practice to have the base line located and monumented prior to any survey of 

 the profile lines. 



Considerable preliminary work in locating the base line can be done in the 

 office to reduce the time spent in the field. The first step is to establish 

 the long-term erosion rate for the area so the base line will be placed far 

 enough landward to ensure a 20- to 30-year lifespan or longer. This is desir- 

 able even in a short-term study to allow the profile lines to be reoccupied in 

 the future or to prevent the lines from being lost during a severe storm. 

 Next, an approximate base line trace should be located on available topographic 

 maps or aerial photography. This trace can be checked and modified on the 

 ground at a later time. Once this tentative trace has been selected, a source 

 check should be made for existing horizontal and vertical control in the study 

 area. Sources of this information include the Corps of Engineers, U.S. Geo- 

 logical Survey, National Ocean Survey, National Park Service, and State and 

 local highway departments. The latter often have an established control tied 

 in to the State coordinate system for building and maintaining roads. Whenever 

 possible, existing monuments should be used to minimize the effort required to 

 establish the base line and ensure the recoverability of the profile lines. In 

 every case that an existing monument is to be used, the control data should be 

 examined to determine whether the monument has been recently recovered or has 

 been affected by a recent disturbance, such as a storm or road construction. A 

 monument should not be considered a permanent part of the control until its 

 worth has been proven. 



The lack of established or permanent monuments nearby need not delay the 

 initial survey of the profile lines. If it is determined that establishing 

 control would be time consuming, an assumed elevation may be assigned to a 

 monument and the rest of the control set from that monument. So that it is 

 obvious, the assumed elevation should be highly exaggerated. The profile lines 

 can then be surveyed, but, before the data are analyzed, control for the base 

 line must be established and the data adjusted. 



Selections of the sites for base line monuments must consider certain re- 

 quirements such as the permanence of the bench marks, wishes of property 

 owners, proximity to the area of interest, and accessibility. The final selec- 

 tions, which will require considerable judgment in determining, will usually be 

 based on a compromise of these requirements. 



First, the base line must be located in a stable area where it is not in 

 danger of immediate damage from erosion. In their report on coastal construc- 

 tion in Florida, Purpura and Sensabaugh (1974) list the following considera- 

 tions for determining a setback line which can also be applied to determining 

 the location of a base line: 



(a) Ground elevation in relation to historical storm and hurricane 

 tides, 



(b) predicted maximum wave uprush, 



(c) beach and offshore ground contours, 



(d) vegetation line, 



(e) erosion trends, 



8 ' 



