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**Affine Transformation Operations in PostGIS**» User defined Function in PL/pgSQL to compute the transformation parameters

# User defined Function in PL/pgSQL to compute the transformation parameters

Based on the results of the proof of concept example developed previously, a single function was developed in the procedural language for the PostgreSQL database system called PL/pgSQL. The only input parameter to this procedure is a text string that results in a table in the following format:

gid (integer), from_x (float), from_y (float), to_x (float), to_y (float)

The use of a SQL query found to be the simplest way to avoid the difficulties in programing the procedure without the need for temporary tables. An added benefit is the control point data can be in almost any format as long as it can be arranged in the format specified above. For example if the "from" points are in a geometry column (the_geom) in a table called from_pts and the corresponding "to" points are in a similar table called to_pts with a common attribute called "link_id", then the query would be:

`SELECT from_pts.link_id AS gid, x(from_pts.the_geom) AS from_x, y(from_pts.the_geom) AS from_y, x(to_pts.the_geom) AS to_x, y(to_pts.the_geom) AS to_y FROM from_pts, to_pts WHERE from_pts.link_id = to_pts.link_id`

Other table layouts and queries are possible depending on the manner in which the control points are collected.

The following is the SQL code to add a new procedure called trans_param() into a PostGIS database:

```
-- Function: trans_param(IN sql text, OUT a double precision, OUT b double precision, OUT d double precision, OUT e double precision, OUT xoff double precision, OUT yoff double precision)
-- DROP FUNCTION trans_param(IN sql text, OUT a double precision, OUT b double precision, OUT d double precision, OUT e double precision, OUT xoff double precision, OUT yoff double precision);
CREATE OR REPLACE FUNCTION trans_param(IN sql text, OUT a double precision, OUT b double precision, OUT d double precision, OUT e double precision, OUT xoff double precision, OUT yoff double precision) AS
$BODY$
DECLARE
cc_row record;
cc_det float;
inv_cc00 float;
inv_cc01 float;
inv_cc02 float;
inv_cc11 float;
inv_cc12 float;
inv_cc22 float;
BEGIN
EXECUTE 'SELECT
count(a.to_x) as cc00,
sum(a.to_x) as cc01,
sum(a.to_y) as cc02,
sum(a.to_x * b.to_x) as cc11,
sum(a.to_x * b.to_y) as cc12,
sum(a.to_y * b.to_y) as cc22,
sum(a.from_y) as aa0,
sum(a.from_y * b.to_x) as aa1,
sum(a.from_y * b.to_y) as aa2,
sum(a.from_x) as bb0,
sum(a.from_x * b.to_x) as bb1,
sum(a.from_x * b.to_y) as bb2
from (' || sql || ') a,(' || sql|| ') b WHERE a.gid = b.gid' INTO cc_row;
SELECT INTO cc_det
cc_row.cc00*cc_row.cc11*cc_row.cc22 +
cc_row.cc01*cc_row.cc12*cc_row.cc02 +
cc_row.cc02*cc_row.cc01*cc_row.cc12 -
cc_row.cc00*cc_row.cc12*cc_row.cc12 -
cc_row.cc01*cc_row.cc01*cc_row.cc22 -
cc_row.cc02*cc_row.cc11*cc_row.cc02;
SELECT INTO inv_cc00 (cc_row.cc11*cc_row.cc22-cc_row.cc12*cc_row.cc12)/cc_det;
SELECT INTO inv_cc01 (cc_row.cc12*cc_row.cc02-cc_row.cc01*cc_row.cc22)/cc_det;
SELECT INTO inv_cc02 (cc_row.cc01*cc_row.cc12-cc_row.cc11*cc_row.cc02)/cc_det;
SELECT INTO inv_cc11 (cc_row.cc00*cc_row.cc22-cc_row.cc02*cc_row.cc02)/cc_det;
SELECT INTO inv_cc12 (cc_row.cc01*cc_row.cc02-cc_row.cc00*cc_row.cc12)/cc_det;
SELECT INTO inv_cc22 (cc_row.cc00*cc_row.cc11-cc_row.cc01*cc_row.cc01)/cc_det;
SELECT INTO a cc_row.bb0*inv_cc01+cc_row.bb1*inv_cc11+cc_row.bb2*inv_cc12;
SELECT INTO b cc_row.bb0*inv_cc02+cc_row.bb1*inv_cc12+cc_row.bb2*inv_cc22;
SELECT INTO d cc_row.aa0*inv_cc01+cc_row.aa1*inv_cc11+cc_row.aa2*inv_cc12;
SELECT INTO e cc_row.aa0*inv_cc02+cc_row.aa1*inv_cc12+cc_row.aa2*inv_cc22;
SELECT INTO xoff cc_row.bb0*inv_cc00+cc_row.bb1*inv_cc01+cc_row.bb2*inv_cc02;
SELECT INTO yoff cc_row.aa0*inv_cc00+cc_row.aa1*inv_cc01+cc_row.aa2*inv_cc02;
END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION trans_param(IN sql text, OUT a double precision, OUT b double precision, OUT d double precision, OUT e double precision, OUT xoff double precision, OUT yoff double precision) OWNER TO postgres;
```

Currently there is no error checking in the code if the determinant is zero.

To use the procedure simply use: `SELECT trans_param('my SQL text')`

The data from this series of articles is stored in a table called link. The id of the points is gid and the "from" values are b_x and b_y. The "to" values are a_x and a_y. Thus the query is:

`SELECT gid,b_x AS from_x, b_y AS from_y, a_x AS to_x, a_y AS to_y FROM link`

The result of the procedure is:

```
SELECT * FROM trans_param(
'SELECT gid,b_x as from_x, b_y as from_y, a_x as to_x, a_y as to_y from link')
```

`0.997546509279282;-0.00917177514909895;0.0135300872142122;1.00246938174737;-5301.39933295548;28155.8822879205`

Which matches the results returned from GRASS and R. Additional queries will be developed to give a table of residuals, the RMS error and the actual transformation of geometry.

## Software

- General Purpose Programming with Scripting Languages
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- PROJ: forward and reverse geographic projections
- GDAL and OGR: geodata conversion and re-projection tools
- R: advanced statistical package
- GRASS GIS: raster, vector, and imagery analysis
- Generic Mapping Tools: high quality map production