我有一个表rbeijing,有空间字段the_geom,但是是基于SRS4326的，我想用900913d的投影，所以就学要添加一个新的空间字段并转换投影坐标。做法如下：

mapdb=# alter table rbeijing add column the_geom_900913 geometry;
mapdb=# create index index_rbeijing_the_geom_900913 on rbeijing  using gist(the_geom_900913);
mapdb=# create index index_rbeijing_the_geom  on rbeijing gist(the_geom);
mapdb=# update table rbeijing set the_geom_900913=st_tranform(the_geom,900913);

mapdb=# update rbeijing set  the_geom_900913 = st_transform(the_geom,900913);

mapdb=# \d rbeijing
                                      Table "public.rbeijing"
     Column      |          Type          |                       Modifiers                      
-----------------+------------------------+--------------------------------------------------------
 gid             | integer                | not null default nextval('rbeijing_gid_seq'::regclass)
 mapid           | character varying(8)   |
 id              | character varying(13)  |
 kind_num        | character varying(2)   |
 kind            | character varying(23)  |
 width           | character varying(3)   |
 direction       | character varying(150) |
 const_st        | character varying(1)   |
 pathname        | character varying(40)  |
 pathpy          | character varying(250) |
 snodeid         | character varying(13)  |
 enodeid         | character varying(13)  |
 pathclass       | character varying(2)   |
 pathno          | character varying(13)  |
 the_geom        | geometry               |
 the_geom_900913 | geometry               |
Indexes:
    "rbeijing_pkey" PRIMARY KEY, btree (gid)
    "index_rbeijing_the_geom" gist (the_geom)
    "index_rbeijing_the_geom_900913" gist (the_geom_900913)
Check constraints:
    "enforce_dims_the_geom" CHECK (ndims(the_geom) = 2)
    "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) = 'MULTILINESTRING'::text OR the_geom IS NULL)
    "enforce_srid_the_geom" CHECK (srid(the_geom) = 4326)

上面的表结构是添加和转换后的。添加空间字段的同时，也为空间字段添加了索引。

操作前提：postgis/postgres能支持900913，如果不支持的话，就学要先添加900913投影支持。





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