A single layer, reduced gravity, double gyre primitive equation model in a 2000 
km by 2000 km square domain is used to test the accuracy and sensitivity of time 
dependent Eulerian velocity fields reconstructed from numerically generated 
drifter trajectories and climatology. The goal is to determine how much 
Lagrangian data is needed to capture the Eulerian velocity field within a 
specified accuracy. The Eulerian fields are found by projecting, on an analytic 
set of divergence free basis functions, drifter data launched in the active 
western half of the basin supplemented by climatology in the eastern domain. The 
time dependent coefficients are evaluated by least squares minimization and the 
reconstructed fields are compared to the original model output. We find that the 
accuracy of the reconstructed fields depends critically on the spatial coverage 
of the drifter observations. With good spatial coverage, the technique allows 
accurate Eulerian reconstructions with under 200 drifters deployed in the 1000 
km by 1400 km energetic western region. The base reconstruction error, achieved 
with full observation of the velocity field, ranges from 5% to 30% depending on 
the number of basis functions. We conclude that with appropriate coverage, 
drifter data could provide accurate basin scale reconstruction of Eulerian 
velocity fields.