Contemporary chroniclers referred to him (in Latin) as "Theodericus de Phlardirtinga" (Vlaardingen) and as margrave thereof.
The exact extent of his domain is not known, but at one time or another it stretched north to at least Rijnsburg and east to roughly Aalburg.
Because of this, Emperor Henry III personally led an expedition against him in 1046, forcing Dirk to return some areas he had occupied.
Shortly after the emperor had left however, Dirk started to plunder the territories of the bishops of Utrecht and Liège and made alliances with Godfrey III, Duke of Lower Lorraine and the counts of Hainaut and Flanders.
During, the retreat however, the imperial army suffered severe losses, causing Dirk's allies to rise in open revolt as well in his support.