[font color=\'#000000\']I am suprised to see over a hundred views of your question without any answers. Here is a link from Luminous Landscape that explores exposure issues.
ExposureA spot meter, either external or if available through the camera might have been the method used. By taking several meter readings, one of the sand, and another of a midtone area (representing the 18% grey), it can be determined that the sand was one stop brighter.
A clear view of the northern sky is close to 18% grey, concrete, grey camera bags, an 18% card in your camera bag, or even the palm if your hand (if fair skined about 1 stop brighter then 18% grey) can be used for your greypoint measurement.[/font]