accessibility standards for Federal outdoor developed areas 17 Measuring Surface Firmness and Stability The rotational pen- etrometer (RP) is a precision surface-in- denter measuring tool for evaluating the firmness and stability of ground and floor surfaces (figure 1). To measure firmness, the precision spring applies force to the penetrator and the caliper measures the vertical displacement of the penetrator into the surface. The pen- etrator is then rotated and the total displace- ment into the surface is measured, indicating surface stability. The Access Board has conducted several research projects using the RP to evaluate the firmness and stability of trail and play area surfaces. Additional information about these projects is available at www.access-board.gov/research /completed-research/accessible-exterior-surfaces. Slip resistance is not required for the surface of trails because leaves, dirt, ice, snow, and other surface debris and weather conditions are part of the natural environment that would be difficult, if not impossible, to avoid. Figure 1—The rotational penetrometer is a portable precision surface indenter that is used for measuring the firmness and stability of surfaces.