PostgreSQL 11 trigger can't select a value in a table to use it in a second table Here's a sample of log :

gid alti  numlog
1   79.55 1
2   79.65 2


Here's a sample of us :

gid numlog numus sommet base alti_sommet alti_base
1   1      1000  0      0.25 (79.55-0)   (79.55-0.25)
2   1      1001  0.25   0.39 (79.55-0.25)(79.55-0.39)
3   1      1002  0.39   0.59 (79.55-0.39) ...
4   2      1000  0      0.15 (79.65-0)
5   2      1001  0.15   0.40 (79.65-0.15)


With a trigger on us, I would like to update alti_sommet and alti_base with the result of the math between ().

I create this function just for alti_base for now :

create or replace function activite.calcul_alti_us()
returns trigger as
$body$
 BEGIN
  IF (TG_OP IN ('insert')) THEN
  NEW.alti_base = l.alti-NEW.base from log l where NEW.numlog = l.numlog ;
 return NEW ;
  END IF ;
 return NULL ;
END ;
$body$
language 'plpgsql' ;    


and the trigger :

CREATE TRIGGER trg_calcul_alti_us
AFTER insert
ON us
FOR EACH ROW
EXECUTE PROCEDURE activite.calcul_alti_us();


The result : alti_surface = 0 everywhere.
I tried :

NEW.alti_base = (select l.alti-NEW.base from activite.log l where NEW.numlog =l.numlog) ;


I tried with a trigger BEFORE as well.
Both failed. 
Thanks for help.
There are many errors:


It must be a BEFORE trigger to modify the row about to be inserted.
There is no column base in table log.
TG_OP is always in upper case, so you must compare with 'INSERT'.