Understanding Latitude and Longitude Coordinates for Map Plotting

Introduction

Latitude and longitude coordinates are essential for creating maps. These coordinates help pinpoint specific locations on Earth’s surface. In this article, we’ll delve into the details of latitude and longitude coordinates, how to use them to create maps, and address a specific issue related to plotting maps within defined boundaries.

Latitude and Longitude Basics

Understanding Latitude and Longitude Scales

Latitude and longitude are two perpendicular lines that converge at the poles (North Pole and South Pole). The latitude line measures distance north or south of the equator (0°), while the longitude line measures east or west of the prime meridian (0°).

The Earth is divided into 360 degrees, with each degree further divided into 60 minutes. Latitude ranges from -90° to +90°, and longitude ranges from -180° to +180°.

Understanding Map Projections

Map projections are mathematical formulas used to transform latitude and longitude coordinates onto a two-dimensional map surface. These projections can distort the Earth’s shape, affecting accuracy depending on the projection type.

Some common map projections include:

• Mercator Projection (most widely used): preserves angles but distorts area
• Gall-Peters Projection: preserves area but distorts angles
• Robinson Projection: balances both angle and area preservation

Understanding Bounding Boxes for Map Plotting

What is a Bounding Box?

A bounding box, also known as an envelope or rectangle, is used to define the region of interest (ROI) on a map. This box represents the minimum and maximum latitude and longitude values that should be plotted.

Using make_bbox Function in R

The make_bbox function in R takes three arguments:

• lon: list of longitude values
• lat: list of latitude values
• f: factor representing the fraction of the Earth’s circumference (between 0 and 1)

By adjusting the value of f, you can change the size of your bounding box, thereby affecting the map’s scale.

Plotting Maps with ggmap

Using ggmap to Plot a Map

The ggmap package in R allows users to plot maps from various sources. To use this package, you’ll need to:

1. Install and load the ggmap package
2. Use get_map function to specify the map source (e.g., Google Maps)
3. Provide the bounding box coordinates using make_bbox
4. Create a ggplot object with ggmap

Here is an example:

# Load required packages
library(ggmap)

# Define the data frame
data <- structure(list(Latitude = c(18.682744, -27.706318, 17.65651, 
                        -21.006735), Longitude = c(46.252432, 43.179057, 100.89364, 104.064258
                        )), class = "data.frame", row.names = c(NA, -4L))

# Define the bounding box coordinates
bbox <- make_bbox(lon = data$Longitude, lat = data$Latitude, f = 0.05)

# Get the map using ggmap
map <- get_map(location = bbox, source = "google", maptype = "satellite")

# Create a ggplot object with ggmap
plot <- ggmap(map) + 
    geom_polygon(data = plot_data(map), aes(x = long/360, y = lat/180, group = group), fill = "lightblue")

Example Use Case: Plotting a Map of the Indian Ocean

In this example, we’ll create a map of the Indian Ocean using ggmap and bounding boxes.

# Load required packages
library(ggmap)

# Define the data frame with latitude and longitude values for the Indian Ocean
data <- structure(list(Latitude = c(0, -27.706318, 17.65651, 21.006735),
                        Longitude = c(90, 43.179057, 100.89364, 104.064258)),
                  class = "data.frame", row.names = c("Indonesia", "Antarctica", "Red Sea",
                                                    "Middle East"))

# Define the bounding box coordinates for the Indian Ocean
bbox <- make_bbox(lon = data$Longitude, lat = data$Latitude, f = 0.05)

# Get the map using ggmap
map <- get_map(location = bbox, source = "google", maptype = "satellite")

# Create a ggplot object with ggmap
plot <- ggmap(map) + 
    geom_polygon(data = plot_data(map), aes(x = long/360, y = lat/180, group = group), fill = "lightblue")

In the resulting plot, you can see that the map is limited to the Indian Ocean region.

Conclusion

This article has covered the basics of latitude and longitude coordinates for map plotting. We’ve also discussed bounding boxes, map projections, and using ggmap to create maps with specific regions of interest. By adjusting the fraction value in the make_bbox function, you can change the size of your bounding box, thus affecting the map’s scale.

I hope this article has provided a comprehensive overview of creating maps within defined boundaries. If you have any further questions or need additional clarification on any concept, feel free to ask!

Last modified on 2024-02-17