From: "Eino Uikkanen" To: "cygnet" , "Dan Jacobson" , Subject: Taiwanese transformation parameters Date: Sat, 20 Sep 2003 23:55:09 +0300 Since I don't have measured coordinate values of a representative set of points in both TWD67 and TWD97, I can't: - Calculate parameters for a seven parameter similarity transformation - Calculate parameters for a three parameter similarity transformation based on measured coordinate values - Check the accuracy of any transformation-parameter set I can however: - Calculate parameters for a three parameter similarity transformation reducing known parameters for a seven parameter similarity transformation to parameters for three parameter similarity transformation. I did that using GeoConv's run 723TWD67.BAT, which was a modified copy of http://www.kolumbus.fi/eino.uikkanen/geoconvgb/723.bat.txt The entire process is explained in http://www.kolumbus.fi/eino.uikkanen/geoconvgb/geo723.htm I used as input values the datum definition I found from Dan's page and one point (23, 121, 0), which is from the centre of Taiwan (hopefully). This datum seemed to give almost the same results as the application wgs84tm2.exe, which Cygnet sent me.Therefore I selected this datum for GeoConv too. Name=ToECEF,Ellipsoid,DX,DY,DZ,rx,ry,rz,m TWD67=1,GRS67,-764.558,-361.229,-178.374,-0.0000011698,0.0000018398,0.0000009822,0.00002329 The result of my calculation was: dx: -765 m dy: -361 m dz: -178 m And since I used TWD67 which uses ellipsoid GRS67: da: -23.00 df: -0.00113048 I have no means to check the accuracy of the result, but the calculation process if thoroughly tested. Therefore the accuracy is mainly dependent on the accuracy of the selected input parameters, that is, the parameters for the seven parameter similarity transformation (above) and the selection of the point (23, 121, 0). For comparison I copied the GARMIN parameters from Dan's page: A: For GARMIN GPS: dx: -767 m dy: -358 m dz: -176 da: -18.00 df: -0.00081204