This paper introduces a novel energy efficient data aggregation algorithm that targets spatially correlated data in sensor networks. Isolines aggregation works by detecting isolines which are the lines in a contour map. Energy efficiency is achieved by having only the nodes that detect the isoline report to the sink. Simulation results show that isoline aggregation can lead to significant energy savings (some scenarios reported that no aggregation can send close to 150% more bytes than isolines aggregation) with adequate data accuracy. We also compared iso-lines against polygon aggregation, our implementation of an approach representing existing spatially-correlated data aggregation mechanisms. Our results report that isolines exhibit higher accuracy with a slight advantage in energy efficiency.