The radio emission from air showers is used to accurately reconstruct the depth of the shower
maximum (Xmax ). We present a method based on using the full two-dimensional radiation profile
as observed on the ground. While the density of shower particles reaching the ground is usually
described with a 1D lateral distribution function, the intensity of the radio pulse is a complex
function of observer position with respect to the shower axis. The CoREAS code simulates these
complicated patterns to very high precision. When the antenna density is sufficiently high, like
for example in the LOFAR core, the 2D approach leads to a resolution on Xmax of < 20 g/cm2.
This is the same level of accuracy that is achieved with fluorescence detection.