This paper introduces an energy-efficient data collection technique that takes advantage of spatial/temporal data correlation to generate maps for continuous monitoring (e.g., of environmental conditions such as temperature, humidity, etc.). In its essence, the proposed technique, isoline aggregation, works by detecting isolines which are the lines that make up a contour map. Energy effi ciency through spatial aggregation is achieved by having only nodes that detect the isoline report to the sink. Simulation results show that isoline aggregation can reduce the amount of bytes transmitted by a factor of 11 compared to when no data aggregation is used and by up to 4 times when compared to an existing spatial-correlation based aggregation mechanism. At the same time, we are able to keep high data accuracy.