detection and ranging), SLAR (side-looking airborne radar), SAR (synthetic 

 aperture radar), SIR (shuttle imaging radar), and passive microwave systems 

 have applications including mapping of bottom contours of coastal waters. 

 Some of these systems, such as LIDAR, are capable of accurate profiling of 

 water depths by using transmission and reflection from a pulsed coherent laser 

 light beneath an aircraft. In operation, a strongly reflected return is recorded 

 from the water surface, followed closely by a weaker return from the bottom 

 of the water body. 



Topographic and Bathymetric Data 



Topographic and bathymetric maps are available from the USGS, many 

 Corps of Engineer District Offices, and the USCGS. USGS topographic maps 

 are generally revised every 20 to 30 years; sometimes more often in areas 

 determined to be of high priority. Nevertheless, the maps may be outdated 

 for some studies because of the ephemeral nature of many coastlines. The 

 USGS quadrangles are the 7.5 minute series (scale 1:24,000) and the 15 min- 

 ute series (scale 1:62,500). The resolution of these maps is typically inade- 

 quate to provide details of surface features, but may be sufficient for 

 examining large landforms and pronounced changes, particularly over long 

 periods. 



Hydrographic survey data are available from the NOS and its predecessor, 

 the USCGS. Archives of all past surveys of these agencies are available from 

 NOS, a division within NOAA. Much of this data can be obtained in the 

 form of preliminary plots that are of larger scale and contain more soundings 

 and bottom notations than the published charts made from them. 



Bathymetric survey maps are sometimes out of date because geomorphic 

 changes in many submarine areas occur rapidly. On some navigation charts, 

 the bathymetry may be more than 50 years old and the marked depths may be 

 quite different from actual depths. The greatest changes can be areas of 

 strong current activity, of strong storm activity, of submarine mass 

 movement, and of dredging near ship channels. The user must also be aware 

 of changes in the datum used in different maps. 



Shoreline Change Maps 



Shoreline changes may be interpreted from navigation maps, topographic 

 maps, aerial photographs, and property records. In some areas, maps show- 

 ing shoreline changes and land loss may have been produced by state and 

 Federal agencies, universities, or engineering firms. However, the user 

 should be aware of potential sources of error that may not have been ade- 

 quately corrected when these maps were prepared. 



Chapter 2 Secondary Sources of Coastal Information 



27 



