There are 2 intrinsic resolutions of DVD. One is 720 X 480 pixels(NTSC) and the other is 720 X 540 pixels(PAL). No matter how pixels are there on DVD, all the DVD will be displayed on 2 aspect ratios. One is for stand screen and it is 4:3, and the other is for widescreen and it is 16:9. And these 2 kinds of DVD are known as 480p videos.
If you want to enjoy DVD in 1080p, then you probably need to buy the blue-ray DVD video, which in 1080p.