Just Cerfing Vol. 7, Issue 8, August 2016 Volume 5, Issue 4, April, 2014 | Page 22

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Coastal Digital Elevation Models

Figure 1. Digital and analog soundings of the 1976 National Ocean Service (NOS)
hydrographic survey H09608 along the southeast coast of Puerto Rico.(A) Digital
soundings, units converted to feet, of one portion of the survey. White soundings
are accurate; yellow soundings at bottom are shoal by a factor of 6. Background
color image and contours are from a bathymetric DEM built from these digital
soundings. (B) Corresponding location on scanned survey smooth sheet. Soundings are in black, separated by multicolored depth contours. Note how the lower
part of the DEM (panel (A) has a shallow east–west ridge with basins behind,
rather than the steep slope depicted on the smooth sheet (panel (B).

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Figure 2. Conceptual illustration of elevation smoothing introduced by DEM conversion between registration formats. (A) Original five-cell by five-cell grid or noderegistered DEM, where data and extents are defined by the center of cells. (B) Derived cell or pixel-registered DEM, where cell edges define the extents. Each cell’s
elevation value is an average of the four original cell quadrants that it overlaps.
Note how central peak has become a four-cell plateau at reduced height. (C) Repeat
conversion back to grid-registered DEM. Peak height is now reduced due to averaging. Note that outermost rows and columns of original DEM have been eliminated.

Just Cerfing Vol. 5, Issue 4, April 2014

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