Plot points in map d3 javascript

问题:

i want to draw on map base on longitude and latitude in csv file called tree.csv on a map that i using an image . My csv file include many lines ,so i will just put some lines here

Longitude Latitude

37.7295482207565 122.392689419827

37.8030467266869 122.425063628702 ...... Here is my code

d3.csv("/trees.csv", function(data) {
    dataset=data.map(function(d) { return [+d["Longitude"],+d["Latitude"] ];});
    console.log(data)
    var width = 750,
    height = width;

    // Set up projection that map is using
    var projection = d3.geo.mercator()
     .center([-122.433701, 37.767683])

     .scale(225000)
     .translate([width / 2, height / 2]);

    var path=d3.geo.path().projection(projection);


    var svgContainer=d3.select("body").append("svg")
    .attr("width",width)
    .attr("height",height);
    svgContainer.append("image")
     .attr("width", width)
     .attr("height", height)
     .attr("xlink:href", "/Ilu.svg");

    var trees=svgContainer.selectAll("circles")
    .data(data).enter()
    .append("circles")

    var treesAttributes=trees
    .attr("cx",function(d) { return projection(d["Longitude"])[0];})
    .attr("cy",function(d) { return projection(d["Latitude"])[1];})
    .attr("r","100px")
    .style("fill","red");

I can see my map but i cant see any points on my map . When i inspect the web. i see that cx is Nan number ,and cy is same number. I think maybe my array havent been read yet. But i am not sure about the problems. I have been stucked. Can you guys solve me the problem ? Thank you

回答1:

Your problem lies in that you aren't providing coordinates to be projected.

A d3 geoProjection takes a longitude latitude pair and projects it to an x,y svg coordinate (a projection returns a coordinate as: [x,y], which is why you use this form in your code: projection(coord)[0] to get the cx value). You are seeking to project only a longitude, and then only a latitude:

.attr("cx",function(d) { return projection(d["Longitude"])[0];})
.attr("cy",function(d) { return projection(d["Latitude"])[1];})

In this case, projection won't return an svg coordinate as you aren't providing a geographical coordinate to project. You need to project both longitude and latitude, becuase x and y values produced in a projection are usually (not always) co-dependent - in any conical projection for example, the output y (or x) value is dependent on both latitude and longitude. Further, as projection() returns [x,y], it requires both longitude and latitude for every projection.

Instead try:

.attr("cx",function(d) { return projection([d["Longitude"],d["Latitude"]])[0];})
.attr("cy",function(d) { return projection([d["Longitude"],d["Latitude"]])[1];})

Remeber that d3 geoprojections expect the form: projection([longitude, latitude]), changing the order of longitude and latitude will produce unexpected results.

var data = [
{longitude:1,latitude:1},
{longitude:-1,latitude:1},
{longitude:1,latitude:-1},
{longitude:-1,latitude:-1}
]

var svg = d3.select("body")
   .append("svg")
   .attr("width",200)
   .attr("height",200);
   
var projection = d3.geoMercator()
  .translate([100,100]);
  
var circles = svg.selectAll("circle")
  .data(data)
  .enter()
  .append("circle")
  .attr("cx",function(d) { return projection([d.longitude,d.latitude])[0];
   })
  .attr("cy",function(d) { return projection([d["longitude"],d["latitude"]])[1];
   })
   .attr("r",2)

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