Environmental sensing data provide crucial information for environment-related decision-making. Formal data are provided by official environmental institutes. Beyond those, however, there is a growing body of so-called informal sensing data, which are contributed by citizens using low-cost sensors. How good are these informal data, and how might they be applied, next to formal environmental sensing data? Could both types of sensing data be gainfully integrated? This paper presents the results of an online survey investigating perceptions within citizen science communities, environmental institutes and their networks of formal and informal environmental sensing data. The results show that citizens and experts had different views of formal and informal environmental sensing data, particularly on measurement frequency and the data information provision power. However, there was agreement, too, for example, on the accuracy of formal environmental sensing data. Furthermore, both agreed that the integration of formal and informal environmental sensing data offered potential for improvements on several aspects, particularly spatial coverage, data quantity and measurement frequency. Interestingly, the accuracy of informal environmental sensing data was largely unknown to both experts and citizens. This suggests the need for further investigation of informal environmental sensing data and the potential for its effective integration with formal environmental sensing data, if hurdles like standardisation can be overcome.