Drawing a Circle with a Radius of a Defined Distance in a Map

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离开以前 2021-01-22 08:47

I am able to plot a map and caption a specific point:

library(maps)
map(\"state\")
text(-80.83,35.19,\"Charlotte\",cex=.6)

I can also plot a ci

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  •  醉梦人生
    2021-01-22 09:30

    The proposition of Gary is well adapted for plane maps, but can not be applied to maps generated by the "maps" package because it does not take care of the projection used to draw the map. Applied strictly as this, it results in drawing of an elipse (see below), because the unit of the circle radius is degrees, but not kilometers or miles. But degrees in latitude and longitude do not correspond to the same physical distance. To draw arround a point a circle, or something that is close to a circle, whose radius is a contant distance in miles or kilometers, you need to compute the corrected coordinates regarding to the projection. Taking your function and adapting it regarding to Chris Veness explanations on http://www.movable-type.co.uk, your function became :

    library(maps)
    library(mapdata)#For the worldHires database
    library(mapproj)#For the mapproject function
    plotElipse <- function(x, y, r) {#Gary's function ;-)
       angles <- seq(0,2*pi,length.out=360)
       lines(r*cos(angles)+x,r*sin(angles)+y)
    }
    plotCircle <- function(LonDec, LatDec, Km) {#Corrected function
        #LatDec = latitude in decimal degrees of the center of the circle
        #LonDec = longitude in decimal degrees
        #Km = radius of the circle in kilometers
        ER <- 6371 #Mean Earth radius in kilometers. Change this to 3959 and you will have your function working in miles.
        AngDeg <- seq(1:360) #angles in degrees 
        Lat1Rad <- LatDec*(pi/180)#Latitude of the center of the circle in radians
        Lon1Rad <- LonDec*(pi/180)#Longitude of the center of the circle in radians
        AngRad <- AngDeg*(pi/180)#angles in radians
        Lat2Rad <-asin(sin(Lat1Rad)*cos(Km/ER)+cos(Lat1Rad)*sin(Km/ER)*cos(AngRad)) #Latitude of each point of the circle rearding to angle in radians
        Lon2Rad <- Lon1Rad+atan2(sin(AngRad)*sin(Km/ER)*cos(Lat1Rad),cos(Km/ER)-sin(Lat1Rad)*sin(Lat2Rad))#Longitude of each point of the circle rearding to angle in radians
        Lat2Deg <- Lat2Rad*(180/pi)#Latitude of each point of the circle rearding to angle in degrees (conversion of radians to degrees deg = rad*(180/pi) )
        Lon2Deg <- Lon2Rad*(180/pi)#Longitude of each point of the circle rearding to angle in degrees (conversion of radians to degrees deg = rad*(180/pi) )
        polygon(Lon2Deg,Lat2Deg,lty=2)
    }
    map("worldHires", region="belgium")#draw a map of Belgium (yes i am Belgian ;-)
    bruxelles <- mapproject(4.330,50.830)#coordinates of Bruxelles
    points(bruxelles,pch=20,col='blue',cex=2)#draw a blue dot for Bruxelles
    plotCircle(4.330,50.830,50)#Plot a dashed circle of 50 km arround Bruxelles 
    plotElipse(4.330,50.830,0.5)#Tries to plot a plain circle of 50 km arround Bruxelles, but drawn an ellipse
    

    Result in image

    (sorry my "reputation" do not allow me to post images ;-). Edit: Added your image.

    I hope this helps. Grégoire

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