NSString *filePath = [[NSBundle mainBundle] pathForResource:@"haidian" ofType:@"txt"];
    NSString *pointsString = [NSString stringWithContentsOfFile:filePath encoding:NSUTF8StringEncoding error:nil];
    NSArray<NSString *> *pointsArray = [pointsString componentsSeparatedByString:@"\n"];
    BMKMapPoint points[pointsArray.count];
    for (NSUInteger i = 0; i < pointsArray.count; ++i) {
        if ([pointsArray[i] rangeOfString:@","].location != NSNotFound) {
            NSArray<NSString *> *tmpPoints = [pointsArray[i] componentsSeparatedByString:@","];
            points[i] = BMKMapPointMake([tmpPoints.firstObject doubleValue], [tmpPoints.lastObject doubleValue]);
        }
    }
    BMKPrismOverlay *_prism = [BMKPrismOverlay prismOverlayWithPoints:points count:pointsArray.count];
    _prism.height = 1000.f;
    [_mapView addOverlay:_prism];

- (BMKOverlayView *)mapView:(BMKMapView *)mapView viewForOverlay:(id<BMKOverlay>)overlay {
    if ([overlay isKindOfClass:[BMKPrismOverlay class]]) {
        BMKPrismOverlayView *prismOverlayView = [[BMKPrismOverlayView alloc] initWithPrismOverlay:(BMKPrismOverlay *)overlay];
        if ([overlay isEqual:_prismOverlay]) {
            prismOverlayView.sideFaceColor = [UIColor colorWithRed:0 green:0 blue:0.5 alpha:1.0];
            prismOverlayView.topFaceColor = [UIColor colorWithRed:0.0 green:0 blue:0.5 alpha:0.4];
            return prismOverlayView;

        }
        return nil;
    }
      
    return nil;
}