Processing geometries

As said in the post about data types, geometries are simple data types at the same level strings are. Typically this means that they are treated as a whole and there is no way to mess around with the internal components of those values.

However, in case of GGL2 there is a special construction to process the geometry coordinates individually: the "process coordinates" block.

g = LINESTRING(0 0, 10 10);

process g coordinates as coordinate {

show coordinate;

}

These construction lets us process all the coordinates of a geometry, but there's more. If a "replace" statement is used inside the code block it will replace the values of that coordinate with the values specified in that same statement. For example, this makes possible to modify internally a geometry to translate all the coordinates of a geometry with an offset:

process geom coordinates as c {

x = ST_X(c) + deltaX;

y = ST_Y(c) + deltaY;

replace x y z;

}

And the previous code conveniently packaged:

alg ST_Translate(geometry geom, double deltaX, double deltaY) returns geometry {

process geom coordinates as c {

x = ST_X(c) + deltaX;

y = ST_Y(c) + deltaY;

z = ST_Z(c) + deltaZ;

replace x y z;

}

return geom;

}

This method can be used with a single geometry:

show ST_Translate(POINT(0 0), 10, 10);

but also in a "select" statement to process all the elements in a table:

show myShape select (f| ST_Translate(f/the_geom, 10, 10));

Another interesting algorithm may be the ST_Transform one, that performs coordinate system transformations on the geometries. As we'll see in the next post, it is possible to call Java code from GGL2 so it would be relatively easy to implement it using geotools... or proj4.