There is still a difference between ISIS and USGSCSM, but it is highly correlated with the sub-pixel location that we are checking. When the sub-pixel line is close to 0, then ISIS and USGSCSM line up almost perfectly. As the sub-pixel line approaches 1, ISIS and USGSCSM line up less well, capping at around 0.15 pixels. This is likely due to how ISIS and USGSCSM handle time within a line.
In USGSCSM, time is constant within a line. So, 1.2 and 1.8 use the same time. In ISIS, time is changes across a line. So, 1.2 and 1.8 use different times.
This uses changes in https://github.com/USGS-Astrogeology/ale/pull/305
There is still a difference between ISIS and USGSCSM, but it is highly correlated with the sub-pixel location that we are checking. When the sub-pixel line is close to 0, then ISIS and USGSCSM line up almost perfectly. As the sub-pixel line approaches 1, ISIS and USGSCSM line up less well, capping at around 0.15 pixels. This is likely due to how ISIS and USGSCSM handle time within a line.
In USGSCSM, time is constant within a line. So, 1.2 and 1.8 use the same time. In ISIS, time is changes across a line. So, 1.2 and 1.8 use different times.