This invention consists of new methods for accurately estimating the surface of the human body or objects that are scanned by active wideband microwave or millimeter-wave imaging systems. The invention improves upon the state of the art by carefully focusing the images to preserve phase information inherent in the propagation of the electromagnetic waves used to form the images, and then exploiting the fact that the phase of the reconstructed image follows the surface. This means that surfaces of constant phase in the reconstruction follow the contours of the body or target. Furthermore, if the image reconstruction is performed in an exacting manner, the surface of the body tracks the zero-phase contour precisely. The surface can therefore be estimated by forming a high-resolution image using backprojection or similar methods and then finding the surface by numerically finding the zero-phase position over a lattice of positions. High-resolution active wideband microwave and millimeter-wave imaging systems are typically formed by mechanically, or electronically scanning a transceiver over a 2D aperture. At each point in the aperture the transceiver emits a wideband signal that interacts with the target and is captured coherently by the receiver. The subsequent data are then three-dimensional consisting of two spatial axes and one frequency axis. These data can then be focused using backprojection or other similar methods. Resolution in microwave imaging is limited by diffraction in the lateral dimensions and by bandwidth in the range or depth dimension. Tracking the surface is typically done after image formation by taking the magnitude image and forming iso-surfaces, or surfaces of constant amplitude. This process causes errors in the surface estimation since it inherently assumes that brightness is related to position. A brighter zone in the image will appear closer than a dimmer zone, even if they are at the same depth. The new methods in this invention achieve high accuracy by eliminating the bias caused by the image amplitude variations and by exploiting the image phase. The image phase varies approximately 360 degrees for every half-wavelength in depth variation. The zero-phase position can be estimated to accuracies of better than a few degrees. Therefore, the surface can be estimated to small fraction of one-half wavelength. Normal methods are limited by the depth resolution, which is typically much larger than one-half wavelength.