--- a/pkg/imports/sr.go	Wed Oct 17 15:22:38 2018 +0200
+++ b/pkg/imports/sr.go	Wed Oct 17 15:29:58 2018 +0200
@@ -3,13 +3,16 @@
 import (
 	"archive/zip"
 	"bufio"
+	"bytes"
 	"context"
 	"database/sql"
+	"encoding/binary"
 	"encoding/json"
 	"errors"
 	"fmt"
 	"io"
 	"log"
+	"math"
 	"os"
 	"path"
 	"strconv"
@@ -47,6 +50,11 @@
 )
 
 const (
+	wkbNDR     byte   = 1
+	wkbPolygon uint32 = 3
+)
+
+const (
 	insertPointsSQL = `
 INSERT INTO waterway.sounding_results (
   bottleneck_id,
@@ -320,9 +328,28 @@
 }
 
 func (p Polygon) AsWBK() []byte {
-	if len(p) == 0 {
+	if p == nil {
 		return nil
 	}
-	// TODO: Implement me!
-	return nil
+	// pre-calculate size to avoid reallocations.
+	size := 1 + 4 + 4
+	for _, ring := range p {
+		size += 4 + len(ring)*2*8
+	}
+
+	buf := bytes.NewBuffer(make([]byte, 0, size))
+
+	binary.Write(buf, binary.LittleEndian, wkbNDR)
+	binary.Write(buf, binary.LittleEndian, wkbPolygon)
+	binary.Write(buf, binary.LittleEndian, uint32(len(p)))
+
+	for _, ring := range p {
+		binary.Write(buf, binary.LittleEndian, uint32(len(ring)))
+		for _, v := range ring {
+			binary.Write(buf, binary.LittleEndian, math.Float64bits(v.X))
+			binary.Write(buf, binary.LittleEndian, math.Float64bits(v.Y))
+		}
+	}
+
+	return buf.Bytes()
 }