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You might consider:
1) geo-referencing your digital image of the Bushnell survey, by identifying points on the image with known latitude and longitude, and saving as a GeoTiff file.
2) creating a georeferenced blank grid covering the area of the lagoon, using GIS software.
3) copy the values of the Bushnell survey soundings, with the shoreline values =>0 into the appropriate cells on the blank grid.
4) Use the software to interpolate values for the empty cells.
The product will be a geo-referenced digital elevation model (DEM) of lagoon, from which you can then use the software to generate contour maps, as well as better appreciate any subtle features that the Bushnell survey may have detected. Additional information can be incorporated into the DEM from later surveys or field work. Further, both the Bushnell survey and the DEM can be overlaid on satellite imagery or charts, which may be helpful in matching visible features, including water color, to the soundings.
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