A light aircraft pilot departing PSP on the CATHEDRAL 9 SID deviated from the charted procedure and felt there could be some improvements in how the clearance was given to avoid the same problem in the future.

Narrative

I departed PSP on Runway 31R on an IFR flight plan. My clearance included: CATHEDRAL 9; join V370; TNP; then as filed. On previous departures I have generally been vectored and have not flown the entire departure. In this instance I flew to PSP and turned on V370 toward TNP. Immediately ATC asked me to confirm I was flying the CATH9 SID. I replied that I thought I was flying my clearance. ATC courteously explained that I was not and that in IMC the terrain to the east might be a factor. I requested a visual climb and the request was granted. I believe that my previous experience of being vectored directly to V370 influenced how I read and understood the departure route description on the chart. With the clearance including the phrase 'join V370' I made an erroneous assumption that I was not to fly the procedure after PSP. In reading the departure route description afterward it is clear that I was mistaken; but it seems that the departure procedure and clearance could be communicated better somehow. There was no other traffic at the time so the Departure Controller and I briefly discussed the miscommunication. He told me I was not alone in my mistake and that many airline pilots had made the same assumption. Clearly the mistake was mine alone and I accept full responsibility. In light of the fact that others seem to make the same mistake; I wonder if there is a better way of communicating the clearance to emphasize the entire departure procedure must be flown.