The following information is excerpted from Doug Barnett's Early Surveys and Settlements in Central Alberta
The DLS System first established controlling lines on which to base the township surveys. It was decided to layout the System on an astronomic basis, that is "square with the world", with north-south and east-west lines following lines of latitude and longitude on the earth's surface. Starting near Winnipeg, Dominion Land Surveyors established six meridians over a period of time.
A meridian is an astronomic north-south line on the earth's surface. The Principal Meridian was followed by successive Initial Meridians (the Second, Third, Fourth, Fifth, and Sixth), each about four degrees of longitude apart. The Fourth Initial Meridian later became the Alberta - Saskatchewan boundary, and the Western Provinces were extended northward from the 49th parallel (international boundary) to the 60th parallel, a distance of about 760 miles. As meridians follow the spherical curve of the earth, they converge as they are produced northward. For example, the distance between the Fourth and Fifth Meridians along the 49th parallel is about 182 miles (293 kilometres); at the 60th parallel, the distance between the same two Initial meridians is reduced to about 139 miles (224 kilometres) due to convergence of the meridians.
The Dominion Lands Survey System is therefore an astronomic system with all north-south lines laid off as true meridians, and all east-west lines established as chords to parallels of latitude.