Display of MapView Object in Shiny

Introduction

In this article, we will explore how to display a MapView object in Shiny. A MapView is a powerful function provided by the mapview package that allows for the creation of interactive maps. One of its key features is the ability to compare multiple maps side-by-side.

However, when trying to integrate a MapView object into a Shiny application using the renderMapview and mapviewOutput functions, we may encounter some issues. In this article, we will delve into the details of these functions and explore alternative approaches to display a MapView object in Shiny.

Understanding MapView

Before we dive into the implementation, let’s take a look at what a MapView is and how it works.

A MapView is an interactive map that can be used for various purposes such as displaying geographical data, creating visualizations, or even performing spatial analysis. The mapview package provides an interface to access these maps using R.

The core of the mapview package is its engine, which is responsible for rendering the map and handling user interactions. This engine is built on top of Leaflet.js, a popular JavaScript library used for creating interactive maps.

When we call the mapview() function in our Shiny application, it creates an instance of the MapView object, which is then rendered to the output area of our application.

Rendering MapView Objects

To render a MapView object in Shiny, we need to use the renderMapview() function in our server.R file. This function takes a MapView object as input and returns an output object that can be used in our UI.

Here’s an example code snippet:

library(shiny)
library(mapview)

ui <- fluidPage(
  mapviewOutput("samplemap")
)

server <- function(input, output, session) {
  output$samplemap <- renderMapview({
    mapview(gadmCHE)
  })
}

shinyApp(ui = ui, server = server)

In this example, we define a MapView object called samplemap and render it using the renderMapview() function. The resulting output is then displayed in our Shiny application.

However, when we run this code, no map appears on the screen, despite the fact that the mapview() function seems to be working correctly. This issue can be attributed to the warning message we receive in the command window:

Warning in spCheckObject(x) : Columns CCN_1 in attribute table contain only NA values and are dropped.

This warning message indicates that there’s an issue with our data, specifically with columns CCN_1 in the attribute table. This can be due to various reasons such as missing or incorrect data.

Troubleshooting MapView Objects

To troubleshoot issues with MapView objects, we need to understand how they work and what could cause problems.

One of the key factors that affect the rendering of a MapView object is its size. If the map is too large or too small, it may not render correctly in our Shiny application.

Another issue can arise when the mapview() function is called from within another function. In this case, the map view may not be rendered correctly due to timing issues.

Finally, we need to consider the data that we’re using with our MapView object. If the data is incorrect or incomplete, it can cause rendering issues or even lead to errors in our application.

Alternative Approaches

Despite the challenges associated with rendering MapView objects directly, there are alternative approaches we can use to achieve similar results.

One approach is to use the sync() function provided by the mapview package. The sync() function allows us to create multiple maps for comparison and display them side-by-side in our Shiny application.

Here’s an example code snippet:

library(shiny)
library(mapview)

ui <- fluidPage(
  fluidRow(
    column(6, mapviewOutput("samplemap1")),
    column(6, mapviewOutput("samplemap2"))
  )
)

server <- function(input, output, session) {
  map1 <- eventReactive(input$button_click, {
    mapview(gadmCHE)
  })
  
  map2 <- eventReactive(input$button_click, {
    mapview(gadmCHE)
  })
  
  output$samplemap1 <- renderMapview({
    sync(map1(), map2())
  })
  
  output$samplemap2 <- renderMapview({
    sync(map1(), map2())
  })
}

shinyApp(ui = ui, server = server)

In this example, we define two MapView objects called samplemap1 and samplemap2. When the user clicks a button, both maps are rendered using the sync() function.

By using the sync() function, we can create multiple maps for comparison without having to worry about rendering issues or data inconsistencies.

Conclusion

Displaying a MapView object in Shiny requires careful consideration of various factors such as size, timing, and data quality. Despite these challenges, there are alternative approaches we can use to achieve similar results, such as using the sync() function provided by the mapview package.

In this article, we explored how to display a MapView object in Shiny using the renderMapview() and mapviewOutput() functions, as well as alternative approaches like the sync() function. We also discussed common issues that can arise when rendering MapView objects, such as size-related problems or data inconsistencies.

By following these guidelines and best practices, you can create effective and engaging interactive maps in your Shiny applications using the mapview package.

Last modified on 2023-10-22