Environmental monitoring is a promising application for sensor networks. Many scenarios produce geographically correlated readings, making them visually interesting and good targets for the isoline query. This query depicts boundaries showing how values change in the network. Temporal and spatial suppression provide opportunities for reducing the cost of maintaining the query result. We combine both techniques for maximal benefit by monitoring node and edge constraints. A monitored node triggers a report if its value changes. A monitored edge triggers a report if the difference between its nodes? values changes. The root collects reports and derives all node values, from which the query result is generated. We fully exploit this strategy in our algorithm, CONCH, which maintains the set of node and edge constraints that minimizes message traffic.