Last Monday, on the 20th of July, I got an email from the organisation comitee of the annual FOSS4G conference stating that my submitted talk was accepted.

I am proud to have the opportunity to share my thoughts on “Comparing apples and oranges: Uncovering the mystery of component selection in WebGIS projects”. The talk will mainly deal with uncertainty when faced with technologigal decisions concerning component selection for WebGIS projects.

This years FOSS4G takes place in Sydney, Australia from 20th to 23rd of October. I eagerly anticipate meeting with many of the big players in the Free and OpenSource GIS Community.

After the conference is over, I want to stay for vacation in Australia and explore the eastern coast… staring out of the window to see the cloudy Bonn, I really wish I was there already.

Recently I was faced with 2D-geometries that should be “spatially clean” but they weren’t. Postgres/Postgis to the rescue, the data could be cleaned and the problem was solved.

“Spatially clean” in the project meant:

- no geometry should overlap another geometry
- the geometries should touch at least one other geometry

Let’s have a look at an example. First we’ll create a table with polygons and insert 4 triangles that together build a rectangle:

```
CREATE TABLE translate_polygons (
"id" SERIAL NOT NULL PRIMARY KEY,
"name" char(1) NOT NULL,
"the_geom" geometry NOT NULL
);
-- the data
INSERT INTO translate_polygons (name, the_geom) VALUES (
'A',
'POLYGON((2 1, 0 3, 2 3, 2 1))'::geometry
), (
'B',
'POLYGON((0 3, 2 5, 2 3, 0 3))'::geometry
), (
'C',
'POLYGON((2 5, 4 3, 2 3, 2 5))'::geometry
), (
'D',
'POLYGON((4 3, 2 1, 2 3, 4 3))'::geometry
);
```

Those geometries are displayed in the following image.

The data inserted matches the criterias noted above:

```
-- no overlapping
SELECT t1.name || ' overlaps ' || t2.name || ' = ' ||
```**ST_Overlaps(t1.the_geom, t2.the_geom)::text** AS overlap_test
FROM translate_polygons AS t1,
translate_polygons AS t2
WHERE t1.name <> t2.name
ORDER BY t1.name, t2.name;

overlap_test ---------------------- A overlaps B = false A overlaps C = false A overlaps D = false B overlaps A = false B overlaps C = false B overlaps D = false C overlaps A = false C overlaps B = false C overlaps D = false D overlaps A = false D overlaps B = false D overlaps C = false

```
-- in this case every triangle touches all other triangles:
SELECT t1.name || ' touches ' || t2.name || ' = ' ||
```**ST_Touches(t1.the_geom, t2.the_geom)::text** AS touch_test
FROM translate_polygons AS t1,
translate_polygons AS t2
WHERE t1.name <> t2.name
ORDER BY t1.name, t2.name;

touch_test -------------------- A touches B = true A touches C = true A touches D = true B touches A = true B touches C = true B touches D = true C touches A = true C touches B = true C touches D = true D touches A = true D touches B = true D touches C = true

Let’s use ST_Translate to move two of the triangles:

```
-- triangle to the lower left
-- move it to "southwest"
-- this produces a gap
UPDATE translate_polygons
SET the_geom =
```**ST_Translate(the_geom, -0.4, -0.2)**
WHERE name = 'A';
-- triangle to the upper right
-- move it to "southwest"
-- this produces overlapping geometries
UPDATE translate_polygons
SET the_geom = **ST_Translate(the_geom, -0.1, -0.4)**
WHERE name = 'C';

Now the geometries look like this:

We now have geometries that overlap and are not touching at least one other geometry:

touch_test --------------------- A touches B = false A touches C = false A touches D = false B touches A = false B touches C = false B touches D = true C touches A = false C touches B = false C touches D = false D touches A = false D touches B = true D touches C = false overlap_test ---------------------- A overlaps B = false A overlaps C = false A overlaps D = false B overlaps A = false B overlaps C = true B overlaps D = false C overlaps A = false C overlaps B = true C overlaps D = true D overlaps A = false D overlaps B = false D overlaps C = true

How can we correct the geometries when we do not want to ST_Translate()-calls with (manually) adjusted Parameters? Postgis provides a handy function **ST_SnapToGrid()**:

Function signatures:

- geometry ST_SnapToGrid(geometry geomA, float originX, float originY, float sizeX, float sizeY);
- geometry ST_SnapToGrid(geometry geomA, float sizeX, float sizeY);
- geometry ST_SnapToGrid(geometry geomA, float size);
- geometry ST_SnapToGrid(geometry geomA, geometry pointOrigin, float sizeX, float sizeY, float sizeZ, float sizeM);

ST_SnapToGrid — Snap all points of the input geometry to the grid defined by its origin and cell size. Remove consecutive points falling on the same cell, eventually returning NULL if output points are not enough to define a geometry of the given type. Collapsed geometries in a collection are stripped from it. Useful for reducing precision.

(From the Postgis-SVN-Manuals)

We can snap the geometries to a 1 by 1-grid when calling

```
UPDATE translate_polygons
SET the_geom =
```**ST_SnapToGrid(the_geom, 1)**

Now the geometries look neat again:

2008-12-15: fixed typos.