For best broadcast TV and radio reception, you should have an unobstructed path between your TV or radio aerial and the transmitter. The yellow and pink lines should not have any ground impeding them. The yellow line is the radio line-of-sight path and is the most important. If the yellow line is unobstructed but the pink line is you may get some multipath/ghosting on analogue TV. (DTT and DAB are less affected by this.) Red sections indicate ground that is obstructing your line-of-sight (yellow) path. If you want to know what it is, hover the mouse over the red section you should see a six-figure Ordnance Survey grid reference of the obstruction. You can then locate it on an OS map to find out what it is, or click on the red section of the chart to open Streetmap in a new window to see the point and a small area around it.
Terrain does not take into account local issues such as trees and buildings - watch out for trees if installing in winter.
Raising the height of your TV aerial is the main option you have to improve your reception. That is, of course, usually the most difficult thing to do. For safety's sake you should really use an aerial installer for roof aerials.
Selecting and siting your TV aerial
Why DTT is different from an analogue install
