Colmap expects the descriptor value to be between 0 and 255. For the SIFT feature descriptor, colmap has a logic to convert the small float value into uint8 value. https://github.com/colmap/colmap/blob/7180b62380e35fea330dde9a325d0c61f0d5111c/src/feature/utils.cc#L65

For the SURF, you should do a similar conversion.