Swath bathymetric sonar systems use seabed backscattered energy to estimate the slant range from the survey instrument to the seafloor. Because the slant range measurements are made over a range of angles, it is possible to estimate the bottom backscattering strength as a function of grazing angle. This approach is used for backscattered data collected at 95 kHz for grazing angles in the range 15 to 90 deg. The data are acquired from a Simrad EM1000 sonar in water depths ranging from 20--200 m at survey speeds of about 16 kn. These measurements have the advantage that they are made over wide regions. Operational problems that limit the quality of the available data include: degradation due to sea state; uncertainty in radiation and receive patterns of the sonar; and spatial variation in the seabed type within the footprint over which the measurements are made. Methods for minimizing the effect of these problems are presented. In order to use the estimates of local seabed angular response, a parametrization is used to relate the control that variations in seabed physical properties (density, velocity, roughness spectrum, volume inhomogeneites) have on the shape and magnitude of the seabed backscatter angular response.