问题
I am using D3.js and TopoJSON libraries to render a flat SVG map of the world in a small div on a web page. I'm also taking some geographic objects (polygons and circles), and plotting them on this map via lat/long coordinates. This all seems to be working pretty well, however, the circle objects that I am plotting on the map contain a radius element which is given in meters. I cannot find or figure out how to convert/scale this measurement appropriately onto the SVG map. Any help would be greatly appreciated.
The snippet of code that is drawing the circle and setting is:
if (formattedGeoObjects[a].shape.indexOf('circle') >= 0) {
//plot point for circle
svg.selectAll('.pin')
.data(formattedGeoObjects).enter().append('circle', '.pin')
.attr({fill: formattedGeoObjects[a].color.toString()})
.attr('r', 5) //formattedGeoObjects[a].radius is in meters
.attr('transform', 'translate(' +
projection([
formattedGeoObjects[a].location[0],
formattedGeoObjects[a].location[1]
]) + ')'
);
}
JSFiddle link for condensed version of the code: https://jsfiddle.net/vnrL0fdc/7/
Here's the full code for reference...
Function that does the bulk of the work:
setupMap: function(mapJson, theElement, geoObject, colorCb, normalizeCb) {
var width = 200;
var height = 120;
//define projection of spherical coordinates to Cartesian plane
var projection = d3.geo.mercator().scale((width + 1) / 2 / Math.PI).translate([width / 2, height / 2]);
//define path that takes projected geometry from above and formats it appropriately
var path = d3.geo.path().projection(projection);
//select the canvas-svg div and apply styling attributes
var svg =
d3.select('#' + theElement + ' .canvas-svg').append('svg')
.attr('width', width)
.attr('height', height)
.attr('class', 'ocean');
//convert the topoJSON back to GeoJSON
var countries = topojson.feature(mapJson, mapJson.objects.countries).features;
//give each country its own path element and add styling
svg.selectAll('.countries')
.data(countries).enter().append('path')
.attr('class', 'country')
.attr('d', path);
//add borders around all countries with mesh
svg.append('path')
.datum(topojson.mesh(mapJson, mapJson.objects.countries, function() {
return true;
}))
.attr('d', path)
.attr('class', 'border');
//if shape data exists, draw it on the map
if (geoObject !== null && geoObject.length !== 0) {
//normalize geoObject into format needed for d3 arc functionality and store each shapes color
var formattedGeoObjects = normalizeCb(geoObject, colorCb);
for (a = 0; a < formattedGeoObjects.length; a++) {
if (formattedGeoObjects[a].shape.indexOf('polygon') >= 0) {
for (b = 0; b < formattedGeoObjects[a].lines.length; b++) {
//plot point for polygon
svg.selectAll('.pin')
.data(formattedGeoObjects).enter().append('circle', '.pin')
.style({fill: formattedGeoObjects[a].color.toString()}).attr('r', 2)
.attr('transform', 'translate(' +
projection([
formattedGeoObjects[a].lines[b].coordinates[0][0],
formattedGeoObjects[a].lines[b].coordinates[0][1]
]) + ')'
);
}
//draw lines for polygon
svg.append('g').selectAll('.arc')
.data(formattedGeoObjects[a].lines).enter().append('path')
.attr({d: path})
.style({
stroke: formattedGeoObjects[a].color.toString(),
'stroke-width': '1px'
});
}
if (formattedGeoObjects[a].shape.indexOf('circle') >= 0) {
//plot point for circle
svg.selectAll('.pin')
.data(formattedGeoObjects).enter().append('circle', '.pin')
.attr({fill: formattedGeoObjects[a].color.toString()})
.attr('r', 5)
.attr('transform', 'translate(' +
projection([
formattedGeoObjects[a].location[0],
formattedGeoObjects[a].location[1]
]) + ')'
);
}
}
}
}
Here is a condensed version of what the formattedGeoObjects looks like:
[
{
"shape": "polygon0",
"color": "#000000",
"lines": [
{
"type": "LineString",
"coordinates": [
[
-24.9609375,
36.5625
],
[
-24.9609375,
55.1953125
]
]
}
..... more coords
]
},
{
"shape": "polygon1",
"color": "#006600",
"lines": [
{
"type": "LineString",
"coordinates": [
[
-42.1875,
26.3671875
],
[
-71.71875,
7.734375
]
]
}
..... more coordindates
]
},
{
"shape": "circle2",
"color": "#FF0000",
"location": [
13.359375,
31.640625
],
"radius": 1881365.33
}
]
And lastly, the CSS/HTML:
.canvas-svg {
.ocean {
background: #85E0FF;
}
.country {
fill: #FFFFFF;
}
.border {
fill: none;
stroke: #777;
stroke-width: .5;
}
}
<div class="canvas-svg"></div>
回答1:
A colleague of mine helped me out by showing me a much simpler way to do this (FYI - there has been an update to the lat/lon for the center of the circle). Plotting two points on the canvas and computing the distance to find the scale works and is accurate - but there is a much more simple way of doing it using the total pixels in the image and the total area of the world, code snippets and JSFiddle below:
var width = 200;
var height = 120;
//variables for scaling circle radius
var totPixels = (width * height);
var totWorldMeters = 510000000000;
var metersPerPixel = (totWorldMeters / totPixels);
var scaledRadius;
//scale the radius given in meters to pixels on the map
scaledRadius = (100 * (formattedGeoObjects[a].radius / metersPerPixel));
if(scaledRadius < 2) {
scaledRadius = 2;
}
Working JSFiddle: https://jsfiddle.net/vnrL0fdc/15/
回答2:
So, I "think" I've found a way to scale the radius (given in meters) to the pixels on the Cartesian plane d3 geo map. I'm probably making this way more complicated than it needs to be - but I'm not sure how else to do it.
The map's height, width, and projection is defined as:
var width = 200;
var height = 120;
var projection =
d3.geo.mercator()
.scale((width + 1) / 2 / Math.PI)
.translate([width / 2, height / 2]);
The geo objects that I'm plotting on the map contain lat/long coordinates for multiple points. By searching on stackoverflow, I found a distance formula:
d = sqrt((x2 - x1)^2 + (y2 - y1)^2)
The formula requires two points from the cartesian plane (x1,y1) and (x2,y2). I picked the circle's point and one of the polygon points, the lat/long coordinates are as follows:
Lat/Long for polygon1, point1: 36.5625, -24.9609375
Lat/Long for circle: 31.640625, 13.359375
I used the following web site to find out how many miles it is between the two coordinates above - http://www.freemaptools.com/how-far-is-it-between.htm
Miles between the two coordinates on the map are: 3020.207
Then I found the projected coordinates (x,y) on the Cartesian plane for the two lat/long coordinates via:
projection([long,lat])
X/Y for polygon1, point1: 86.0634765625, 38.040230671805666
X/Y for circle: 107.458984375, 41.36090550209383
So, I then plugged these values into the formula to calculate the pixel distance between the two points:
d = sqrt ( (107.458984375 - 86.0634765625)^2 + (41.36090550209383 - 38.040230671805666)^2 )
result = 21.651665891641176 pixels
miles per pixel = 139.49074473599643 (calculated by: 3020.207/21.651665891641176)
meters per pixel = 224488.5930964074505 (calculated by 139.49074473599643 * 1609.34)
Working JSFiddle: https://jsfiddle.net/vnrL0fdc/8/
This seems like an awfully roundabout way to scale meters to a mercator map projection. If anyone has a much simpler solution - please share!
来源：https://stackoverflow.com/questions/31573480/scale-a-circles-radius-given-in-meters-to-d3-js-d3-geo-mercator-map