In order to control a discrete-event system (DES), the supervisory controller observes event sequences. Observability, together with controllability, is required for obtaining the exact specification language via supervisory control. By extending observability, relative observability was recently investigated. In this article, relative network observability, which accommodates observation incompleteness and delay in relative observability, is defined. An algorithm is proposed to transform relative network observability to network observability. This leverages existing solutions to problems concerning network observability to solve problems concerning relative network observability. In particular, we propose techniques for checking relative network observability and calculating supremal controllable and relatively network observable sublanguage.