Il modo sbagliato

Quando viene creato un oggetto Spatial***DataFrame, ci sono due modi in cui gli oggetti spaziali (per esempio Polygons) possono essere associati alle righe dei dati. Il metodo più robusto è usare gli ID degli oggetti spaziali, abbinati ai nomi di riga nei dati. Questo assicura che, se qualcosa non corrisponde, te ne accorgi subito. L’altro modo è semplicemente per ordine: si assume che il primo oggetto spaziale corrisponda alla prima riga dei dati.

Una volta creato, l’abbinamento si basa unicamente sull’ordine. Se modifichi lo slot data, non c’è alcun controllo che gli oggetti spaziali corrispondano ancora alle righe giuste. Cosa significa in pratica? È molto rischioso manipolare direttamente lo slot data!

Per creare il grafico del reddito, devi abbinare il data frame dei redditi con il SpatialPolygonsDataFrame dei tract. Per mostrare il pericolo di modificare direttamente @data, vediamo cosa succede se provi a forzare nyc_income dentro nyc_tracts.

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Visualizzare dati geospaziali in R

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Esercizio pratico interattivo

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library(sp)

# Use str() on nyc_income 


# ...and on nyc_tracts@data

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Questo esercizio fa parte del corso

Visualizzare dati geospaziali in R

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We'll dive in by displaying some spatial data -- property sales in a small US town -- using ggplot2 and we'll introduce you to the ggmap package as a quick way to add spatial context to your plots. We'll talk about what makes spatial data special and introduce you to the common types of spatial data we'll be working with throughout the course.

Exercise 1: Introduction to spatial data Exercise 2: Grabbing a background map Exercise 3: Putting it all together Exercise 4: Insight through aesthetics Exercise 5: Useful get_map() and ggmap() options Exercise 6: Different maps Exercise 7: Leveraging ggplot2's strengths Exercise 8: A quick alternative Exercise 9: Common types of spatial data Exercise 10: Drawing polygons Exercise 11: Choropleth map Exercise 12: Raster data as a heatmap

You can get a long way with spatial data stored in data frames, but it makes life easier if they are stored in special spatial objects. In this chapter we'll introduce you to the spatial object classes provided by the sp package, particularly for point and polygon data. You'll learn how to explore and subset these objects by exploring a world map. The reward for learning about these object classes: we'll show you the package tmap which requires spatial objects as input, but makes creating maps really easy! You'll finish up by making a map of the world's population.

Exercise 1: Introducing sp objects Exercise 2: Let's take a look at a spatial object Exercise 3: What's inside a spatial object?Exercise 4: A more complicated spatial object Exercise 5: sp and S4 Exercise 6: Walking the hierarchy Exercise 7: Further down the rabbit hole Exercise 8: More sp classes and methods Exercise 9: Subsetting by index Exercise 10: Accessing data in sp objects Exercise 11: Subsetting based on data attributes Exercise 12: tmap, a package that works with sp objects Exercise 13: Introduction to tmap Exercise 14: Building a plot in layers Exercise 15: Why is Greenland so big?Exercise 16: Saving a tmap plot

While the sp package provides some classes for raster data, the raster package provides more useful classes. You'll be introduced to these classes and their advantages and then learn to display them. The examples continue with the theme of population from Chapter 2, but you'll look at some much finer detail datasets, both spatially and demographically. In the second half of the chapter you'll learn about color -- an essential part of any visual display, but especially important for maps.

Exercise 1: The raster package Exercise 2: What's a raster object?Exercise 3: Some useful methods Exercise 4: A more complicated object Exercise 5: A package that uses Raster objects Exercise 6: Color scales Exercise 7: Pick the right palette Exercise 8: Adding a custom continuous color palette to ggplot2 plots Exercise 9: Custom palette in tmap Exercise 10: More about color scales Exercise 11: An interval scale example Exercise 12: A diverging scale example Exercise 13: A qualitative example

In this chapter you'll follow the creation of a visualization from raw spatial data files to adding a credit to a map. Along the way, you'll learn how to read spatial data into R, more about projections and coordinate reference systems, how to add additional data to a spatial object, and some tips for polishing your maps.

Exercise 1: Lettura dei dati spaziali Exercise 2: Lettura di uno shapefile Exercise 3: Lettura di un file raster Exercise 4: Ottenere dati usando un pacchetto Exercise 5: Sistemi di riferimento di coordinate Exercise 6: Unire dati con CRS/proiezioni differenti Exercise 7: Conversione da un CRS/proiezione a un altro Exercise 8: Aggiungere dati agli oggetti spaziali Exercise 9: Il modo sbagliato

Esercizio in corso

Exercise 10: Verificare la corrispondenza dei dati Exercise 11: Unire gli attributi dei dati Exercise 12: Un primo grafico Exercise 13: Rifinire una mappa Exercise 14: Sotto-selezionare i neighborhood Exercise 15: Aggiungere le etichette dei quartieri Exercise 16: Sistemare la legenda e alcuni ritocchi finali Exercise 17: Riepilogo