Parking lot islands can often be a problem to model especially if they were designed on a downhill or uphill slope with a cross slope. You are given callout elevations at many of the critical points on the curb, but when you use the elevated curb to form a surface it can often be incorrect because the curb elevation interpolation is computed around the curb itself and not from the slopes defined by the framework of the grading of the parking lot.
It seems like the obvious thing to do to elevate the curb where you are given the spot elevations and then to use those curb lines to create the surface for finished grade. This video however shows a better and a faster approach to modeling parking lots where we show you how to build the grading framework and then elevate the islands of the parking lot to the surface defined by the grading framework - it takes a bit of lateral thinking sometimes to come up with a better / faster / more reliable way to build out a parking lot - this way all your grades match all of the provided spot elevations, but you get a better overall result and the islands are way better defined than simply elevating the spot elevations.
Take a look - it will likely be a good use of the 30 minutes it takes to watch the video.
Alan