我使用Google Maps API创建了一个地图，突出显示了所有明尼苏达县。基本上，我使用一组经度/纬度坐标创建了县边界多边形。以下是生成的地图截图：
用户要求之一是能够将类似的地图作为图像嵌入到他们的PowerPoint/keynote幻灯片中。我找不到任何有用的Google Maps API，使我能够像原来一样保存我的自定义地图（如果你知道方法，请告诉我），所以我想应该在Java中使用Graphics2D进行绘制。
阅读有关将经度/纬度转换为X/Y坐标的公式后，我最终得到了以下代码：
private static final int EARTH_RADIUS = 6371;
private static final double FOCAL_LENGTH = 500;
...
BufferedImage bi = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
Graphics2D g = bi.createGraphics();
for (Coordinate coordinate : coordinates) {
double latitude = Double.valueOf(coordinate.getLatitude());
double longitude = Double.valueOf(coordinate.getLongitude());
latitude = latitude * Math.PI / 180;
longitude = longitude * Math.PI / 180;
double x = EARTH_RADIUS * Math.sin(latitude) * Math.cos(longitude);
double y = EARTH_RADIUS * Math.sin(latitude) * Math.sin(longitude);
double z = EARTH_RADIUS * Math.cos(latitude);
double projectedX = x * FOCAL_LENGTH / (FOCAL_LENGTH + z);
double projectedY = y * FOCAL_LENGTH / (FOCAL_LENGTH + z);
// scale the map bigger
int magnifiedX = (int) Math.round(projectedX * 5);
int magnifiedY = (int) Math.round(projectedY * 5);
...
g.drawPolygon(...);
...
}
生成的地图与使用相同经度/纬度集合的Google Maps API生成的地图相似。然而，它看起来有点倾斜并且有些不一样，我不确定如何解决。
如何使县的形状看起来与上面由Google Maps API生成的地图完全一样？
非常感谢。
最终解决方案
我终于找到了解决方案，感谢@QuantumMechanic和@Anon。
这里使用墨卡托投影真正做到了。我使用Java地图投影库（Java Map Projection Library）来执行墨卡托投影的计算。
private static final int IMAGE_WIDTH = 1000;
private static final int IMAGE_HEIGHT = 1000;
private static final int IMAGE_PADDING = 50;
...
private List<Point2D.Double> convertToXY(List<Coordinate> coordinates) {
List<Point2D.Double> xys = new ArrayList<Point2D.Double>();
MercatorProjection projection = new MercatorProjection();
for (Coordinate coordinate : coordinates) {
double latitude = Double.valueOf(coordinate.getLatitude());
double longitude = Double.valueOf(coordinate.getLongitude());
// convert to radian
latitude = latitude * Math.PI / 180;
longitude = longitude * Math.PI / 180;
Point2D.Double d = projection.project(longitude, latitude, new Point2D.Double());
// shift by 10 to remove negative Xs and Ys
// scaling by 6000 to make the map bigger
int magnifiedX = (int) Math.round((10 + d.x) * 6000);
int magnifiedY = (int) Math.round((10 + d.y) * 6000);
minX = (minX == -1) ? magnifiedX : Math.min(minX, magnifiedX);
minY = (minY == -1) ? magnifiedY : Math.min(minY, magnifiedY);
xys.add(new Point2D.Double(magnifiedX, magnifiedY));
}
return xys;
}
...
通过使用生成的XY坐标，地图看起来是倒置的，这是因为我相信graphics2D的0,0从左上角开始。因此，我需要通过从图像高度中减去该值来反转Y轴，类似于这样：
...
Polygon polygon = new Polygon();
for (Point2D.Double point : xys) {
int adjustedX = (int) (IMAGE_PADDING + (point.getX() - minX));
// need to invert the Y since 0,0 starts at top left
int adjustedY = (int) (IMAGE_HEIGHT - IMAGE_PADDING - (point.getY() - minY));
polygon.addPoint(adjustedX, adjustedY);
}
...
这是生成的地图：
非常完美！
更新 01-25-2013
以下是根据宽度和高度（以像素为单位）创建图像映射的代码。在这种情况下，我没有依赖Java Map Project Library，而是从中提取了相关公式并将其嵌入到我的代码中。与上面的代码示例相比，这使您更加控制地图生成，后者依赖于任意缩放值（上面的示例使用6000）。
public class MapService {
// CHANGE THIS: the output path of the image to be created
private static final String IMAGE_FILE_PATH = "/some/user/path/map.png";
// CHANGE THIS: image width in pixel
private static final int IMAGE_WIDTH_IN_PX = 300;
// CHANGE THIS: image height in pixel
private static final int IMAGE_HEIGHT_IN_PX = 500;
// CHANGE THIS: minimum padding in pixel
private static final int MINIMUM_IMAGE_PADDING_IN_PX = 50;
// formula for quarter PI
private final static double QUARTERPI = Math.PI / 4.0;
// some service that provides the county boundaries data in longitude and latitude
private CountyService countyService;
public void run() throws Exception {
// configuring the buffered image and graphics to draw the map
BufferedImage bufferedImage = new BufferedImage(IMAGE_WIDTH_IN_PX,
IMAGE_HEIGHT_IN_PX,
BufferedImage.TYPE_INT_RGB);
Graphics2D g = bufferedImage.createGraphics();
Map<RenderingHints.Key, Object> map = new HashMap<RenderingHints.Key, Object>();
map.put(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC);
map.put(RenderingHints.KEY_RENDERING, RenderingHints.VALUE_RENDER_QUALITY);
map.put(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
RenderingHints renderHints = new RenderingHints(map);
g.setRenderingHints(renderHints);
// min and max coordinates, used in the computation below
Point2D.Double minXY = new Point2D.Double(-1, -1);
Point2D.Double maxXY = new Point2D.Double(-1, -1);
// a list of counties where each county contains a list of coordinates that form the county boundary
Collection<Collection<Point2D.Double>> countyBoundaries = new ArrayList<Collection<Point2D.Double>>();
// for every county, convert the longitude/latitude to X/Y using Mercator projection formula
for (County county : countyService.getAllCounties()) {
Collection<Point2D.Double> lonLat = new ArrayList<Point2D.Double>();
for (CountyBoundary countyBoundary : county.getCountyBoundaries()) {
// convert to radian
double longitude = countyBoundary.getLongitude() * Math.PI / 180;
double latitude = countyBoundary.getLatitude() * Math.PI / 180;
Point2D.Double xy = new Point2D.Double();
xy.x = longitude;
xy.y = Math.log(Math.tan(QUARTERPI + 0.5 * latitude));
// The reason we need to determine the min X and Y values is because in order to draw the map,
// we need to offset the position so that there will be no negative X and Y values
minXY.x = (minXY.x == -1) ? xy.x : Math.min(minXY.x, xy.x);
minXY.y = (minXY.y == -1) ? xy.y : Math.min(minXY.y, xy.y);
lonLat.add(xy);
}
countyBoundaries.add(lonLat);
}
// readjust coordinate to ensure there are no negative values
for (Collection<Point2D.Double> points : countyBoundaries) {
for (Point2D.Double point : points) {
point.x = point.x - minXY.x;
point.y = point.y - minXY.y;
// now, we need to keep track the max X and Y values
maxXY.x = (maxXY.x == -1) ? point.x : Math.max(maxXY.x, point.x);
maxXY.y = (maxXY.y == -1) ? point.y : Math.max(maxXY.y, point.y);
}
}
int paddingBothSides = MINIMUM_IMAGE_PADDING_IN_PX * 2;
// the actual drawing space for the map on the image
int mapWidth = IMAGE_WIDTH_IN_PX - paddingBothSides;
int mapHeight = IMAGE_HEIGHT_IN_PX - paddingBothSides;
// determine the width and height ratio because we need to magnify the map to fit into the given image dimension
double mapWidthRatio = mapWidth / maxXY.x;
double mapHeightRatio = mapHeight / maxXY.y;
// using different ratios for width and height will cause the map to be stretched. So, we have to determine
// the global ratio that will perfectly fit into the given image dimension
double globalRatio = Math.min(mapWidthRatio, mapHeightRatio);
// now we need to readjust the padding to ensure the map is always drawn on the center of the given image dimension
double heightPadding = (IMAGE_HEIGHT_IN_PX - (globalRatio * maxXY.y)) / 2;
double widthPadding = (IMAGE_WIDTH_IN_PX - (globalRatio * maxXY.x)) / 2;
// for each country, draw the boundary using polygon
for (Collection<Point2D.Double> points : countyBoundaries) {
Polygon polygon = new Polygon();
for (Point2D.Double point : points) {
int adjustedX = (int) (widthPadding + (point.getX() * globalRatio));
// need to invert the Y since 0,0 starts at top left
int adjustedY = (int) (IMAGE_HEIGHT_IN_PX - heightPadding - (point.getY() * globalRatio));
polygon.addPoint(adjustedX, adjustedY);
}
g.drawPolygon(polygon);
}
// create the image file
ImageIO.write(bufferedImage, "PNG", new File(IMAGE_FILE_PATH));
}
}
结果：图片宽度为600像素，图片高度为600像素，图片内边距为50像素
结果：图片宽度为300像素，图片高度为500像素，图片内边距为50像素
