Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_6864 ( "Number on map" real, "Name" text, "Colour" text, "Construction commenced" real, "Population" real )

### Question ###
What is the average number for Northgate which commenced its construction before 1951?

### Accurate SQL ###

SELECT AVG("Number on map") FROM table_6864 WHERE "Name" = 'northgate' AND "Construction commenced" < '1951'