ex2.Rmd

---
title: "互联网地图学 — ex 2"
author: "Jianghao Wang"
date: "`r Sys.Date()`"
output:
  prettydoc::html_pretty:
    theme: architect # architect cayman leonids tactile hpstr
    highlight: github # vignette github
    toc: true
    toc_depth: 4
    number_sections: true
---

# R + leaflet 互联网制图

## 基本用法 
1. 通过 `leaflet()` 初始化一个地图.
2. 利用以下函数 (e.g. `addTiles`, `addMarkers`, `addPolygons`) 来添加图层.
3. 重复第二步完成你想要的设置.
4. 输出地图.

一个简单的例子：

```{r warning=FALSE}
library(leaflet)
# library(magrittr)

m <- leaflet() %>%
  addTiles() %>%  # Add default OpenStreetMap map tiles
  addMarkers(lng=116.67498, lat=40.40652, popup="The University of Chinese Academy of Sciences")
m  # Print the map
```

如果你对R中的管道操作（`%>%`）不熟悉，可以采用以下方式：

```{r eval=FALSE}
m <- leaflet()
m <- addTiles(m)
m <- addMarkers(m, lng=116.67498, lat=40.40652, popup="The University of Chinese Academy of Sciences")
m
```
leaflet中可以接受的输出数据包括以下几种：

- From base R:

- From the [**maps** package](http://cran.rstudio.com/package=maps):
    - the data frame from returned from `map()`

## 数据输入

**第一种：输入数据为经纬度坐标**

- lng/lat matrix
- data frame with lng/lat columns

```{r warning=FALSE}
m <- leaflet() %>% addTiles()
df <- data.frame(
  lat = rnorm(100),
  lng = rnorm(100),
  size = runif(100, 5, 20),
  color = sample(colors(), 100)
)
m <- leaflet(df) %>% addTiles()
m %>% addCircleMarkers(radius = ~size, color = ~color, fill = FALSE)
m %>% addCircleMarkers(radius = runif(100, 4, 10), color = c('red'))
```

**第二种：空间数据格式**

[**sp** package](http://cran.rstudio.com/package=sp):

- `SpatialPoints[DataFrame]`
- `Line`/`Lines`
- `SpatialLines[DataFrame]`
- `Polygon`/`Polygons`
- `SpatialPolygons[DataFrame]`

**第三种：`maps`包中的数据**

[**maps** package](http://cran.rstudio.com/package=maps):

- the data frame from returned from `map()`

```{r warning=FALSE}
# install.packages("maps")
library(maps)
mapStates = map("state", fill = TRUE, plot = FALSE)
leaflet(data = mapStates) %>% addTiles() %>%
  addPolygons(fillColor = topo.colors(10, alpha = NULL), stroke = FALSE)
```

## 地图选择

### 第三方地图
leaflet支持多种类型的底图，包括Google Map，Openstreetmap等，默认为OSM的底图。也可以添加第三方的底图。

改变底图的函数：`addProviderTiles()`, 也可以通过改变[leaflet-providers plugin](https://github.com/leaflet-extras/leaflet-providers)来改变，更全面的例子见[这里](http://leaflet-extras.github.io/leaflet-providers/preview/index.html)。

```{r fig.height=4}
m <- leaflet() %>% setView(lng=116.67498, lat=40.40652, zoom = 12)
m %>% addTiles()
m %>% addProviderTiles(providers$Stamen.Toner)
m %>% addProviderTiles(providers$CartoDB.Positron)
m %>% addProviderTiles(providers$Esri.NatGeoWorldMap)
```

### WMS Tiles
可以利用`addWMSTiles()`来添加WMS (Web Map Service) tiles。

The map below shows the Base Reflectivity (a measure of the intensity of precipitation occurring) using the WMS from the [Iowa Environmental Mesonet ](http://mesonet.agron.iastate.edu):

```{r fig.height=4}
leaflet() %>% addTiles() %>% setView(-93.65, 42.0285, zoom = 4) %>%
  addWMSTiles(
    "http://mesonet.agron.iastate.edu/cgi-bin/wms/nexrad/n0r.cgi",
    layers = "nexrad-n0r-900913",
    options = WMSTileOptions(format = "image/png", transparent = TRUE),
    attribution = "Weather data © 2012 IEM Nexrad"
  )
```

### 多种底图叠加
还可以通过设置透明色的方式，将多种底图进行叠加显示

```{r fig.height=4}
m %>% addProviderTiles(providers$MtbMap) %>%
  addProviderTiles(providers$Stamen.TonerLines,
    options = providerTileOptions(opacity = 0.35)) %>%
  addProviderTiles(providers$Stamen.TonerLabels)
```