Hybrid geoids are created from gravimetric geoids and GPS-derived ellipsoidal heights at spirit-leveled Bench Marks (GPSBM's). Modeling of the residuals between the GPSBM's and gravimetric values was previously accomplished nationally using single values for correlation length and signal amplitude in Least Squares Collocation (LSC). The most recent high resolution hybrid geoid (GEOID99) converts heights between the NAD 83 and NAVD 88 datums at about 2.5 cm RMS accuracy, which represents the remaining correlated signal in the residuals. While this signal is lower in power compared to the initial residuals (21 cm RMS) and the features implied by it are generally narrow, these features can have great lateral extent (100's to 1000's of km). Hence it is desirable to further reduce this correlated signal, and more elaborate LSC modeling techniques were explored to do this. The two most promising are the of progressively reduced correlation lengths & signal amplitudes and also combination of multiple covariance matrices in a single pass. The results of these tests demonstrate that the correlated signal of the residuals can be reduced to 1.5 - 2.0 cm RMS. These results are discussed in relation to error sources deriving from the observed heights above the NAVD 88 and NAD 83 datums (the GPSBM's) as well as those from the gravimetric geoid model.