Question

我想知道是否有人使用ggmap绘制一个纬度/长点周围的圆半径？例如，我想绘制一个给定的点，然后在这个点周围2,500英尺的半径内绘制和阴影。我有一个想法，如何使用更大的圆周公式，但我想先在这里检查。

Answer 1

我致力于构建一个简单的黑客，不是我想象的那个圈子，但它现在可以工作。

library(ggmap)
##__________________________________________________________________
### earth.dist I found on r-bloggers. I believe it now belongs to the fossil package. 
earth.dist <- function ( lat1,long1,lat2, long2)
{
  rad <- pi/180
  a1 <- lat1 * rad
  a2 <- long1 * rad
  b1 <- lat2 * rad
  b2 <- long2 * rad
  dlon <- b2 - a2
  dlat <- b1 - a1
  a <- (sin(dlat/2))^2 + cos(a1) * cos(b1) * (sin(dlon/2))^2
  c <- 2 * atan2(sqrt(a), sqrt(1 - a))
  R <- 6378.145
  d <- R * c
  # this I changed to return feet instead of KM
  return(d* 3280.8)
}

##__________________________________________________________________
## Function to output polygon to map
BoxGon <- function(Lat,Long,feet){

    for(i in 1:1000){
    point = Long - i/1000
    Dist <- earth.dist(Lat,Long, Lat,point)
    if(Dist >  feet){
                    West <- cbind(Lat,point)
                    break}
    }

    for(i in 1:1000){
    point = Long + i/1000
    Dist <- earth.dist(Lat,Long, Lat,point)
    if(Dist >  feet){
                    East <- cbind(Lat,point)
                    break}
    }

    for(i in 1:1000){
    point = Lat + i/1000
    Dist <- earth.dist(Lat,Long, point,Long)
    if(Dist >  feet){
                    North <- cbind(point,Long)
                    break}
    }   

    for(i in 1:1000){
    point = Lat - i/1000
    Dist <- earth.dist(Lat,Long, point,Long)
    if(Dist >  feet){
                    South <- cbind(point,Long)
                    break}
    }   

    return(rbind(West,North,East,South,West))
}

##__________________________________________________________________
df = BoxGon(37.295844, -121.898057,5000)
df = as.data.frame(df)
colnames(df) <- c('Latitude','Longitude')

map <- get_map(location = 'san,jose', zoom = 12)
map <-  ggmap(map)

# we select - 1 because once we map in pairs. IE once we have the last record there is nothing for that record to map to
for(i in 1:nrow(df)-1){
latlon <-  head(df,2)
map <-  map + geom_polygon(data=latlon,aes(x=Longitude,y=Latitude),alpha=0.1,size = 1,colour="green",fill="green")
df = df[-1,]
print(i)
}
map <- map + geom_polygon(aes(x=-121.898057,y=37.295844),alpha=0.1,size = 6,colour='Purple',fill='Purple')

ggmap创建的地块

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