The conterminous U. S. has 21 States with ocean coastline, and it is 
estimated that over 40 percent of the population lives in the coastal zone 
close to the wetlands. The coastal zone consists of open water, salt marshes, 
mudflats, twisting tidal channels, intracoastal waterways, islands, bays, 
natural snd manmade levees, mosquito ditches, oceanfront and intracoastal 
developments, and estuaries. Within this diverse panorama the coastal wet- 
lands are classified according to several State and Federal definitions and 
interpretations. They are generally composed of coastal vegetation and are 
under the influence, if not the daily flood, of the tide. The wetlands are 
the transition zone between the mainland and the sea and are influenced by 
both. The tide usually covers the wetlands at some stage; it is often 
measurable in inches and may be of different phase and amplitude than the 
tide on open beach only a few miles away. Tidal rivers, streams, and surface @ % 
runoff mix with the ocean water to form a brackish zone of changing salinity. . 
The freshwater flow causes a hydrodynamic stage related more to the hydrologic 
cycle than to the lunar cycle. Grass several feet tall precludes photogram- 
metric measurement of the ground surface where elevation differences of a few 
inches are critical. It is not unusual for grass tops on the levees to be 
level with grass tops on lower ground behind the levee, giving the appearance 
of a continuous level surface. Tidal flow may reach an area by a long circuitous 
path through channels and breaks in natural levees. 
MAPS OF THE COASTAL WETLANDS 
There are approximately 2,500 standard 73-minute, 1:24,000-scale topographic 
maps that cover the landward portion of the coastal zone of the conterminous 
States. USGS is responsible for their preparation and publication. At present, 
2,000 maps are complete; the remaining 500 are in various stages of production. 
These maps are the primary cartographic base for the coastal wetlands. They are 
produced according to National Map Accuracy Standards (NMAS), i.e., not more than 
10 percent of the well-defined points tested shall be in error horizontally by 
more than 1/50 inch (40 feet at 1:24,000) or 1/30 inch at scales larger than o ® 
1:20,000. The relative internal accuracy of the map is likely to be better 
than NMAS. 
In the 1:24,000 series, coastal zone features such as marsh, sand, levees, 
reefs, and flats are compiled primarily by photointerpretation using field data 
and available nautical charts. Some of the symbols used to depict these features 
are illustrated in figure 1. The coastal shoreline shown in blue is the approxi- 
mate line of mean high water, except in areas of marsh grass, mangrove, or other 
similar marine vegetation where the apparent shoreline at the outer edge of 
vegetation is shown in lieu of the mean high-water line. So many factors 
affect the horizontal position of the mean high-water line that it is not 
feasible to map the entire shoreline within NMAS. For this reason, a state- 
ment that explains the shoreline mapped appears in the margin, along with the 
mean range of tide if available. Although standard accuracy cannot be*achieved 
in mapping some parts of the shoreline, all available information and methods 
are used in an effort to obtain the current position. Photographs taken at 
high tide are used when available. Field inspection of vegetation and wave- 
deposited debris is standard procedure.