Contained within the 3rd Edition (1957) of the Atlas of Canada is a plate that shows four maps, circa 1955. On the left of this plate, the top map shows the extent of township and boundary surveys. The bottom left map shows the extent and type of air-photographic surveys. Air-photographic surveys have greatly facilitated the mapping of the country and serve many other uses as well. For purposes of accuracy, vertical photographs are preferable in that distortion is reduced to a minimum. Where speed has been a factor, trimetrogon surveys have been carried out. In this type of survey the aircraft carries three cameras, one of which takes vertical photographs and the other two oblique photographs on either side of the aircraft's track. Because of the wide coverage, fewer flights are required for oblique than vertical surveys. On the right of this plate, the upper map entitled Geodetic Surveys shows the triangulation areas in which precise control points have been established by the triangulation method, while the shoran trilateration symbols indicate precise control points established by that method. The bottom right map also entitled Geodetic Surveys shows precise and exploratory astronomical fixations which differ only in the amount of time and instrumentation required in taking the fixes. Both types of fixations are used as control points for surveys.