The propagation of a fast coronal mass ejection through the interplanetary medium results in compression and draping of existing heliospheric magnetic fields, forming an upstream sheath. Such sheaths are a common context for planar magnetic structures, where the magnetic field is ordered into laminar sheets. A technique is presented to estimate the location of spacecraft with respect to the large-scale structure of ejecta which they encounter. The technique employs the characteristics of sheath planar magnetic structures, assuming that they are formed by field-line draping. The technique's results for four magnetic clouds encountered by the Ulysses spacecraft are compared to those of a flux rope model. Very good agreement is found, suggesting that this is a promising technique for estimating an ejection's location from single-spacecraft data.