  LOPES is a digital radio interferometer located at Karlsruhe Institute of
Technology (KIT), Germany, which measures radio emission from extensive air
showers at MHz frequencies in coincidence with KASCADE-Grande. In this article,
we explore a method (slope method) which leverages the slope of the measured
radio lateral distribution to reconstruct crucial attributes of primary cosmic
rays. First, we present an investigation of the method on the basis of pure
simulations. Second, we directly apply the slope method to LOPES measurements.
Applying the slope method to simulations, we obtain uncertainties on the
reconstruction of energy and depth of shower maximum Xmax of 13% and 50 g/cm^2,
respectively. Applying it to LOPES measurements, we are able to reconstruct
energy and Xmax of individual events with upper limits on the precision of
20-25% for the primary energy and 95 g/cm^2 for Xmax, despite strong human-made
noise at the LOPES site.
