<p>This study introduces a single-camera synthetic Schlieren technique for measuring free-surface topography in fluid layers over a smoothly varying solid bed. By modeling light refraction through weakly deformed air–liquid and weakly varying liquid–solid interfaces, we establish a linear relationship between free-surface gradients and pattern displacements that yields an explicit bed-independent formulation for upward-looking configurations. The proposed framework incorporates coordinate mapping to correct refraction-induced parallax distortion influenced by the bed shape. Validation experiments featuring sinusoidal bed topography and wedge-shaped sloped bed topography achieve accurate spatiotemporal reconstruction of both static capillary meniscus profiles and dynamic water drop ripple evolution. The present method advances experimental capabilities for quantifying interfacial hydrodynamics in multi-layer fluid systems.</p>