Move doc from wiki to repo, closes #85

2015-07-01 07:01:50 -07:00
parent ac8203eec4
commit 509944ea6d
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+Function to "safely" transform to webmercator. This function is most useful for rendering custom geometries using the CartoDB tiler. Often, transforming a projection like WGS84 can cause issues with extents beyond what are actually valid in webmercator, this attempts to fix those issues.
+
+#### Using the function
+
+Using a box that is nearly the full globe,
+
+```sql
+ST_SetSRID(
+  ST_Envelope(
+    ST_Collect(
+      ST_MakePoint(-180,60),
+      ST_MakePoint(180,-60)
+    )
+   ),
+   4326
+)
+```
+
+We can then convert it to a renderable webmercator geometry.
+
+```sql
+SELECT CDB_TransformToWebmercator(
+ ST_SetSRID(
+  ST_Envelope(
+    ST_Collect(
+      ST_MakePoint(-10,60),
+      ST_MakePoint(300,-60)
+    )
+   ),
+   4326
+ )
+)
+```
+
+Would give you back a single valid rectangle in webmercator. Since a longitude of 300 would convert to an unallowed webmercator coordinate, it gets clipped first. Valid extent is WGS84 (-180, -89, 180, 89)
+
+![valid geom](http://i.imgur.com/EFdXiqt.png)
+
+
+#### Arguements
+
+CDB_TransformToWebmercator(geom)
+
+* **geom** geometry