How to attach a simple data.frame to a Spatial Polygon DataFrame in R?

In spatial data analysis it is often necessary to combine spatial geometries with the attribute data. In R the SpatialPolygonDataFrame class from the sp package is used to represent spatial polygons with the associated data. Sometimes, we may have simple data. frame with the attributes that we want to attach to the SpatialPolygonDataFrame. Here, we will discuss the process of attaching data. frame to the SpatialPolygonDataFrame.

What is a Spatial Polygon DataFrame?

A Spatial Polygon DataFrame is a specific data structure in R, used to represent and manage spatial data. It is particularly useful in geographic information systems (GIS) and spatial analysis. A Spatial Polygon DataFrame combines the spatial data of polygon shapes with associated attribute data, allowing for comprehensive spatial analysis and visualization.

Before starting to make sure we have the necessary packages installed, we'll need the sp package for handling spatial objects and rgdal for spatial data manipulation.

install.packages(c("sp", "rgdal"))

Now we will discuss step by step of How to attach a simple data.frame to a Spatial Polygon DataFrame in R Programming Language.

Step 1: Load the Required Libraries

First we will Load the Required Libraries.

# Load necessary libraries
library(sp)

Step 2: Create a Spatial Polygons Object

Assume we have a set of polygons and want to create a SpatialPolygons object. For demonstration, we'll use a simple example with two polygons.

# Define coordinates for two polygons
coords1 <- matrix(c(0, 0, 0, 1, 1, 1, 1, 0, 0, 0), ncol = 2, byrow = TRUE)
coords2 <- matrix(c(2, 2, 2, 3, 3, 3, 3, 2, 2, 2), ncol = 2, byrow = TRUE)

# Create Polygon objects
polygon1 <- Polygon(coords1)
polygon2 <- Polygon(coords2)

# Create Polygons objects
polygons1 <- Polygons(list(polygon1), ID = "1")
polygons2 <- Polygons(list(polygon2), ID = "2")

# Create a SpatialPolygons object
sp_polygons <- SpatialPolygons(list(polygons1, polygons2))

Step 3: Create a data.frame with Attribute Data

Create a data.frame with the attributes that we want to attach to the polygons and also the IDs match those of the SpatialPolygons object.

# Create a data.frame with attributes
attributes <- data.frame(
  ID = c("1", "2"),
  Name = c("Polygon1", "Polygon2"),
  Area = c(1, 1)
)

Step 4: Combine the SpatialPolygons Object with the Data Frame

The Combine the SpatialPolygons object with the data.frame to the create a SpatialPolygonsDataFrame.

# Combine the SpatialPolygons object with the data.frame
sp_polygons_df <- SpatialPolygonsDataFrame(sp_polygons, data = attributes)

Step 5: Verify the Results

Check the resulting SpatialPolygonsDataFrame to the ensure that the attributes have been correctly attached.

# Print the SpatialPolygonsDataFrame object
print(sp_polygons_df)

Output:

An object of class "SpatialPolygonsDataFrame"
Slot "data":
  ID     Name Area
1  1 Polygon1    1
2  2 Polygon2    1

Slot "polygons":
[[1]]
An object of class "Polygons"
Slot "Polygons":
[[1]]
An object of class "Polygon"
Slot "labpt":
[1] 0.5 0.5

Slot "area":
[1] 1

Slot "hole":
[1] FALSE

Slot "ringDir":
[1] 1

Slot "coords":
     [,1] [,2]
[1,]    0    0
[2,]    0    1
[3,]    1    1
[4,]    1    0
[5,]    0    0



Slot "plotOrder":
[1] 1

Slot "labpt":
[1] 0.5 0.5

Slot "ID":
[1] "1"

Slot "area":
[1] 1


[[2]]
An object of class "Polygons"
Slot "Polygons":
[[1]]
An object of class "Polygon"
Slot "labpt":
[1] 2.5 2.5

Slot "area":
[1] 1

Slot "hole":
[1] FALSE

Slot "ringDir":
[1] 1

Slot "coords":
     [,1] [,2]
[1,]    2    2
[2,]    2    3
[3,]    3    3
[4,]    3    2
[5,]    2    2



Slot "plotOrder":
[1] 1

Slot "labpt":
[1] 2.5 2.5

Slot "ID":
[1] "2"

Slot "area":
[1] 1



Slot "plotOrder":
[1] 2 1

Slot "bbox":
  min max
x   0   3
y   0   3

Slot "proj4string":
Coordinate Reference System:
Deprecated Proj.4 representation: NA 

• Class of Object -The object is a SpatialPolygonsDataFrame, combining spatial geometries and attribute data.
• Data Slot -
  • ID: Unique identifiers for each polygon.
  • Name: Descriptive names for each polygon.
  • Area: The area of each polygon.
• Polygons Slot -
  • Label Point (labpt): Central coordinates of each polygon, useful for labeling or placing markers.
  • Area: The area of each polygon.
  • Hole: Indicates if the polygon contains internal voids (all FALSE, meaning no holes).
  • Ring Direction (ringDir): Direction of the polygon's boundary (standard direction).
  • Coordinates (coords): Vertex coordinates defining the shape of each polygon.
  • Plot Order: Sequence for drawing polygons.
• Bounding Box (bbox) -
  • Min and Max Coordinates: The spatial extent of the dataset, showing the minimum and maximum x and y values.
• Coordinate Reference System (CRS) - Details about how the spatial data is projected. Remember that the CRS information is deprecated in this example.

Conclusion

Attaching a data.frame to a SpatialPolygonsDataFrame in R is crucial for merging map shapes with additional information. This process includes by creating the spatial shapes, preparing a data.frame with matching IDs, and combining them with the SpatialPolygonsDataFrame function. Once combined, it’s important to check that the information is correctly linked to each shape. It ensures that our map data and additional details are well-integrated, making it easier to analyze and visualize our spatial data.