在 R 中识别传单中光栅的点击位置 [英] Identify position of a click on a raster in leaflet, in R

问题描述

我正在使用 shinydashboard 在 R leaflet 地图上绘制一个大型 lat-lon NetCDF raster.当我单击地图时，会弹出一个弹出窗口，显示行、列、经纬度位置和单击栅格点的值.(请参阅下面的可重现代码)

I am plotting a large lat-lon NetCDF raster over an R leaflet map using shinydashboard. When I click on the map, a popup comes out and shows row, column, lat-lon position and value of the clicked raster point. (See reproducible code below)

问题是，如果栅格足够大，我会遇到栅格变化.例如，这里我点击了一个应该有值的点，但结果是识别的点就是上面那个.

The problem is that I am experiencing a shift in the raster if the raster is large enough. For example, here I clicked on a point which should have a value, but the result is that the identified point is the one above.

我认为这与 leaflet 使用的栅格是投影的事实有关，而我用来识别点的原始数据是 Lat-Lon，因为单击的点返回为leaflet 的 Lat-Lon.我不能使用投影文件(depth)，因为它的单位是米，而不是度！即使我试图将这些米重新投影到度数，我也得到了转变.

I believe this has to do with the fact that the raster used by leaflet is projected, while the raw data I use to identify the points is Lat-Lon, since the clicked point is returned as Lat-Lon by leaflet. I cannot use the projected file (depth) since its units are in meters, not degrees! Even if I tried to reproject those meters to degrees, I got a shift.

这是一个基本的可运行代码示例:

Here is a basic runnable example of the code:

#Libraries
library(leaflet)
library(raster)
library(shinydashboard)
library(shiny)

#Input data
download.file("https://www.dropbox.com/s/y9ekjod2pt09rvv/test.nc?dl=0", destfile="test.nc")
inputFile    = "test.nc"
inputVarName = "Depth"
lldepth <- raster(inputFile, varname=inputVarName)
lldepth[Which(lldepth<=0, cells=T)] <- NA #Set all cells <=0 to NA
ext <- extent(lldepth)
resol <- res(lldepth)
projection(lldepth) <- "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"

#Project for leaflet
depth <- projectRasterForLeaflet(lldepth)

#Prepare UI
sbwidth=200
sidebar <- dashboardSidebar(width=sbwidth)
body <- dashboardBody(
          box( #https://stackoverflow.com/questions/31278938/how-can-i-make-my-shiny-leafletoutput-have-height-100-while-inside-a-navbarpa
            div(class="outer",width = NULL, solidHeader = TRUE, tags$style(type = "text/css", paste0(".outer {position: fixed; top: 50px; left: ", sbwidth, "px; right: 0; bottom: 0px; overflow: hidden; padding: 0}")),
            leafletOutput("map", width = "100%", height = "100%")
            )
          )
        )
ui <- dashboardPage(
  dashboardHeader(title = "A title"),
  sidebar,
  body
)
#
#Server instance
server <- function(input, output, session) {
  output$map <- renderLeaflet({#Set extent
    leaflet()  %>%
      fitBounds(ext[1], ext[3], ext[2], ext[4])
  })

  observe({#Observer to show Popups on click
    click <- input$map_click
    if (!is.null(click)) {
      showpos(x=click$lng, y=click$lat)
    }
  })

  showpos <- function(x=NULL, y=NULL) {#Show popup on clicks
    #Translate Lat-Lon to cell number using the unprojected raster
    #This is because the projected raster is not in degrees, we cannot use it!
    cell <- cellFromXY(lldepth, c(x, y))
    if (!is.na(cell)) {#If the click is inside the raster...
      xy <- xyFromCell(lldepth, cell) #Get the center of the cell
      x <- xy[1]
      y <- xy[2]
      #Get row and column, to print later
      rc <- rowColFromCell(lldepth, cell)
      #Get value of the given cell
      val = depth[cell]
      content <- paste0("X=",rc[2],
                        "; Y=",rc[1],
                        "; Lon=", round(x, 5),
                        "; Lat=", round(y, 5),
                        "; Depth=", round(val, 1), " m")
      proxy <- leafletProxy("map")
      #add Popup
      proxy %>% clearPopups() %>% addPopups(x, y, popup = content)
      #add rectangles for testing
      proxy %>% clearShapes() %>% addRectangles(x-resol[1]/2, y-resol[2]/2, x+resol[1]/2, y+resol[2]/2)
    }
  }

  #Plot the raster
  leafletProxy("map") %>%
    addRasterImage(depth, opacity=0.8, project=FALSE, group="Example", layerId="Example", colors=colorNumeric(terrain.colors(10), values(depth), na.color = "black"))
}


print(shinyApp(ui, server))

如果栅格很大，如何正确识别点?

How can I correctly identify the points, if the raster is large?

我还想提供一些(可能)相关文档或问题的附加链接:

I also wanted to provide some additional links to (possibly) related documentation or questions:

• 光栅图像似乎是使用 R 的传单转移
• R 用于单击光栅图像时的传单重定向
• https://gis.stackexchange.com/questions/183918/is-it-possible-to-use-a-rasterclick-event-within-an-interactive-leaflet-map
• 标记鼠标点击事件在 R 传单中用于闪亮

推荐答案

我发现我可以重新投影回 input$map_click 给出的 X-Y(经纬度)位置.在这种情况下，我假设输入投影为 Lon-Lat，但我认为不一定必须如此.它只需要有经纬度单位.

I have found that I can reproject back the X-Y (lon-lat) position given by input$map_click. In this case I assumed the input projection to be Lon-Lat, but I think it doesn't necessarily have to be. It just needs to have Lat-Lon units.

#Set projections
inputProj <- "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"
leafletProj <- "+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378137 +b=6378137 +towgs84=0,0,0,0,0,0,0 +units=m +nadgrids=@null +wktext +no_defs"
#Note that for some reason "+nadgrids=@null +wktext" is very important
  #as hinted to by other questions and answers linked in my question.
xy <- SpatialPoints(data.frame(x,y))
proj4string(xy) <- inputProj
xy <- as.data.frame(spTransform(xy, leafletProj))
#Get the cell number from the newly transformed metric X and Y.
cell <- cellFromXY(depth, c(xy$x, xy$y))

#At this point, you can also retrace back the center of the cell in
  #leaflet coordinates, starting from the cell number!
xy <- SpatialPoints(xyFromCell(depth, cell))
proj4string(xy) <- leafletProj
xy <- as.data.frame(spTransform(xy, inputProj))
#Here XY will again be in lat-lon, if you projection says so,
  #indicating the center of the clicked cell

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