Add notes on reprojecting EPSG:2136 to 4326

Reference notes for transforming Accra / Ghana National Grid geometry to WGS 84
in PostGIS: ST_Transform, ST_SetSRID for columns reporting SRID 0, updating a
column in place, and altering a column whose type modifier locks it to 2136.

Relevant wherever data arrives on the national grid while the API and the
application work in 4326 throughout.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/Transform2136To4326.txt

@@ -0,0 +1,52 @@
+Yes, that's a standard PostGIS operation. You use the `ST_Transform` function, which reprojects geometry from one spatial reference system (SRID) to another.
+
+The basic command looks like this:
+
+```sql
+SELECT ST_Transform(geom, 4326) AS geom_wgs84
+FROM your_table;
+```
+
+Here `geom` is your geometry column, which PostGIS assumes is already in EPSG:2136. If your column doesn't have its SRID set (i.e. it reports SRID 0), you first need to tell PostGIS what it actually is before transforming:
+
+```sql
+SELECT ST_Transform(ST_SetSRID(geom, 2136), 4326) AS geom_wgs84
+FROM your_table;
+```
+
+`ST_SetSRID` just labels the geometry as 2136 without changing coordinates; `ST_Transform` does the actual reprojection to 4326.
+
+If you want to permanently update the column in place rather than just query it:
+
+```sql
+UPDATE your_table
+SET geom = ST_Transform(geom, 4326);
+```
+
+Note that if the column has a type modifier locking it to SRID 2136 (e.g. `geometry(Point, 2136)`), you'll need to alter the column type first, otherwise the update will be rejected:
+
+```sql
+ALTER TABLE your_table
+ALTER COLUMN geom TYPE geometry(Point, 4326)
+USING ST_Transform(geom, 4326);
+```
+
+A couple of things worth knowing:
+
+Both 2136 and 4326 must exist in the `spatial_ref_sys` table for the transform to work. 4326 (WGS 84) is always there in a standard install. EPSG:2136 is Accra / Ghana National Grid and is included in the standard EPSG dataset that PostGIS ships with, so it should be present. You can confirm with:
+
+```sql
+SELECT srid FROM spatial_ref_sys WHERE srid IN (2136, 4326);
+```
+
+If 2136 is somehow missing, you can insert its definition manually from spatialreference.org or epsg.io.
+
+You can quickly test a single transformation without touching your table:
+
+```sql
+SELECT ST_AsText(ST_Transform(ST_SetSRID(ST_MakePoint(400000, 300000), 2136), 4326));
+```
+
+That returns the WGS 84 longitude/latitude for a sample projected coordinate, which is a good sanity check that the datum shift is being applied correctly.
+
+Want me to tailor the exact command to your table and column names?