If you still own the land (easement grant), the easement represents a reduction in your cost basis and not an actual sale. To report the 1099-S enter in the Investments, Other as it is reported on your form. Then enter the cost basis in an equal amount so that the net effect is zero.
This reports the form, but the gain will be zero. Keep the forms for your records and the reduced basis comes into play later when you sell the property.