mirror of
https://github.com/CartoDB/crankshaft.git
synced 2024-11-01 10:20:48 +08:00
30 lines
1.2 KiB
Markdown
30 lines
1.2 KiB
Markdown
|
### Aereal Weighting
|
||
|
|
|
||
|
|
Aereal weighting is a simple interpolation technique to assign a value
|
||
|
|
to a polygon given a set of polygons with one value assigned to each one.
|
||
|
|
|
||
|
|
The value is assigned by averaging the values of intersecting areas
|
||
|
|
weighted by the intersection area.
|
||
|
|
|
||
|
|
Its accuracy depends on the values assigned to reference areas being
|
||
|
|
homogeneous over each area.
|
||
|
|
|
||
|
|
The `cdb_overlap_function` takes three required parameters:
|
||
|
|
|
||
|
|
* `geometry` a Polygon geometry which defines the area where a value will be
|
||
|
|
estimated.
|
||
|
|
* `table_name`: name of the values table that provides the source values;
|
||
|
|
this table must have a geometric column `the_geom` containing the polygons
|
||
|
|
to which values are assigned.
|
||
|
|
* `column_name`: name of the column that contains the values in the values
|
||
|
|
table (should be a numeric column)
|
||
|
|
|
||
|
|
There's also an additional optional parameter to define the schema to which
|
||
|
|
the values table belongs. This is necessary only if it is not in the
|
||
|
|
`search_path`. Note that `table_name` should never include the schema in it.
|
||
|
|
|
||
|
|
* `schema_name` name of the schema that contains the values table
|
||
|
|
|
||
|
|
This function returns a numeric value resulting from the aggregation
|
||
|
|
of the polygons in
|