Bathymetric integer grid generated from interpolating original point sounding and depth line features acquired from NOAA Electronic Navigational Charts [ENCs].
ENC data is available for various geographic regions from the Office of Coast Survey [<http://chartmaker.ncd.noaa.gov/>]>. Lake Superior data was identified, downloaded, and the ENC data imported into geodatabase format.
The narrow coastal zone corresponding to the depth area outlined by depth contours was interpolated from DepthLineFeature contours. The rest of the lake bathymetry data came from SoundingFeature point interpolations.
RELATIONSHIP BETWEEN SCALE AND SPATIAL RESOLUTION
Vector to raster interpolation requires that certain assumptions are made regarding an appropriate raster cell size corresponding to a vector map scale. A literature review was performed in order to try and identify some sort of industry standard, logic, or methodology, which could be used to relate vector scale to raster spatial resolution. The literature review revealed a few common relationships between cell sizes and original map scales, but the conclusion to this literature review is there is no definite industry standard with defined rules. Some of the sources found along the way are as follows:
<http://srmwww.gov.bc.ca/gis/gisscale.html>
<http://cbc.rs-gis.amnh.org/guides/basic_concepts/scale_res.html>
<http://www.profc.udec.cl/~gabriel/tutoriales/giswb/vol2/cp1/t1-2.gif>
<http://listes.ulaval.ca/cgi-bin/wa?A2=ind0503&L=applied-gis-rs&F=&S=&P=3331>
The goal is to get the most information from the sounding features as possible, but also not to use an output cell size which assumes more accuracy than is actually present in the data, as the cell size should not be smaller than the uncertainty in the vector linework. In order to determine what output cell size was most appropriate for the production of bathymetry grids, the vector map scale [i.e., CSCL or compilation scale given as a subfield of the Data Set Parameter [DSPM] field] of each NOAA ENC would be identified. And because with a vector data model resolution is tied to the scale of the data, the cell size of the vector data set can be calculated using this scale. Sources say that the smallest line that can be drawn on a map which is still recognizable is about .5 millimeters. On a 1:24,000 scale map, 1 millimeter equals 24,000 millimeters on the ground. Since you can only draw something as small as .5 mm, the resolution of the data set is 12,000 mm or 12 meters. Using this logic, the output cell size of the bathymetry grids was selected.
This methodology was proposed via email to Adeline Wong [Adeline.Wong@noaa.gov] with the NOAA National Ocean Service [NOS] IN Silver Spring, MD. Ms. Wong provided the following statement regarding my proposed methodology, "Your approach is sensible with regard to ENC soundings, which are often downsampled and scaled by chart." Ms. Wong was also asked about how to determine the accuracy of the ENC data. During the same literature review, it was discovered in section 7.3.4, Data Set Accuracy Record Structure, of "IHO Transfer Standard for Digital Hydrographic Data, Edition 3.1 - November 2000" [i.e., 31Main.pdf], there is mention of a field table named SDAC or Data Set Accuracy. However, such a table has never been found with any of the ENCs downloaded at MED. It appears as though the information in this table would answer a lot of questions regarding ENC accuracy. Ms. Wong responded by saying, "Your question on the accuracy of charted features is on the cutting edge of research. We're currently developing methods to attribute accuracy (SDAC field) for ENCs!"
The compilation scale of all 19 NOAA ENC charts available for Lake Superior ranged from 1:15,000 to 1:600,000, or 8 meters to 300 meters.
Lake NOAA Chart NOAA Cell CSCL Scale Resolution [m]
Superior 14965 US4MI70M 1:120,000 60
Superior 14963 US4MI76M 1:120,000 60
Superior 14962 US4MI77M 1:120,000 60
Superior 14966 US4MN11M 1:120,000 60
Superior 14967 US4MN21M 1:120,000 60
Superior 14968 US4MN22M 1:120,000 60
Superior 14970 US5MI75M 1:15,000 8
Superior 14975 US5WI11M 1:15,000 8
Superior 14974 US5WI22M 1:15,000 8
Superior 14964 US4MI71M 1:180,000 90
Superior 14971 US5MI72M 1:30,000 15
Superior 14972 US5MI73M 1:30,000 15
Superior 14969 US5MI74M 1:30,000 15
Superior 14882 US5MI61M 1:40,000 20
Superior 14883 US5MI62M 1:40,000 20
Superior 14884 US5MI63M 1:40,000 20
Superior 14976 US5MI78M 1:40,000 20
Superior 14973 US4WI21M 1:60,000 30
Superior 14961 US3MI79M 1:600,000 300