Translating Keypoints To Orignal Image After Pyrdown

edit

I have a full HD image (1920 x 1080).

In an effort to improve keypoint detection, I am using pyrdown to reduce the image size. Then, I run SURF on the reduced image.

My question is: what is the best way to translate the keypoints found on the reduced image to where they would be on the original image for display?

Thanks for any help.