VFR GA pilot reported the Tower assigned them headings which reportedly placed them in unsafe proximity to terrain and less than 500 ft above the ground in violation of FAR 91.119.

Narrative

TTD is a towered class D airport located under the PDX Class C at an elevation of 39 feet. Terrain to the South rises to 1;100 feet within 6 miles; and to several thousand feet to the East. The floor of class C is 1;700 feet. I was approaching from the southwest at 1;500 feet in order to remain well under PDX Class C and heading roughly north east. when I contacted Tower at 8 miles. I was instructed to maintain at or below 1;200 feet and report a right base to Runway 7. The terrain in this area and along the path to the airport is at about 200 feet. Immediately to my right (East) terrain rises to over 1;000 feet. After a few minutes Tower advised a 10 degree right turn for traffic. A very short time later they advised another 10 degree turn. Another very short time later they advised 'HEAD DUE EAST for traffic.' This sent me towards hills which were just barely below my altitude; and I was flying with an 'at or below 1;200 feet' restriction.I contemplated replying 'unable' but decided not to. While the hills were nearly my elevation; they were small and I would cross in a very short time. There was no ceiling other than those imposed by my instructions; and the Class C floor I knew that if I began to sink or encountered turbulence I could throttle up and climb. I would not fly into an unsafe situation and do not believe that I was at any risk at any time. There are houses on these hills and I wondered what the occupants think of such low flying aircraft. Then I began to wonder about minimum altitude and clearance restrictions. Part 91.119 states that 'Except when necessary for takeoff or landing' altitude must exceed 500 feet above the ground. I was still far enough away from the runway; approximately 6 miles; that except for the artificial restriction to remain below 1;200 feet; there was no reason. it was not 'necessary' to be this low. I checked later on charts and determined that I had been within 100 feet of the ground; although still 1;160 feet above the airport.Given the altitude restriction; I would not have chosen a route over those hills; but that is where the tower sent me. Unlike clouds; which the controller cannot see; (and into which I would decline to fly) the hills are in a permanent location. I presumed that the controller knew and understood that he was sending me into that situation. On reflection; however; I realize that in a situation like this the tower may be issuing 'advisories' not vectors. I'm not sure that it was legal for me to follow those advisories.