postgis计算矢量切片

    没写错，是使用postgis计算出来矢量切片。在这之前先准备一个数据:一个GIS数据表(本例中数据为一百万的点数据，坐标：4326),并在表中添加x,y字段，方便后面的数据筛选。sql中用到了
ST_AsMVT和ST_AsMVTGeom。

    本文中创建矢量切片很简单,就是使用下方的一个sql,运行结果如下图。接着写一个矢量切片的http服务(参考go-vtile-example，这个例子中矢量切片压缩率更高)，并且使用mapbox进行前端展示（小贴士:sql中‘points’的字符串与渲染中mapbox里的source-layer一致）.代码见最下方

SELECT ST_AsMVT(tile,'points') tile  FROM(
SELECT ST_AsMVTGeom(geom,ST_MakeEnvelope(100,10,125,22, 4326),4096, 0, true)	
 AS geom FROM grid20180322 ) 
AS  tile where  tile.geom is not null

package main

import 
(   
    _ "github.com/lib/pq"
    "database/sql"
    "time"
    "log"
    "math"
    "errors"
	"fmt"
    "net/http"
    "regexp"
	"strconv"
	"strings"
)

func tilePathToXYZ(path string) (TileID, error) {
	xyzReg := regexp.MustCompile("(?P[0-9]+)/(?P[0-9]+)/(?P[0-9]+)")
	matches := xyzReg.FindStringSubmatch(path)
	if len(matches) == 0 {
		return TileID{}, errors.New("Unable to parse path as tile")
	}
	x, err := strconv.ParseUint(matches[2], 10, 32)
	if err != nil {
		return TileID{}, err
	}
	y, err := strconv.ParseUint(matches[3], 10, 32)
	if err != nil {
		return TileID{}, err
	}
	z, err := strconv.ParseUint(matches[1], 10, 32)
	if err != nil {
		return TileID{}, err
	}
	return TileID{x: uint32(x), y: uint32(y), z: uint32(z)}, nil
}
type LngLat struct {
	lng float64
	lat float64
}
type TileID struct {
	x uint32
	y uint32
	z uint32
}

func tile2lon( x int,  z int)(a float64) {
	return float64(x) /math.Pow(2, float64(z)) * 360.0 - 180;
 }

 func tile2lat( y int,  z int)(a float64) {
   n := math.Pi - (2.0 * math.Pi * float64(y)) / math.Pow(2, float64(z));
   return math.Atan(math.Sinh(n))*180/math.Pi;
 }
 

func FloatToString(input_num float64) string {
    // to convert a float number to a string
    return strconv.FormatFloat(input_num, 'f', 6, 64)
}

 func loadData(xyz TileID)(a []byte){
	ymax :=FloatToString(tile2lat(int(xyz.y), int(xyz.z)));
	ymin := FloatToString(tile2lat(int(xyz.y+1), int(xyz.z)));
	xmin := FloatToString(tile2lon(int(xyz.x), int(xyz.z)));
	xmax := FloatToString(tile2lon(int(xyz.x+1), int(xyz.z)));
	fmt.Println("ymax: ", ymax)
	fmt.Println("ymin: ",ymin)
	fmt.Println("xmin : ",xmin )
	fmt.Println("xmax : ",xmax )
	connStr := "dbname=xx user=xx password=xx host=localhost port=5433  sslmode=disable"
	db, err := sql.Open("postgres", connStr)
	if err != nil {
        panic(err)
    }
    defer db.Close()

    err = db.Ping()
    if err != nil {
        panic(err)
    }

    fmt.Println("Successfully connected!")
	var tile []byte
	 s := []string{xmin,ymin,xmax,ymax}
	 maxmin:=strings.Join(s, ",") 
	 s2 := []string{" where (x between", xmin,"and",xmax,") and ( y between",ymin,"and",ymax,")"}
	wmaxmin:=strings.Join(s2, " ") 
	sql:="SELECT ST_AsMVT(tile,'points') tile  FROM(SELECT ST_AsMVTGeom(w.geom,ST_MakeEnvelope("+maxmin+", 4326),4096, 0, true)	 AS geom FROM (select geom from grid20180322"+wmaxmin+") w) AS  tile where  tile.geom is not null"
	rows1:= db.QueryRow(sql)
	err1 := rows1.Scan(&tile)
	if err1 != nil {
		log.Fatal(err1)
	}
    fmt.Println(sql)
    //defer rows1.Close()
	return tile
 }
func main(){
    //t1 := time.Now() 
	mux := http.NewServeMux()
	tileBase := "/tiles/"
	mux.HandleFunc(tileBase, func(w http.ResponseWriter, r *http.Request) {
		t2 := time.Now() 
		log.Printf("url: %s", r.URL.Path)
		tilePart := r.URL.Path[len(tileBase):]
		fmt.Println("tilePart: ", tilePart)
		xyz, err := tilePathToXYZ(tilePart)
		fmt.Println("xyz: ", xyz)
		
		if err != nil {
			http.Error(w, "Invalid tile url", 400)
			return
		}
		tile:=loadData(xyz)
		
		// All this APi to be requests from other domains.
		w.Header().Set("Content-Type", "application/x-protobuf")
		w.Header().Set("Access-Control-Allow-Origin", "*")
		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, OPTIONS")
		w.Write(tile)
		elapsed2 := time.Since(t2)
	
	fmt.Println("耗时: ", elapsed2)
	})
	log.Fatal(http.ListenAndServe(":8081", mux))
}