diff --git a/doc/images/profile.png b/doc/images/profile.png
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diff --git a/doc/reference_guide.rst b/doc/reference_guide.rst
index 2a7cc7dc..bfc0ab0d 100644
--- a/doc/reference_guide.rst
+++ b/doc/reference_guide.rst
@@ -1176,10 +1176,44 @@ Volume
 ......
 
 Calculate volume above a reference plain.
+The user have to enter the height of the reference plain. The volume is
+calculated in the prisms above the reference plain. The result is written into 
+the *Calculation results* window.
+
+.. code::
+
+    2020.11.02 19:52 - Volume ... - test
+    Base height  Volume        Above        Below       Area     Surface area
+     100.000    1267409.3    1267409.3          0.0     178543.3     179311.4
+
+The *Volume* = *Above* - *Below*.
 
 Volume difference
 .................
 
+Calculate volume difference between the loaded and another TIN. GRIDs are
+generated from both TINs using the same GRID steps. Only the common area of the
+two generated GRIDs are considered.
+
+.. figure:: rg_images/ascii_grid.png
+
+First the user have to give the grid step (step for north and east axis). 
+Next in the file selection dialogue a previously saved DTM can be selected.
+
+.. code::
+
+    2020.11.02 19:40 - Volume difference ... testa - testb
+    Grid step: 50.00
+    Lower left  corner: 655886.80 222996.11
+    Upper right corner: 656417.18 223621.18
+    Cut  Volume: 32125.1 m3 Area: 125000.0 m2
+    Fill Volume: 10376.4 m3 Area:  45000.0 m2
+    Same: 10000.0 m2
+
+Beside the results in the *Calculation results* window a new difference grid is
+generated in the folder of the loaded TIN. This file is an ESRI ASCII GRID
+which can be loaded into several sortware among others into QGIS.
+
 VRML/X3D export
 ...............
 
diff --git a/doc/rg_images/ascii_grid.png b/doc/rg_images/ascii_grid.png
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diff --git a/src/sqlgeo.tcl b/src/sqlgeo.tcl
new file mode 100644
index 00000000..e7b07eb7
--- /dev/null
+++ b/src/sqlgeo.tcl
@@ -0,0 +1,47 @@
+#
+#   Save coordinates to postgis SQL 
+#   @param fa name of geo data set
+#   @param rn name of data file (.csv)
+#   @return 0 on success
+proc SavePSql {fn rn} {
+    global ${fn}_coo
+    global geoLoaded
+    global geoCodes geoEasyMsg
+
+    if {[info exists geoLoaded]} {
+        set pos [lsearch -exact $geoLoaded $fn]
+        if {$pos == -1} {
+            return -8           ;# geo data set not loaded
+        }
+    } else {
+        return 0
+    }
+    set f [open $rn w]
+    # get epsg code
+    set epsg [GeoEntry "$geoCodes(140):" $geoCodes(140)]
+    # create table
+    puts $f "CREATE TABLE $fn ("
+    puts $f "  geom geometry(Pointz, $epsg),"
+    puts $f "  psz varchar(20) PRIMARY KEY,"
+    puts $f "  code varchar(20)"
+    puts $f ");"
+
+    foreach pn [lsort -dictionary [array names ${fn}_coo]] {
+        set x [GetVal {38} [set ${fn}_coo($pn)]]
+        set y [GetVal {37} [set ${fn}_coo($pn)]]
+        set z [GetVal {39} [set ${fn}_coo($pn)]]
+        set code [GetVal {4} [set ${fn}_coo($pn)]]
+        if {[string length $code]} {
+            set code "'$code'"
+        } else {
+            set code "NULL"
+        }
+        if {[string length $x] && [string length $y]} {
+            if {[string length $z] == 0} { set z 0 }
+            puts $f "INSERT INTO $fn "
+            puts $f "  VALUES (ST_SetSRID(ST_MakePoint($x,$y,$z), $epsg), '$pn', $code);"
+        }
+    }
+    close $f
+    return 0
+}