digital data were available as of January 1981 are indicated in Figures 1 and 

 2. Data from the squares in Figures 1 and 2 were digitized from surveys con- 

 ducted between 1930 and 1965. 



III. PROGRAM ZGRID1 



ZGRID1 is a modification of the program supplied by NGSDC to perform two- 

 dimensional interpolation-extrapolation. Irregularly spaced bathymetry data 

 from the NOS digital tapes or from independently obtained surveys are read by 

 ZGRID1. ZGRID1 then determines the coordinates of each sounding on a map pro- 

 jection preselected by the user and interpolates-extrapolates this data to 

 regularly spaced locations on the projection. 



As an option of ZGRIDl, a plot of the irregularly spaced soundings can be 

 made. Figures 3 and 4 are examples of such plots. Figure 3 shows the sound- 

 ings offshore of Great Egg Harbor Inlet, New Jersey, on a 0.03° (1.8 minute) 

 latitude-longitude square. Soundings were obtained from the NGSDC tape covering 

 the 1° latitude-longitude square 39° to 40° N. latitude, 74° to 75° W. longitude. 

 Figure 4 consists of bed elevations near CERC's Field Research Facility (FRF) at 

 Duck, North Carolina. The FRF bed elevations were derived from a survey made by 

 CERC in September 1978. 



The estimated bed elevation or sounding value at each regularly spaced loca- 

 tion can be written on a magnetic disk or tape for subsequent use. At the 

 user's discretion, the regularly spaced values can be printed on a linepr inter 

 or plotted. Figure 5 is a plot of the estimated regularly spaced depth values 

 for the same region covered by Figure 3. For this example, there are 30 regu- 

 larly spaced (on the map projection) points in both the x and y directions for 

 a total of 900 regularly spaced points in the whole region. 



IV. PROGRAM CONTOR 



CONTOR determines and draws contours of any variable from a two-dimensional 

 array of variable values. These values are assumed to be located at regularly 

 spaced grid points. CONTOR was developed at the Carnegie-Mellon University, 

 Pittsburgh, Pennsylvania, by Robinson and Scarton (1972). When used to contour 

 bed elevation or water depth fields, CONTOR is usually run in conjunction with 

 ZGRIDl. ZGRID1 produces the depth or bed elevation values at regularly spaced 

 locations needed by CONTOR. The contours to be plotted are specified by the 

 user. 



Figures 6 and 7 are examples of contour plots done by CONTOR. Figure 6 

 corresponds to the region off Great Egg Harbor Inlet covered by Figures 3 and 

 5. Figure 7 corresponds to the FRF region covered by Figure 4. Figure 6 was 

 produced from the ZGRIDl computed values in Figure 5 . Figure 7 was produced 

 from ZGRIDl computed regularly spaced values (not shown) located 82.02 feet 

 (25 meters) apart in both x and y directions. 



V. LIMITATIONS 



Limited experience indicates that the data on the NGSDC tapes contain a 

 few gross errors in most 0.25° by 0.25° squares. One satisfactory way to 

 detect these errors is through the use of contour plots. Gross data errors 

 are easily located on these plots since they appear as small "bull's-eyes." 



