solvePnP with fixed camera coordinates

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I have a camera on a building, and I know the camera coordinates accurately from a survey. My surveyed coordinates are in the world frame, so I use an estimate of rvec to calculate tvec in the camera frame.

I am using the solvePnP function with an initial estimate for tvec, zeros for rvec, and useExtrinsicGuess=True. My result always has tvec different to the initial estimate. How to I constrain the solution so that tvec is not changed?