Chapter 7 Maps as Processes: Null Landscapes, Spatial Processes, and Statistical Maps

NOTE: The source files for this book are available with companion package {isdas}. The source files are in Rmarkdown format and packed as templates. These files allow you execute code within the notebook, so that you can work interactively with the notes.

In last practice your learning objectives were:

  1. How to obtain a descriptive summary of a dataframe.
  2. Factors and how to use them.
  3. How to subset a dataframe.
  4. Pipe operators and how to use them.
  5. How to improve your maps.

Please review the previous practices if you need a refresher on these concepts.

7.1 Learning Objectives

In this chapter, you will learn:

  1. How to generate random numbers with different properties.
  2. About Null Landscapes.
  3. About stochastic processes.
  4. How to create new columns in a dataframe using a formula.
  5. How to simulate a spatial process.

7.2 Suggested Readings

  • Bivand RS, Pebesma E, Gomez-Rubio V (2008) Applied Spatial Data Analysis with R, Analysing Spatial Data (pp. 169-171). Springer: New York.
  • O’Sullivan D and Unwin D (2010) Geographic Information Analysis, 2nd Edition, Chapter 4. John Wiley & Sons: New Jersey.

7.3 Preliminaries

As usual, it is good practice to clear the working space to make sure that you do not have extraneous items there when you begin your work. The command in R to clear the workspace is rm (for “remove”), followed by a list of items to be removed. To clear the workspace from all objects, do the following:

rm(list = ls())

Note that ls() lists all objects currently on the workspace.

Load the libraries you will use in this activity:

library(tidyverse)

7.4 Random Numbers

Colloquially, we understand random as something that happens in an unpredictable manner. The same word in statistics has a precise meaning, as the outcome of a process that cannot be predicted with any form of certainty.

The question whether random processes exist is philosophically interesting. In the early stages of the invention of science, there was much optimism that humans could one day understand every aspect of the universe. This notion is well illustrated by Laplace’s Demon, a hypothetical entity that could predict the state of the universe in the future based on an all-encompassing knowledge of the state of the universe at any past point in time (see here).

There are two important limitations to this perspective. First, there is the assumption that the mechanisms of operation of phenomena are well understood (in the case of Laplace’s Demon, it was somewhat naively assumed that classical Newtonian mechanics were sufficient). And secondly, the assumption that all relevant information is available to the observer.

There are many processes in reality that are not fully understood, which make Laplace’s Demon an interesting, but unreliable source on predicting the state of the universe. Furthermore, there are often constraints in terms of how much and how accurately information can be collected with respect to any given phenomenon.

##Types of Processes

A process can be deterministic. However, When limited knowledge/limited information prevent us from being able to make certain predictions, we assume that the process is random.

It is important to note that “random” does not mean that just any outcome is possible. For instance, if you flip a coin, there are only two possible outcomes. If you roll a dice, there are only six possible outcomes. The concentration of a pollutant cannot be negative. The height of a human adult cannot be zero or 10 meters. And so on. It is the result of the possible outcomes that is random, as there is no process controlling the respective outcome.

Over time, many formulas have been devised to describe different types of random processes. A random probability distribution function describes the probability of observing different outcomes.

For instance, a formula for processes similar to coin flips was discovered by Bernoulli in 1713 (see here).

The following function reports a random binomial variable. The number of observations n is how many random numbers we require. The size is the number of trials. For instance, if the experiment was flipping a coin, it would be how many times we get heads in size flips. The probability of success prob is the probability of getting heads in any given toss. Execute the chunk repeatedly to see what happens.

#This function simulates the outcome of flipping a coin. 
# Here, we are simulating the result for flipping heads, 
# which has a probability of 0.5. The value of `n` is the 
# number of experiments and `size` is the number of trials 
# in each experiment 
rbinom(n = 1, 
       size = 1, 
       prob = 0.5)
## [1] 1

It can be noted that although there are only two outcomes, we do not have control over the result of the process, making the result random. If you tried this “experiment” repeatedly, you would find that “heads” (1s) and “tails” (0s) appear each about 50% of the time. A way to implement this is to increase n- think of this as recruiting more people to do coin flips at the same time:

n <- 1000 # Number of people tossing the coin one time.
coin_flips <- rbinom(n = n, 
                     size = 1, 
                     prob = 0.5)
sum(coin_flips)/n
## [1] 0.547

What happens if you change the size to 0, and why?

The binomial function is an example of a discrete probability distribution function, because it can take only one of a discrete (limited) number of values (i.e., 0 and 1).

Other random probability distribution functions are for continuous variables, variables that can take any value within a predefined range. The most famous of this distributions is the normal distribution, which you may know also as the bell curve. This probability distribution is attributed to Gauss (see here).

The normal distribution is defined by a centering parameter (the mean of the distribution) and a spread parameter (the standard deviation). In the normal distribution, 68% of values are within one standard deviation from the mean, 95% of values are within two standard deviations from the mean, and 99.7% of values are within three standard deviations from the mean.

The following function reports a value taken at random from a normal distribution with mean zero and standard deviation sd of one. Execute this chunk repeatedly to see what happens:

# This function generates random numbers based on 
# the normal distribution conditional on the given 
# arguments, i.e., the mean and the standard deviation `sd`. 
rnorm(1, 
      mean = 0, 
      sd = 1)
## [1] 0.7611103

Let’s say that the average height of men in Canada is 170.7 cm and the standard deviation is 7 cm. The height of a random person in this population would be:

rnorm(1, 
      mean = 170.7, 
      sd = 7)
## [1] 176.97

And the distribution of heights of n men in this population would be:

#Creating a data frame using the random numbers generated 
# from n=1000 people. The results in the data frame are then 
# plotted using ggplot. The end result is a distribution of 
# heights of 1000 men. You are able to see which heights are 
# most common out of the sample.
n <- 1000
height <- rnorm(n, 
                mean = 170.7, 
                sd = 7)
height <- data.frame(height)

# `geom_histogram()` is a geometric object in `ggplot2` that 
# represents the frequency of values in a vector as a bar chart
ggplot(data = height, 
       aes(x = height)) + 
  geom_histogram()
## `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

Men shorter than 150 cm would be extremely rare, as well as men taller than 190 cm.

7.5 Null Landscapes

So what do random variables have to do with maps?

Random variables can be used to generate purely random maps. These are called null landscapes or neutral landscapes in spatial ecology (With and King 1997) (Paper is available to download).

The concept of null landscapes is quite useful. They provide a benchmark to compare the results of statistical maps. Let us see how to generate a null landscape of events.

Suppose that there is a landscape with coordinates in the unit square, that is divided in very small discrete units of land. Each of these units of land can be the location of an event. For example, a tree might be present; or a case of a disease.

Let’s first create a landscape. For this, we will use the expand.grid function to find all combinations of two sets of coordinates in the unit interval, using small partitions:

# expand.grid created a set of coordinates by obtaining 
# all the combinations of the input variables. Here, our 
# landscape ranges in the x-axis from 0 to 1, increasing 
# by 0.05, and the y-axis also from 0 to 1, increasing by 0.05
coords <- expand.grid(x = seq(from = 0, 
                              to = 1, 
                              by = 0.05),
                      y = seq(from = 0, 
                              to = 1, 
                              by = 0.05))

Now, let’s generate a binomial random variable to go with these coordinates.

# `nrow()` returns the number of rows that are present 
# in a data frame. Here, it returns the number of rows 
# in the data frame `coords` 
events <- rbinom(n = nrow(coords), 
                 size = 1, 
                 prob = 0.5)

We will collect the coordinates and the random variable in a dataframe for plotting:

# `data.frame()` collects the inputs in a data frame;
# they must have the same number of rows
null_pattern <- data.frame(coords, 
                           events)

We can plot the null landscape we just generated as follows:

ggplot() + 
  geom_point(data = filter(null_pattern, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

By changing the probability prob in the function rbinom you can make the event more or less likely, i.e., frequent. If you are working with the notebook version of this document you can try changing the parameters to see what happens.

A continuous random variable can also be used to generate a null landscape. For instance, imagine that a group of individuals are asked to stand in formation, and that they arrange themselves purely at random. What would a map of their heights look like? First, we will generate a random variable using the same parameters we mentioned above for the height of men in Canada:

#heights will be random numbers generated based on 
# the average height of men, 7 standard deviations, 
# and the null landscape "coords" created previously.
heights <- rnorm(n = nrow(coords), 
                 mean = 170.7, 
                 sd = 7)

The random values that were generated can be collected in a dataframe with the coordinates for the purpose of plotting:

null_trend <- data.frame(coords, 
                         heights)

One possible map of heights when the individuals stand in formation at random would look like this:

# Our plot is created based on the dataframe of coords and heights. 
# The value of `x` is plotted to the x-axis, the value of `y` is plotted 
# to the y-axis, and the color of the points depends on the values of 
# `heights`. We can change the _scale_ of colors by means of 
# `scale_color_distiller()`. There, palette `spectral` associates higher 
# values of `heights` as red (taller men), while lower values of `heights` 
# (i.e., shorter men) are appear in blue. More generally, we can control 
# the scale of aesthetic aspects of the plot by means of scale_*something* 
# (scale_shape, scale_size, etc.) 
ggplot() + 
  geom_point(data = null_trend, 
             aes(x = x, 
                 y = y, 
                 color = heights), 
             shape = 15) +
  scale_color_distiller(palette = "Spectral") +
  coord_fixed()

These two examples illustrate only two of many possible techniques to generate null landscapes. We will discuss other strategies to work with null landscapes later in the course.

7.6 Stochastic Processes

Some processes are random, such as the ones used above to create null landscapes. These processes take values with some probability, but cannot be predicted with any certainty.

We will illustrate this, using again a unit square:

# Remember that `expand.grid()` will find all combinations of values in the inputs
coords <- expand.grid(x = seq(from = 0, 
                              to = 1, 
                              by = 0.05), 
                      y = seq(from = 0, 
                              to = 1, 
                              by = 0.05))

Here is an example of a random pattern of events:

# Create a random variable and join to the coordinates to generate a null landscape
events <- rbinom(n = nrow(coords), 
                 size = 1, 
                 prob = 0.5)
null_pattern <- data.frame(coords, 
                           events)

# Plot the null landscape you just created
ggplot() + 
  geom_point(data = subset(null_pattern, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

A systematic or deterministic process is one that contains no elements of randomness, and can therefore be predicted with complete certainty. For instance (note the use of xlim to set the extent of x axis in the plot):

# Copy the coordinates to a new object
deterministic_point_pattern <- coords

# `mutate()` adds new variables to a data frame while preserving 
# existing variables. Here, we create a new column in our data frame, 
# called `events` that will take the value of `x` (the position of an 
# observation along the x-axis) and will `round()` it, i.e., if it is 
# less than 0.5 it will round it to zero, and if it is equal to or 
# greater than 0.5 it will round to 1
deterministic_point_pattern <- mutate(deterministic_point_pattern, 
                                      events = round(x))

# Plot the new landscape: `filter()` keeps the rows in a dataframe 
# that meet a condition (for example, that the value of `events` is 1),
# and discards the rest
ggplot() + 
  geom_point(data = filter(deterministic_point_pattern, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  xlim(0, 1) +
  coord_fixed()

In the process above, we used the function round() and the coordinate x. The function gives a value of one for all points with x > 0.5, and a value of zero to all points with x <= 0.5. The pattern is fully deterministic: if I know the value of the x coordinate I can predict whether an event will be present.

A stochastic process, on the other hand, is a process that is neither fully random or deterministic, but rather a combination of the two. Let’s illustrate:

# Copy the coordinates to a new object 
stochastic_point_pattern <- coords

# Here, we combine the function `round()`, which is deterministic operation, 
# and `rbinom()` to generate a random number
stochastic_point_pattern <- 
  mutate(stochastic_point_pattern, 
         events = round(x) - round(x) * rbinom(n = nrow(coords), 
                                               size = 1, 
                                               prob = 0.5))

# Plot the new landscape
ggplot() + 
  geom_point(data = subset(stochastic_point_pattern,
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  xlim(0, 1) +
  coord_fixed()

The process above has a deterministic component (the probability of an event is zero if x <= 0.5), and a random component (the probability of a coordinate being an event is 0.5 when x > 0.5). The landscape is not fully random, but also it is not fully deterministic. Instead, it is the result of a stochastic process, a process that combines deterministic and random elements.

7.7 Simulating Spatial Processes

Null landscapes are interesting as a benchmark. More interesting are landscapes that emerge as the outcome of a non-random process - either a systematic/deterministic or stochastic process. Here we will see more ways to introduce a systematic element into a null landscape to simulate spatial processes.

Let’s begin with the point pattern, using the same landscape that we used above. We will first copy the coordinates of the landscape to a new dataframe, that we will call pattern1:

# Copy the coordinates to a new object, called `pattern1`
pattern1 <- coords

Next, we will use the function mutate from the dplyr package that is part of the tidyverse. This function adds a column to a data frame that could be calculated using a formula. For instance, we will now make the probability prob of the random binomial number generator a function of the coordinates:

# Remember, mutate adds a new column to a data frame. In this 
# example, mutate creates a new column, `events` using random
# binomial values; however, notice that the `prob` is not 0.5! 
# Instead, it depends on `x` the position of the event on the x-axis 
pattern1 <- mutate(pattern1, 
                   events = rbinom(n = nrow(pattern1), 
                                   size = 1, 
                                   prob = (x)))

Plot this pattern:

ggplot() + 
  geom_point(data = subset(pattern1, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

Since the probability of a “success” in the binomial experiment is proportional to the value of x (the coordinate of the event), now the events are clustered to the right of the plot. The underlying process in this case can be described in simple terms as “the probability of an event increases in the east direction”. In a real process, this could be possibly as a result of wind conditions, soil fertility, or other environmental factors that follow a trend.

Let us see what happens when we make this probability a function of the y coordinate:

# Overwrite the `events`, now the probability of success 
# in the random binomial number generator is a function of 
# `y`, the position of the event on the y-axis 
pattern1 <- mutate(pattern1, 
                   events = rbinom(n = nrow(pattern1), 
                                   size = 1, 
                                   prob = (y)))

# Plot the new events
ggplot() + 
  geom_point(data = subset(pattern1, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

Since the probability of a “success” in the binomial experiment is proportional to the value of y (the coordinate of the event), now the events are clustered to the top. The probability could be the interaction of the two coordinates:

# Now the probability is the product of `x` and `y`
pattern1 <- mutate(pattern1, 
                   events = rbinom(n = nrow(pattern1), 
                                   size = 1, 
                                   prob = (x * y)))

# Plot
ggplot() + 
  geom_point(data = subset(pattern1, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

Which of course means that the events cluster on the top-right corner.

A somewhat more sophisticated example could make the probability a function of distance from the center of the region:

# Copy the coordinates to the object `pattern1`
pattern1 <- coords

# In this case, `mutate()` creates a new variable, `distance`, 
# which is the straight line distance from the center of the 
# region (at coordinates x = 0.5 and y = 0.5). Now the probability 
# of success in the random binomial number generator depends on this `distance` 
pattern1 <- mutate(pattern1, 
                   distance = sqrt((0.5 - x)^2 + (0.5 - y)^2), 
                   events = rbinom(n = nrow(pattern1), 
                                   size = 1, 
                                   prob = 1 - exp(-0.5 * distance)))

Do not worry too much about the formula that I selected to generate this process; we will see different tools to describe a spatial process. In this particular example, I selected a function that makes the probability increase with distance from the center of the region.

Plot this pattern:

ggplot() + 
  geom_point(data = subset(pattern1, 
                           events == 1), 
             aes(x = x, 
                 y = y), 
             shape = 15) +
  coord_fixed()

As you would expect, there are few events near the center, and the number of events tends to increase away from the center.

To conclude this practice, let’s revisit the example of the people standing in formation. Now, taller people are asked to stand towards the back of the formation (assuming that the back is in the positive direction of the y-axis). As a result of this instruction, now the sorting is not random, since taller people tend to stand towards the back. However, people are not able to assess the height of each other exactly, so there will be some random variation in the distribution of heights. We can simulate this by making the height a function of position.

First, we copy the coordinates to a new dataframe for our trend experiment:

trend1 <- coords

Again we use mutate to add a column to a data frame that could be calculated using a formula. For instance, we will now make the probability prob of the random binomial number generator a function of the coordinates:

trend1 <- mutate(trend1,
                 heights = 160 + 20 * y  + rnorm(n = nrow(pattern1), 
                                                 mean = 0, 
                                                 sd = 7))

If people have a preference for standing next to people about their same height, and shorter people have a preference for standing near the front, this is a possible map of heights in the formation:

ggplot() + 
  geom_point(data = trend1, aes(x = x, 
                                y = y, 
                                color = heights), 
             shape = 15) +
  scale_color_distiller(palette = "Spectral") +
  coord_fixed()

As expected, shorter people are towards the “front” (bottom of the plot) and taller people towards the back. It is not a uniform process, since there is still some randomness, but a trend can be clearly appreciated.

7.8 Processes and Patterns

O’Sullivan and Unwin (2010) make an important distinction between processes and patterns. A process is like a recipe, a sequence of events or steps, that leads to an outcome, that is, a pattern.

You can think of the simulation procedures above as having two components: the process is the formula, function, or algorithm used to simulate a pattern. For instance, a random process could be based on the binomial distribution, whereas a stochastic process would have in addition to a random component some deterministic elements. The pattern is the outcome of the process. In the case of spatial processes, the outcome is typically a statistical map.

The procedures in the preceding sections illustrate just a few different ways to simulate spatial processes with the aim of generating statistical maps that display spatial patterns. There are in fact many more ways to simulate spatial processes, and articles (e.g., Geyer and Møller 1994) - and even books (e.g., Moller and Waagepetersen 2003) - have been written on this topic! Simulation is a very valuable tool in spatial statistics, as we shall see in later chapters.

It is important to note, however, that in the vast majority of cases we do not actually know the process; that is precisely what we wish to infer. Understanding process generation in a statistical sense, as well as null landscapes, is a useful tool that can help us to infer processes in applications with empirical (as opposed to simulated) data. In this sense, spatial statistics is often a tool used to make decisions about spatial patterns: are they random? And, if they are not random, can we infer the underlying process?

References

Bivand, R. S., E. J. Pebesma, and V. Gómez-Rubio. 2008. Applied Spatial Data Analysis with r. Book. New York: Springer Science+Business Media.
Geyer, Charles J, and Jesper Møller. 1994. “Simulation Procedures and Likelihood Inference for Spatial Point Processes.” Scandinavian Journal of Statistics, 359–73.
Moller, Jesper, and Rasmus Plenge Waagepetersen. 2003. Statistical Inference and Simulation for Spatial Point Processes. Chapman; Hall/CRC.
O’Sullivan, David, and David Unwin. 2010. Geographic Information Analysis. Book. 2nd. Edition. Hoboken, New Jersey: John Wiley & Sons.
With, Kimberly A, and Anthony W King. 1997. “The Use and Misuse of Neutral Landscape Models in Ecology.” Oikos, 219–29.