我正在尝试为MKPolygon制作区域计算类别。 我发现了一些带有算法链接的JS代码https://github.com/mapbox/geojson-area/blob/master/index.js#L1:http://trs-new.jpl.nasa.gov/dspace/handle/2014/40409。 它说:
这是我的代码,它给出了错误的结果(比实际的数千倍):
#define kEarthRadius 6378137
@implementation MKPolygon (AreaCalculation)
- (double) area {
double area = 0;
NSArray *coords = [self coordinates];
if (coords.count > 2) {
CLLocationCoordinate2D p1, p2;
for (int i = 0; i < coords.count - 1; i++) {
p1 = [coords[i] MKCoordinateValue];
p2 = [coords[i + 1] MKCoordinateValue];
area += degreesToRadians(p2.longitude - p1.longitude) * (2 + sinf(degreesToRadians(p1.latitude)) + sinf(degreesToRadians(p2.latitude)));
}
area = area * kEarthRadius * kEarthRadius / 2;
}
return area;
}
- (NSArray *)coordinates {
NSMutableArray *points = [NSMutableArray arrayWithCapacity:self.pointCount];
for (int i = 0; i < self.pointCount; i++) {
MKMapPoint *point = &self.points[i];
[points addObject:[NSValue valueWithMKCoordinate:MKCoordinateForMapPoint(* point)]];
}
return points.copy;
}
double degreesToRadians(double radius) {
return radius * M_PI / 180;
}
@end
我错过了什么?
答案 0 :(得分:6)
整个算法在 Swift 3.0 :
中实现(1,A,10,C,120) (2,B,20,C,40) (3,A,10,B,50)
答案 1 :(得分:3)
循环中缺少i = N-1
和i+1 = 0
(环绕)的最后一步。
答案 2 :(得分:1)
这可能会对某人有所帮助...... 您需要将形状边缘点传递给下面的方法,并返回多边形的正确区域
static double areaOfCurveWithPoints(const NSArray *shapeEdgePoints) {
CGPoint initialPoint = [shapeEdgePoints.firstObject CGPointValue];
CGMutablePathRef cgPath = CGPathCreateMutable();
CGPathMoveToPoint(cgPath, &CGAffineTransformIdentity, initialPoint.x, initialPoint.y);
for (int i = 1;i<shapeEdgePoints.count ;i++) {
CGPoint point = [[shapeEdgePoints objectAtIndex:i] CGPointValue];
CGPathAddLineToPoint(cgPath, &CGAffineTransformIdentity, point.x, point.y);
}
CGPathCloseSubpath(cgPath);
CGRect frame = integralFrameForPath(cgPath);
size_t bytesPerRow = bytesPerRowForWidth(frame.size.width);
CGContextRef gc = createBitmapContextWithFrame(frame, bytesPerRow);
CGContextSetFillColorWithColor(gc, [UIColor whiteColor].CGColor);
CGContextAddPath(gc, cgPath);
CGContextFillPath(gc);
double area = areaFilledInBitmapContext(gc);
CGPathRelease(cgPath);
CGContextRelease(gc);
return area;
}
static CGRect integralFrameForPath(CGPathRef path) {
CGRect frame = CGPathGetBoundingBox(path);
return CGRectIntegral(frame);
}
static size_t bytesPerRowForWidth(CGFloat width) {
static const size_t kFactor = 64;
// Round up to a multiple of kFactor, which must be a power of 2.
return ((size_t)width + (kFactor - 1)) & ~(kFactor - 1);
}
static CGContextRef createBitmapContextWithFrame(CGRect frame, size_t bytesPerRow) {
CGColorSpaceRef grayscale = CGColorSpaceCreateDeviceGray();
CGContextRef gc = CGBitmapContextCreate(NULL, frame.size.width, frame.size.height, 8, bytesPerRow, grayscale, kCGImageAlphaNone);
CGColorSpaceRelease(grayscale);
CGContextTranslateCTM(gc, -frame.origin.x, -frame.origin.x);
return gc;
}
static double areaFilledInBitmapContext(CGContextRef gc) {
size_t width = CGBitmapContextGetWidth(gc);
size_t height = CGBitmapContextGetHeight(gc);
size_t stride = CGBitmapContextGetBytesPerRow(gc);
// Get a pointer to the data
unsigned char *bitmapData = (unsigned char *)CGBitmapContextGetData(gc);
uint64_t coverage = 0;
for (size_t y = 0; y < height; ++y) {
for (size_t x = 0; x < width; ++x) {
coverage += bitmapData[y * stride + x];
}
}
// NSLog(@"coverage =%llu UINT8_MAX =%d",coverage,UINT8_MAX);
return (double)coverage / UINT8_MAX;
}