--- a/pkg/controllers/gauges.go	Thu Mar 21 17:31:03 2019 +0100
+++ b/pkg/controllers/gauges.go	Thu Mar 21 18:07:49 2019 +0100
@@ -19,10 +19,12 @@
 	"fmt"
 	"log"
 	"net/http"
+	"sort"
 	"strconv"
 	"time"
 
 	"github.com/gorilla/mux"
+	"gonum.org/v1/gonum/stat"
 
 	"gemma.intevation.de/gemma/pkg/common"
 	"gemma.intevation.de/gemma/pkg/middleware"
@@ -56,12 +58,191 @@
 SELECT
   measure_date,
   water_level,
-  predicted
+  predicted,
 FROM waterway.gauge_measurements
 WHERE
 `
+	selectWaterlevelsMeasuredSQL = `
+SELECT
+  measure_date,
+  water_level
+FROM waterway.gauge_measurements
+WHERE
+  NOT predicted
+  AND fk_gauge_id = (
+    $1::char(2),
+    $2::char(3),
+    $3::char(5),
+    $4::char(5),
+    $5::int
+  ) 
+  AND
+    measure_date BETWEEN
+	  $6::timestamp with time zone AND $7::timestamp with time zone
+ORDER BY measure_date
+`
 )
 
+func averageWaterlevels(rw http.ResponseWriter, req *http.Request) {
+	gauge := mux.Vars(req)["gauge"]
+
+	isrs, err := models.IsrsFromString(gauge)
+	if err != nil {
+		http.Error(
+			rw, fmt.Sprintf("error: Invalid ISRS code: %v", err),
+			http.StatusBadRequest)
+		return
+	}
+
+	var from, to time.Time
+
+	if t := req.FormValue("to"); t != "" {
+		var err error
+		if to, err = time.ParseInLocation(common.DateFormat, t, time.UTC); err != nil {
+			http.Error(
+				rw, fmt.Sprintf("error: bad from date: %v", err),
+				http.StatusBadRequest)
+			return
+		}
+	} else {
+		y, m, d := time.Now().Date()
+		to = time.Date(y, m, d, 0, 0, 0, 0, time.UTC)
+	}
+
+	if f := req.FormValue("from"); f != "" {
+		var err error
+		if from, err = time.ParseInLocation(common.DateFormat, f, time.UTC); err != nil {
+			http.Error(
+				rw, fmt.Sprintf("error: bad from date: %v", err),
+				http.StatusBadRequest)
+			return
+		}
+	} else {
+		from = to.AddDate(-1, 0, 0)
+	}
+
+	to = to.AddDate(0, 0, 1).Add(-time.Nanosecond)
+
+	if to.Before(from) {
+		from, to = to, from
+	}
+
+	conn := middleware.GetDBConn(req)
+
+	ctx := req.Context()
+
+	rows, err := conn.QueryContext(
+		ctx,
+		selectWaterlevelsMeasuredSQL,
+		isrs.CountryCode,
+		isrs.LoCode,
+		isrs.FairwaySection,
+		isrs.Orc,
+		isrs.Hectometre,
+		from, to,
+	)
+	if err != nil {
+		http.Error(
+			rw, fmt.Sprintf("error: %v", err),
+			http.StatusInternalServerError)
+		return
+	}
+	defer rows.Close()
+
+	rw.Header().Add("Content-Type", "text/csv")
+
+	out := csv.NewWriter(rw)
+
+	var last time.Time
+	var values []float64
+
+	record := []string{
+		"#date",
+		"#min",
+		"#max",
+		"#mean",
+		"#median",
+		"#q25",
+		"#q75",
+	}
+
+	if err := out.Write(record); err != nil {
+		log.Printf("error: %v\n", err)
+		// Too late for an HTTP error code.
+		return
+	}
+
+	format := func(v float64) string {
+		return strconv.FormatFloat(v, 'f', -1, 64)
+	}
+
+	write := func() error {
+		if len(values) > 0 {
+			sort.Float64s(values)
+			// date
+			record[0] = last.Format(common.DateFormat)
+			// min
+			record[1] = format(values[0])
+			// max
+			record[2] = format(values[len(values)-1])
+			// mean
+			record[3] = format(stat.Mean(values, nil))
+			// median
+			record[4] = format(values[len(values)/2])
+			// Q25
+			record[5] = format(
+				stat.Quantile(0.25, stat.Empirical, values, nil))
+			// Q75
+			record[6] = format(
+				stat.Quantile(0.75, stat.Empirical, values, nil))
+
+			err := out.Write(record)
+			values = values[:0]
+			return err
+		}
+		return nil
+	}
+
+	for rows.Next() {
+		var (
+			date  time.Time
+			value float64
+		)
+		if err := rows.Scan(&date, &value); err != nil {
+			log.Printf("error: %v\n", err)
+			// Too late for an HTTP error code.
+			return
+		}
+		oy, om, od := last.Date()
+		ny, nm, nd := date.Date()
+		if oy != ny || om != nm || od != nd {
+			if err := write(); err != nil {
+				log.Printf("error: %v\n", err)
+				// Too late for an HTTP error code.
+				return
+			}
+			last = date
+		} else {
+			values = append(values, value)
+		}
+	}
+	write()
+
+	if err := rows.Err(); err != nil {
+		log.Printf("error: %v", err)
+		// Too late for an HTTP error code.
+		return
+	}
+
+	out.Flush()
+	if err := out.Error(); err != nil {
+		log.Printf("error: %v", err)
+		// Too late for an HTTP error code.
+		return
+	}
+
+}
+
 func nashSutcliffe(
 	_ interface{},
 	req *http.Request,