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In the COMPASS experiment at CERN, large straw drift tube detectors are used for large-angle tracking. To minimize the total a real density, a self supporting structure of thin-walled plastic tubes was chosen and, hence, a loss in mechanical precision was accepted. A complete mapping of the anode wire coordinate grid was required. An X-ray apparatus using a charge-coupled device (CCD) as imaging detector was built to investigate the mechanical properties and to calibrate (offline) the wire positions. Deviations of typically 200-400 [micro]m from the nominal positions, defined by equal spacing, are found across the detector area of 8 [m.sup.2]. With a calibration method based on high-resolution CCD imaging and pattern recognition algorithms, the absolute wire coordinates are determined with an accuracy better than 30 [micro]m across the whole detector area. Temperature effects are clearly seen. Their inhomogenity limits the achievable accuracy to about 50 [micro]m under realistic experimental conditions, which is sufficient in view of the intrinsic straw resolution of 200 [micro]m for minimum ionizing particles. The offline calibration was checked with particle tracks in the experimental setup, running COMPASS with 160 GeV/c muons. Tracks reconstructed with other detectors that cover a central angular range were used for this comparison. Good agreement is found between these in situ measurements and the X-ray calibration. Index Terms--Calibration, charge-coupled device (CCD), particle tracking, straw detector, X-ray.