 The difference between the two , the differential gravitational effect of the unevenness of the terrain , is called the terrain effect.
<meaning-changed>  The difference between the two , the differential gravitational effect of the unevenness of the terrain , is called the terrain effect.
 The difference between the two is called the terrain effect and is due to the differential gravitational effect of the unevenness of the terrain , is called the terrain effect.