# approximateWcs¶

lsst.meas.astrom.approximateWcs(wcs, bbox, order=3, nx=20, ny=20, iterations=3, skyTolerance=4.84814e-09 rad, pixelTolerance=0.02, useTanWcs=False)

Approximate an existing WCS as a TAN-SIP WCS

The fit is performed by evaluating the WCS at a uniform grid of points within a bounding box.

Parameters: wcs : lsst.afw.geom.SkyWcs wcs to approximate bbox : lsst.geom.Box2I the region over which the WCS will be fit order : int order of SIP fit nx : int number of grid points along x ny : int number of grid points along y iterations : int number of times to iterate over fitting skyTolerance : lsst.geom.Angle maximum allowed difference in world coordinates between input wcs and approximate wcs (default is 0.001 arcsec) pixelTolerance : float maximum allowed difference in pixel coordinates between input wcs and approximate wcs (default is 0.02 pixels) useTanWcs : bool send a TAN version of wcs to the fitter? It is documented to require that, but I don’t think the fitter actually cares fitWcs : lsst.afw.geom.SkyWcs the fit TAN-SIP WCS