I am looking into a way to calculate the elevation of the ground floor of many buildings in a study area. My plan is to run the building centroids against an elevation grid to get an elevation measurement for each point. I know that by using an elevation source such as NED, the elevation that will be assigned to each point will be somewhat less than that of the actual ground floor of the building. My plan was to apply some sort of factor to the extracted elevations to hopefully increase the accuracy of my data. How would I go about determining such a factor?