This study experimentally investigates the flow characteristics of a high-pressure air jets impinging on a flat plate and an inclined plate with various nozzle-to-plane gaps of 10 mm, 20 mm, and 30 mm. Full-field measurements of flow characteristics in the central plane of the nozzle and near the impinging surface are performed using two-dimensional two-component (2D2C) particle image velocimetry (PIV) technique. This paper presents results from the nozzle pressure ratio (NPR) of 2.77, approximately yielding the sonic jet with Mach number of 1.2. Flow characteristics obtained from the 2D2C-PIV measurements with various spatial gaps are compared and presented. Results including the first- and second-order flow statistics, such as mean velocity and turbulent kinetic energy, and effects of the impinging surface to the flow patterns are investigated. Finally, proper orthogonal decomposition (POD) analysis is applied to reveal the statistically dominant flow structures that capture the highest amount flow kinetic energy and play important roles to the flow dynamics and heat transfers.