A Wide-Area Sensor Network (WASN) is a collection of heterogeneous sensor networks and data repositories spread over a wide geographic area. The diversity of sensor types and the regional differences over which WASNs operate result in semantic interoperability mismatches among sensor data, and a difficulty in agreeing on methods for sensor data access and exchange. We assume that sensors and their associated data have an explicit spatio-temporal basis (or tagging) in their representation. In this paper, we describe a spatio-temporal loosely-coupled federated database model for the WASN data storage problem that of unifying query and data representation given a heterogeneous WASN - and propose a conceptual schema to ease the problem of integration of sensor data representations. This is a continuing and critical challenge as sensor networks become more ubiquitous and data interoperation becomes increasing vital for a variety of applications (such as homeland security, transportation, environmental monitoring, etc.). We employ a top-down ontology-driven software development methodology. We use the SNAP/SPAN ontology as a sample framework for the conceptual schema. We compare our methodology of conceptual schema development with a bottom-up entity-oriented schema construction and discuss the differences in the two approaches. A unique contribution is the discussion of deployment experiences to evaluate proposed approaches in the context of a concrete WASN testbed.