The Geological Survey of Canada (GSC) has been acquiring aeromagnetic data since 1947. Over the years, more than 500 surveys have been carried out, generally with a flight-line spacing of 800 m and an altitude of 305 m above the ground. These aeromagnetic surveys have been levelled to each other to correct for arbitrary datums, slow variations of the Earth's magnetic field over time, and differing survey specifications. These data are archived in the Canadian Aeromagnetic Data Base.

Airborne magnetic surveys are conducted with constant flight-line orientations, usually perpendicular to the regional geological strike, and with constant line spacing. The GSC has been acquiring aeromagnetic data since 1947 and current holdings comprise over 11 million line kilometres of data. Most aeromagnetic data were acquired at an altitude of 305 m mean terrain clearance, although over mountainous areas some surveys were flown at a constant barometric altitude, i.e. at a constant level above the highest peak in the survey area. The standard flight-line spacing for regional surveys is 800 m. In areas of deep sedimentary basins, the flight-line spacing was generally increased to 1600 m. In some areas of Canada, particularly in the Western Canada Basin and the Arctic, data were provided by oil and gas exploration companies and other non-GSC sources. In these areas, the flight-line spacing may exceed 6 km. Detailed, high-resolution surveys are also acquired, with flight-line spacings between 150 m and 300 m.

Most offshore magnetic surveying is done aboard ships, with a magnetometer towed at a sufficient distance from the ship to make the ship's magnetic effect negligible. The ship-track spacing and orientation depend on the purpose of the survey and possibly also on other geophysical measurements that are usually done at the same time (e.g. gravity, depth sounding); spacing is typically in the order of 5 to 10 km over deep water and less over the continental shelves.

Aeromagnetic surveys over Canada must be levelled to a common datum and to each other to account for secular variations in the orientation and strength of the geomagnetic field, arbitrary magnetometer datums in older surveys, differences in flight-line spacing and orientation, and differences in flying height and data quality.

Most data acquired before the advent of digital data recording (late 1970s) were in the form of analogue profiles and compiled as 1:63 360 or 1:50 000 scale contour maps. These maps have been digitized along the flight lines at intersections with contour lines, gridded to an interval of 812.8 m, and levelled to adjacent surveys. This project began in the late 1970s and lasted about 10 years. The levelling of individual surveys was performed by first subtracting the International Geomagnetic Reference Field for the date and altitude of the survey for each grid. The difference at the boundary of adjacent surveys was removed using a low order polynomial. The remaining errors were locally smoothed out where required (Teskey et al.,1982). The unlevelled line data were archived on a survey by survey basis and this accumulated data resulted in the creation of the Canadian Aeromagnetic Data Base.

The levelling of Canadian aeromagnetic survey profile data was initiated in 1989 by the Ontario Geological Survey in co-operation with the GSC. The project involved making a single master aeromagnetic grid for the province of Ontario at a uniform grid spacing of 200 m (Reford et al., 1990). This required the regridding of the digitized line data to a finer grid cell size and the subsequent transfer of the levelling adjustment that had been applied to the regional grid in the first phase of the project. The existing 812.8 m cell size grid was regridded to match the unlevelled 200 m grid. The grid of the level adjustments was then subtracted from the original, unlevelled, total field grid. The line data for the digitized surveys were extracted by interpolation from the 200 m levelled grid. The levelling adjustments for the digitally acquired surveys were calculated from the adjustment grid and applied directly to the line data. Subsequently, a similar procedure was applied to aeromagnetic survey data from the Atlantic provinces, Manitoba, and Saskatchewan. The procedure was modified slightly for the processing of surveys from Quebec and the Northwest Territories. For these surveys, the levelling adjustment was calculated systematically from the adjustment grid. The adjustment was then applied to the line data, thus avoiding the regeneration of profile data from the levelled grid.

Constant barometric altitude surveys have been flown over the mountainous areas of Western Canada and northern Baffin Island. Linking of the drape-flown, levelled, aeromagnetic data to the constant-altitude, aeromagnetic survey data has been performed by computational draping of the constant-altitude surveys to an idealized 305 m altitude surface. The method used for draping is based on a Taylor series expansion of the magnetic field on the measurement surface (Pilkington and Roest, 1992). The computationally draped data and the unlevelled, drape-flown data for Manitoba, Saskatchewan, British Columbia, and the Yukon Territory were levelled in-house to the national datum and stored as profile data in the Canadian Aeromagnetic Data Base.