of the maps so that the colors always refer to the same numbers on any map?This is a case study of creating a colorful interactive choropleth map of US States Population Density with the help of GeoJSON and some custom controls that will hopefully convince all the remaining major news and government websites that do not use Leaflet yet to start doing so.
As the amount of data state shapes and the density value for each state is not very big, the most convenient and simple way to store and then display it is GeoJSON. Now we need to color the states according to their population density. Using the values we got from it, we create a function that returns a color based on population density:.
Here we get access to the layer that was hovered through e. The handy geojson.
See this example stand-alone.Most often the variable is quantitative, with a color associated with an attribute value. Though not as common, it is possible to create a choropleth map with nominal data. Choropleth maps illustrate the value of a variable across the landscape with color that changes across the landscape within a particular geographic area. The earliest known choropleth map was created in by Baron Pierre Charles Dupin.
Red states and blue states
In a well-crafted choropleth map, each of these elements is carefully developed to clearly portray the natural geographic variation in the variable. That is, map readers should be able to intuitively see the real-world patterns without needing to work to decipher the elements themselves. Choropleth maps are based on statistical data summarized over a set of districts such as counties.
This summarization is called "standardization of data" which sets either a geographic or numerical standard to base the data. In choropleth data, the boundaries of the districts are defined a priori before adding data to the map and are not based on patterns in the variable being mapped; that is they are arbitrary with respect to the data. Typically, the value of the given variable for each district is a summary of a large number of individuals or smaller regions within that district, and any variation within the district is not reported.
For example, a choropleth map of median family income by county reports a single value for a county, which may contain neighborhoods of very high and very low income.
In contrast, chorochromatic area-class and isarithmic maps use regions that are defined by patterns in the phenomenon being mapped. The popularity of choropleth maps is largely due to the convenience of obtaining this kind of data since governments typically report statistical information e. Census that has been aggregated into well-known districts such as cities, counties, and provinces.
Where the defined regions are important to a discussion as in an election map divided by constituent jurisdictions or making policy for the regionschoropleth maps are ideal.
However, when real-world patterns in the variable may not conform to the chosen regions, a choropleth map can mask the true pattern and give rise to interpretation issues like the ecological fallacy and the modifiable areal unit problem MAUPso other techniques may be preferable. Unfortunately, choropleth maps are frequently used in inappropriate applications due to the abundance of data in this form and the ease of choropleth map creation using Geographic Information Systems.
While these issues are inherent to the a priori nature of the districts and cannot be eliminated, the problem can be mitigated by choosing districts that are very small with respect to the scale of the map, so that map readers are more likely to make interpretations based on large collections of districts rather than looking at a single large district and making assumptions about the variability therein.
For example, a choropleth map of the 3, counties in the United States is likely to be misinterpreted far less frequently than a choropleth map of the 50 states although there are counties in the West that are as large as states in the East and may still be misread. The dasymetric technique can be thought of as a solution to the districting problem in some situations. This technique uses other data sources to adjust the district boundaries.
Because the effective area of the district is changed, some variables such as population density need to be adjusted. Technically, GIS software can create a choropleth map from any statistical variable aggregated into districts. However, some variables are preferred while others are generally inappropriate. As with the choice of districts, this distinction is based on avoiding the likelihood of misinterpretation. The best variables for choropleth maps are those that can be conceptualized as continuous fields also called statistical surfaces or spatially intensive variablesin which the variable could be theoretically measured at an arbitrary point or small region.
Thus, a choropleth map is a discrete representation of a continuous field. For example, population density, median family income, and annual precipitation are all fields that can be appropriately mapped this way. Nominal field-type variables, such as "most prevalent primary language," are also appropriate for choropleth maps, as they are also statistical aggregations.
However, a colored map of a nominal variable that has only a single value for each district, such as the religious affiliation of the representative of each legislative constituency, are not technically choropleth maps because they do not represent statistical aggregate summaries of more detailed data, and because the district boundaries and the variable are intimately related in this case, because a single legislator represents that district.
Choropleth Maps in Python
Instead, these should be considered chorochromatic maps. Conversely, variables that are only meaningful for the entire district spatially extensive variables, such as total countsare typically avoided because they can be easily misinterpreted.
Representing data types such as total counts is not ideal because "large areas as a consequence of their size, are likely to include more of whatever the map is about, and thus contain darker symbols than smaller areas with equal or greater density" . Other thematic mapping techniques, such as proportional symbols are much more appropriate for visualizing total count variables. A simple method for determining whether a variable is spatially intensive appropriate for choropleth maps or spatially extensive problematic for choropleth maps is the "addition test.
Next, suppose you realign the districts so that these two become a single district. If you would expect the new district to have a value of 50 e. If you expect it to have a value of e. The problem with total counts arises when the districts are not all the same size in either area or total populationas in the figure at right.A Choropleth Map is a map composed of colored polygons. It is used to represent spatial variations of a quantity. This page documents how to build outline choropleth maps, but you can also build choropleth tile maps using our Mapbox trace types.
The GeoJSON data is passed to the geojson argument, and the data is passed into the z argument of choropleth traces.
Here we load unemployment data by county, also indexed by FIPS code. Note In this example we set layout. If the GeoJSON you are using either does not have an id field or you wish you use one of the keys in the properties field, you may use the featureidkey parameter to specify where to match the values of locations.
Note and disclaimer: cultural as opposed to physical features are by definition subject to change, debate and dispute. Plotly includes data from Natural Earth "as-is" and defers to the Natural Earth policy regarding disputed borders which read:. Natural Earth Vector draws boundaries of countries according to defacto status.
Everywhere in this page that you see figyou can display the same figure in a Dash for R application by passing it to the figure argument of the Graph component from the built-in dashCoreComponents package like this:. Black Lives Matter. Please consider donating to Black Girls Code today. GeoJSON with feature. CODE 1 Afghanistan Ask a question or support this project. Check out the Blog for more! For historical maps please visit our other website Historical Map Chart.
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World Simple. With Microstates. Europe Countries. United States States. USA and Canada. Historic Counties. Election Map. Census Divisions. France Regions. Asia Countries. The Americas Countries. Africa Countries.Since the United States presidential electionred states and blue states have referred to states of the United States whose voters predominantly choose either the Republican Party red or Democratic Party blue presidential candidates.
All states contain both liberal and conservative voters i.
The colors red and blue also feature on the United States flag. Traditional political mapmakers, at least throughout the 20th century, had used blue to represent the modern-day Republicans, as well as the earlier Federalist Party. This may have been a holdover from the Civil Warduring which the predominantly Republican north was considered "blue.
Later, in the presidential electionGrover Cleveland and Benjamin Harrison used maps that coded blue for the Republicans, the color perceived to represent the Union and " Lincoln 's Party", and red for the Democrats.
There was one historical use, associated with boss ruleof blue for Democrats and red for Republicans: in the late 19th century and early 20th century, Texas county election boards used color-coding to help Spanish-speaking and illiterate voters identify the parties;  however, this system was not applied consistently in Texas and was not replicated in any other state.
InThe New York Times printed a special color map, using blue for Democrats and yellow for Republicans, to detail Theodore Roosevelt 's electoral victory. The choice of colors reverses a long-standing convention of political colors whereby red symbols such as the red flag or red star are associated with left-wing politics and right-wing movements often choose blue as a contrasting color.
The advent of color television in America in the late s and early s prompted television news reporters to rely on color-coded electoral maps, though sources conflict as to the conventions they followed. One source claims that in the elections prior to every state that voted for Democratic candidates but one had been coded red. It further claims that from to in an attempt to avoid favoritism in color-coding the broadcast networks standardized on the convention of alternating every four years between blue and red the color used for the incumbent president 's party.
According to another source, inJohn Chancellorthe anchorman for NBC Nightly Newsasked his network's engineers to construct a large illuminated map of the United States. The map was placed in the network's election-night news studio. If Jimmy Carterthe Democratic candidate that year, won a state, it lit up in red whereas if Gerald Fordthe incumbent Republican President, carried a state, it was in blue.
NBC continued its color scheme blue for Republicans until ABC used yellow for Republicans and blue for Democrats inthen red for Republicans and blue for Democrats in, and Inwhen John Anderson ran a relatively high-profile campaign as an independent candidate, at least one network provisionally indicated that they would use yellow if he were to win a state. Similarly, at least one network would have used yellow to indicate a state won by Ross Perot in andthough neither of them did claim any states in any of these years.
In the days following the election, whose outcome was unclear for some time after election day, major media outlets began conforming to the same color scheme because the electoral map was continually in view, and conformity made for easy and instant viewer comprehension.
On election night that year, there was no coordinated effort to code Democratic states blue and Republican states red; the association gradually emerged. Partly as a result of this eventual and near-universal color-coding, the terms "red states" and "blue states" entered popular use in the weeks following the presidential election. After the results were final with the Republican George W. Bush winning, journalists stuck with the color scheme, as The Atlantic ' s December cover story by David Brooks entitled, "One Nation, Slightly Divisible", illustrated.
Thus, red and blue became fixed in the media and in many people's minds, despite the fact that the Democratic and Republican parties had not officially chosen colors. On March 14,the California Republican Party officially rejected red and adopted blue as its color. Archie Tse, The New York Times graphics editor who made the choice when the Times published its first color presidential election map inprovided a nonpolitical rationale for retaining the red—Republican link, explaining that "Both 'Republican' and 'red' start with the letter 'R.
There are several problems in creating and interpreting election maps. Popular vote data are necessarily aggregated at several levels, such as counties and states, which are then colored to show election results. Maps of this type are called choropleth mapswhich have several well-known problems that can result in interpretation bias.
One problem arises when areal units differ in size and significance, as is the case with election maps. These maps give extra visual weight to larger areal units, whether by county or state.
This problem is compounded in that the units are not equally significant. A large county or state in area may have fewer voters than a small one in area, for example. Some maps attempt to account for this by using cartogram methods, but the resulting distortion can make such maps difficult to read.Choropleth maps provide an easy way to visualize how a measurement varies across a geographic area or show the level of variability within a region.
A heat map or isarithmic map is similar but does not use a priori geographic areas. They are the most common type of thematic map because published statistical data from government or other sources is generally aggregated into well-known geographic units, such as countries, states, provinces, and counties, and thus they are relatively easy to create using GISspreadsheetsor other software tools.
The earliest known choropleth map was created in by Baron Pierre Charles Dupin. The term "choroplethe map" was introduced in by the geographer John Kirtland Wright in "Problems in Population Mapping".
Choropleth maps are based on statistical data aggregated over previously defined regions e. Thus, where defined regions are important to a discussion, as in an election map divided by electoral regions, choropleths are preferred. Choropleth maps are generalizations of an areal distribution where real-world patterns may not conform to the regional unit symbolized. While the use of smaller and more specific regional units such as census tracts, zip codes, or county boundaries can decrease the risk of an ecological fallacy and MAUP, it may inadvertently cause the map or the data displayed to appear more sophisticated than reality.
Although representing specific data in large regions can be misleading, it can make the map clearer and easier to interpret and remember. The dasymetric technique can be thought of as a compromise approach in many situations. Broadly speaking, choropleths represent two types of data: spatially extensive or spatially intensive.
Another common error in choropleth maps are the use of raw data values to represent magnitude rather than normalized values to produce a map of densities. The problem with using data in total counts arises when the polygons are not all the same size in area or total populationas in the figure at right.
Because a single color, representing a single value, is spread over the entire area of the district, large areas will be more dominant in the visual hierarchy than they should be, and are commonly misinterpreted as having larger values than smaller districts with the same color. To solve this issue, one can normalize the variable by dividing it by the total area, thus deriving densitywhich is a field.
Another solution is to represent total amounts using a proportional symbol map. Other valid forms of normalization for choropleth maps can be derived by computing ratios between two total amounts, such as rates of change e. When mapping quantitative data, a specific color progression should be used to depict the data properly. There are several different types of color progressions used by cartographers.
The following are described in detail in Robinson et al. Single-hue progressions fade from a dark shade of the chosen color to a very light or white shade of relatively the same hue. This is a common method used to map magnitude.
The darkest hue represents the greatest number in the data set and the lightest shade representing the least number. Two variables may be shown through the use of two overprinted single color scales. The hues typically used are from red to white for the first data set and blue to white for the second, they are then overprinted to produce varying hues.
These type of maps show the magnitude of the values in relation to each other. Bi-polar progressions are normally used with two opposite hues to show a change in value from negative to positive or on either side of some either central tendency, such as the mean of the variable being mapped or other significant value like room temperature.
For example, a typical progression when mapping temperatures is from dark blue for cold to dark red for hot with white in the middle. When one extreme can be considered better than the other as in this map of life expectancy  then it is common to denote the poor alternative with shades of red, and the good alternative with green.
Complementary hue progressions are a type of bi-polar progression.
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