Curvilinear features extracted from a 2D user-sketched feature map have been used successfully to constraint a patch-based texture synthesis of real landscapes. This map-based user interface does not give fine control over the height profile of the generated terrain. We propose a new texture-based terrain synthesis framework controllable by a terrain sketching interface. We enhance the realism of the generated landscapes by using a novel patch merging method that reduces boundary artefacts caused by overlapping terrain patches. A more constrained synthesis process is used to produce landscapes that better match user requirements. The high computational cost of texture synthesis is reduced with a parallel implementation on graphics hardware. Our GPU-accelerated solution provides a significant speedup depending on the size of the example terrain. We show experimentally that our framework is more successful in generating realistic landscapes than current example-based terrain synthesis methods. We conclude that texture-based terrain synthesis combined with sketching provides an excellent solution to the user control and realism challenges of virtual landscape generation.