APPENDIX A 

 Preparation of Charts for Digitization 



The 5-degree square unit, around which the data base is 

 created, has been explained in the "Outline of the System" and 

 in figure 3, 4, 5, and 6. Paper copies of the contour charts, 

 which are on a Mercator projection, are used to prepare the 

 basic manuscripts for digitizing. Sufficient overlap around 

 each 5-degree square is required to provide 5 minutes on all 

 sides for the MSQLOC area and an additional 5 minutes on' all 

 sides for interpolation of the track input data (fig. 6): The 

 manuscript size is then at least 320 minutes bv 320 minutes 

 regardless of the chart scale. Ideally, the manuscript should 

 consist of one easy-to-handle document. However, because chart 

 form.ats vary, this is not always possible. A case in point is 

 the addition of large scale survey of a nev/ly discovered seamount 

 to a regional chart. 



One method of handling this is to digitize the two charts 

 separately, then, substitute the synthetic tracks from the new 

 seamount chart for those in the corresponding section of the 

 older regional chart. A second method is to prepare a contour 

 selection overlay for the seamount chart, photographically reduce 

 it to the scale of the regional chart, make a print at that scale, 

 attach the print to the regional chart and match the contours. 

 This method also can be used with transparent media. 



The smallest cell selected for SYNBAPS is a 5-minute (meridional 

 part) square with a depth value at the four corner intersections. 

 The synthetic tracks of input depth points are usually taken at a 

 5-minute spacing on a Mercator projection. In high frequency data 

 areas, additional tracks of data at 1-, 2-, 3-, or 4-minute spacing 

 can be input so as to improve the four cell depth values. However, 

 there is a limit to how much improvement can be made without 

 losing some of the high-frequency detail. One improvement would 

 use a smaller cell size, but this makes random- access storage device 

 data storage requirements very large. Thus, small features that 

 fall within a 5-degree cell can be lost to the data base, 

 especially if they are not picked up at the input or structuring 

 phase. 



It is necessary to interpolate the beginning and end points 

 for each track in the overlap areas. This is not a requirement 

 for short tracks within the body of the MSQLOC area. These 

 points may be visually interpolated by the analyst or by an 

 experienced digitizer operator. This interpolation need only 

 be to the nearest 20 fathoms or about one-tenth the contour 

 interval . 



A-1 



