[GIS] Plotting polygon on Google map

ggmapggplot2polygonr

I have a dataset with points in NYC. I extract the bounding box (max and min lat/lon) and plot it on a map. It works for the basemap stamen watercolor, but not for google's. Why?

Here is the code to create the bounding box polygon:

library(ggmap)

bbx <- c(left=-74.06077,bottom=40.63317,right=-73.76324,top=40.84902)
x <- c(bbx["left"], bbx["left"], bbx["right"], bbx["right"])
y <- c(bbx["bottom"], bbx["top"], bbx["top"], bbx["bottom"])
df <- data.frame(x, y)

Here is the code to plot the bbox on the stamen+watercolor map, and it works:

myMap <-get_map(location=bbx,source="stamen",maptype="watercolor", crop=FALSE)
ggmap(myMap)+geom_polygon(aes(x=x, y=y), data=df, fill="red", alpha=.5)

Here is the code to plot the bbox on the google+satellite map, and it doesn't work:

myMap2 <-get_map(location=bbx,source="google",maptype="satellite")
ggmap(myMap2)+geom_polygon(aes(x=x, y=y), data=df, fill="red", alpha=.5)

Best Answer

You used an experimental function usage:

Warning: bounding box given to google - spatial extent only approximate.
converting bounding box to center/zoom specification. (experimental)

I recommend you to use get_googlemap function. Use center of location instead boundary box:

myMap2 <- get_googlemap(center = c(lon=mean(bbx[c(1,3)]), lat=mean(bbx[c(2,4)])),
                        zoom =11, maptype = "satellite")
ggmap(myMap2) + geom_polygon(aes(x=x, y=y), data=df, fill="red", alpha=.5)

Related Solutions

Open-Source GIS – Converting Tax Map Polygons from Shapefile to Table of Numbers and Corner Coordinates

In QGIS you can use "Extract nodes" to first get the corner points of your polygons. Then, using "Export/Add geometry columns" you can add x and y coordinates to those points. The results will be four lines of data though.

Another way is to copy paste the geometries from QGIS' map window into a text editor. You'll get the WKT representation of the polygons, e.g.

wkt_geom
POLYGON((-0.971664 0.453443,-0.516714 0.827044,-0.245552 0.432353,-1.001793 -0.224462,-0.971664 0.453443))

OpenLayers 2 – Getting Coordinates of a Circle Vector after Draw

var polygonLayer = new OpenLayers.Layer.Vector("Polygon Layer");

polygonLayer.events.on({
    featuresadded: onFeaturesAdded
});

And do this on the function:

function onFeaturesAdded(event){
    var bounds = event.features[0].geometry.getBounds();
    var answer = "bottom: " + bounds.bottom  + "\n";
    answer += "left: " + bounds.left  + "\n";
    answer += "right: " + bounds.right  + "\n";
    answer += "top: " + bounds.top  + "\n";
    alert(answer);
}

or to get all vertices:

function onFeaturesAdded(event) {
    var vertices = event.features[0].geometry.getVertices();
    var strVertices = "";
    for (var x in vertices) {
        strVertices += "(" + vertices[x].x + "," + vertices[x].y + ")";
    }

    alert(strVertices);
}

Best Answer

Related Solutions

Open-Source GIS – Converting Tax Map Polygons from Shapefile to Table of Numbers and Corner Coordinates

OpenLayers 2 – Getting Coordinates of a Circle Vector after Draw

Related Question