Marcus C. England wrote:
Hi all,
I have spent the last three hours trying to find a plain-English
description of how you change the projection of a PostGIS layer, and I
am now giving up. I have found quite a few questions about this on the
internet, but not a single answer that allows for a newbie to SQL. Every
answers states something like "transform (arguments)"... what arguments?
So, here it goes. I have a layer called "california_counties". The
layer's current SRID is correctly set at 3309. I want to transform the
layer into SRID 26911. How do I do that? It seems it should be so simple.
There are a copy of different concepts and issues here which are
probably tripping you up.
o you have a table and it has column of data in some project, in your
case SRID 3309
Typically you do not change the projection of a column. It is possible
by can be tricky and complicated. But that should not stop you, you have
a couple of choices:
1) create a new column and load it from the old column but in the new
projection
select addgeometrycolumn('schema', 'table', 'new_geom', 26911,
'MULTIPOLYGON', 2);
update "schema"."table" set new_geom = transform(the_geom, 26911);
now you have the column loaded with in the new projection. You might
want to index it, and probably vaccum analyze "schema"."table";
2) just use the original column but transform it on the fly to the
projection you want.
In the example UPDATE statement we did on the fly transform of the data.
You can do the same thing in SELECT statements also.
Hope this helps some.
-Steve
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