d3.js：为什么是d3.geo.path（）给NaN？ [英] d3.js: why is d3.geo.path() giving NaN?

问题描述

我试图做一些在线映射与d3，但遇到一个问题，当我尝试绘制两点之间的线。

I am trying to do some online mapping with d3, but running into a problem when I try to plot a line between two points.

我计算了两个多边形（源和目标）的质心

I have calculated the centroid of two polygons (source and target)

在代码中：

var projection = d3.geo.mercator()
    .scale(width)
    .translate([0, 0]);

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

从JS控制台：

> path({type: "LineString",
      coordinates: [path.centroid(source_country), path.centroid(target_country)]});

"M277.05056877663407,121.67976219138909L-694.1792414247936,NaN"

，质心计算似乎工作正常（绘制这些点在地图上显示它们）

Yet, the centroid calculations seem to be working fine (plotting those points shows them on the map)

> [path.centroid(source_country), path.centroid(target_country)]

[
Array[2]
0: 103.89396329123777
1: -41.453727169465765
length: 2
__proto__: Array[0]
, 
Array[2]
0: -260.3172155342976
1: -245.57309459883245
length: 2
__proto__: Array[0]

任何想法为什么NaN出现在

Any ideas why that NaN is appearing at the end of the path generated for my LineString?

推荐答案

这里的问题是你投射lat / lon坐标两次。 path（）运算符期望将lat / lon和project投影到像素; path.centroid（）方法也期望一个纬度/经度几何，并且还产生一个基于像素的投影。

The problem here is that you're projecting the lat/lon coordinates twice. The path() operator expects to take lat/lon and project to pixels; the path.centroid() method also expects a lat/lon geometry, and also produces a pixel-based projection.

所以当你在 [path.centroid（...））上调用 path path.centroid（...）] ，您试图投影已投影的坐标。你得到 NaN ，因为像素坐标的y位置-245超出了经度值的范围。

So when you call path on [path.centroid(...), path.centroid(...)], you're trying to project already-projected coordinates. You get at NaN because the y-position of the pixel coordinates, -245, is out of bounds for a longitude value.

修复此问题的最简单方法是使用 d3.svg.line 创建质心中心路径。我没有测试这个，但我认为它会像：

The easiest way to fix this is probably to use d3.svg.line to create the centroid-centroid path. I haven't tested this, but I think it would look like:

var line = d3.svg.line();
line([path.centroid(source_country), path.centroid(target_country)]);

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