and the east-west side is in longitudinal minutes. On a 

 Mercator projection contour chart this is a 5-minute rectangular 

 grid. The bathymetric data are logically formatted to place 

 depth values at the intersection of each 5-minute grid crossing 

 as shown in figure 2. 



The next level of structuring is to index the 5-minute cells 

 into 5-degree squares called Marsden Square Locator numbers (MSQLOC) 

 using the Marsden square system which divides the earth surface 

 into 5-degree squares (fig. 3). Further subdivision of the 

 Marsden square by quadrants is shown in figure 4. The MSQLOC is 

 the quadrant number followed by the Marsden square number as 

 follows : 



Marsden square number+quadrant = MSQLOC 



Example: 036+2 = 0362 



The MSQLOC is a unique worldwide reference to each 5-degree square 

 of gridded bathymetric data. The MSQLOC area includes a 5-minute 

 overlap of all sides as shown in figure 5 for MSQLOC 0362. 



The gridded bathymetric data base is created following the 

 procedure used by Davis and Kontis (1970), However, accurate 

 synthetic data derived from large and medium-scale bathymetric 

 charts are used instead of original survey data. The synthetic 

 track data are derived from charts by superimposing parallel track 

 lines, 5 minutes apart, over the MSQLOC area. Extraction of the 

 data usually starts from the lower left corner. The orientation 

 of the track lines can be any direction from west-east (90° bearing) 

 to nearly south-north (1° bearing) , but not true north, which 

 necessitates changing several statements in the gridding program. 

 The only other restriction is that the first track be a west-east 

 track across the MSQLOC area. The remaining tracks may be of any 

 orientation and in any order. 



The data are extracted from the chart by digitizing the 

 intersections of the synthetic track with the contours sequentially 

 along the track. Interpolated points must be extracted for the 

 beginning and end of each full track. These tracks must extend 

 5 minutes beyond the MSQLOC area on all sides as shown in figure 6. 

 Short tracks may be added to emphasize certain topographic 

 characteristics such as spot elevations. These can be extracted 

 at any orientation except true north-south as shown in figure 6B. 



Each digitized track is assigned a sequence number, but the 

 physical order of the tracks in the card deck is arbitrary after 

 the first track. These digitized tracks are inputs to the gridding 

 program. The output from that program ns a punched deck of gridded 

 bathymetric data with the point or origin in the lower left corner. 



