The Great Bay Estuary, located between Portsmouth and Durham, New Hampshire, has been surveyed several times in the past century. Most recently, CCOM/JHC conducted hydrographic surveys in portions of Great Bay and Little Bay using single beam, multibeam, and side scan sonars. These surveys were acquired as part of the Center's NOAA-funded educational and research programs and through a grant from CICEET. Maps and data from this project are available for download on this web site. We have also analyzed and compared historic bathymetric data sets from the U.S. Coast and Geodetic Survey (a NOAA predecessor agency) dating from 1913 and from 1953-54. To compare historic data and to understand whether the changes observed are real physical changes or the result of different survey methods used, we considered the general problem of combining multiple hydrographic data sets collected over many years with different techniques and standards, and therefore, different associated errors. The techniques developed for this data comparison are presented on this site together with maps and graphs illustrating the results.