  The magnification effects of clustered matter produce variations in the image
sizes and number density of galaxies across the sky. This paper advocates the
use of these effects in wide field surveys to map large-scale structure and the
profiles of galaxy and cluster sized halos. The magnitude of the size variation
as a function of angular scale is computed and the signal-to-noise is estimated
for different survey parameters. Forthcoming surveys, especially well designed
space-based imaging surveys, will have high signal-to-noise on scales of about
0.1 arcminute to several degrees. Thus the clustering of matter could be
measured on spatial scales of about 50 Kpc to 100 Mpc. The signal-to-noise is
dominated by sample variance rather than shot-noise due to the finite number
density of galaxies, hence the accuracy of the measurements will be limited
primarily by survey area, sampling strategy and possible systematics. Methods
based on magnification are compared with the use of shape distortions and the
contrasts and complementarities are discussed. Future work needed to plan
survey strategy and interpret measurements based on magnification is outlined.
