开发笔记:PostGIS计算矢量切片唯一值渲染

????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))
}


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