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| National Transformation version 2 | |
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Like the United States, Canada uses a grid-based method to convert between NAD
1927 and NAD 1983. The National Transformation version 2 (NTv2) method is quite
similar to NADCON. A set of binary files contains the differences between the
two geographic coordinate systems. A bilinear interpolation is used to calculate
the exact values for a point. Unlike NADCON, which can only use one grid at a time, NTv2 is designed to check multiple grids for the most accurate shift information. A set of low-density base grids exists for Canada. Certain areas such as cities have high-density local subgrids that overlay portions of the base, or parent, grids. If a point is within one of the high-density grids, NTv2 will use the high-density grid; otherwise, the point falls through to the low-density grid. The parent grids for Canada have spacings ranging from five to 20 minutes. The high-density grids usually have a cell size of 30 seconds, or 0.08333333°. Unlike NADCON grids, NTv2 grids list the accuracy of each point. Accuracy values can range from a few centimeters to around a meter. The high-density grids usually have subcentimeter accuracy. Australia and New Zealand adopted the NTv2 format to convert between geographic coordinate systems as well. Australia has released several state-based grids that convert between either Australian Geodetic Datum of 1966 (AGD 1966) or AGD 1984 and Geocentric Datum of Australia of 1994 (GDA 1994). The state grids have been merged into countrywide grids. New Zealand has released a countrywide grid to convert between New Zealand Geodetic Datum of 1949 (NZGD 1949) and NZGD 2000. |
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