This paper explains some of the methods used by the New Jersey Geological Survey (NJGS) for generating, managing, analyzing, displaying, and distributing digital geologic data. These methods include a geographic information system (GIS) running on Sun microcomputers to produce digital maps and geo-referenced data and the use of personal computer (PC) software for managing outcrop and remotely-sensed structural geology data. The geological data layers (coverages) are documented according to federal standards, archived as electronic data, and distributed to the public through a publication sales office and the Internet.
For a State/USGS cooperative mapping project, the NJGS began designing a digital geologic database using Arc/Info geographic information system (GIS) software. The Survey also modernized the GIS laboratory with Sun SPARCstation microcomputers (workstations) connected to desktop PCs using an ethernet local-area network. This effort included adding an E-format (34 inch-wide media) optical scanner, raster-to-vector conversion software, and an electrostatic plotter. By 1994 the NJGS had a modern GIS laboratory for creating and maintaining digital geological databases and publishing geological maps using digital cartography. More recent efforts include the development of an electronic information archive with
a distribution outlet on the Internet's World-Wide Web at http://www.state.nj.us/dep/njgs/.
The Survey started building geology coverages in 1987. In 1992, mylar sheets were produced of machine-drafted lines representing geologic contacts and oriented map symbols in lieu of scribing lines on peel coats as part of the standard cartographic process for producing geologic maps. ARC Macro Language (AML) scripts were written for use in ARCPLOT for automatically plotting oriented geologic-map symbols. This work formed the basis for other digital-cartographic tools that the NJGS later developed to produce full-color, bedrock geologic maps. The advance of these digital-cartographic methods also spurred the development of other PC-DOS geologic data input/output (I/O) programs.
Most geologic coverages produced by the NJGS are initially generated in NAD27 State Plane Coordinate (SPC) feet because United States Geological Survey (USGS)
7' topographic maps are based on this datum and because the New Jersey Department of Environmental Protection (NJDEP) uses SPC feet as the default geographic projection. Each map is geo-referenced (registered) to the NAD27 projection grid using at least four corresponding reference points (tics) for each map. The tics usually correspond to the corners of 1:24,000-scale' quadrangles, or 2-1/2' gradicules corresponding to the corners of 1:12,000-scale quarter quadrangles. The NJDEP Bureau of Geographic Information Analysis maintains a reference set of tics.
All archived data sets at the NJGS are projected into NAD83 SPC feet upon completion. This projection typically results in map rotations of about (+-)0.5 percent, and translation shifts of about .015" (+-120 SPC feet) for 1:100,000-scale coverages scanned and traced from NAD27 base maps.
The NJGS normally maintains a maximum root-mean-square (RMS) error of .006 (about 12 ft. at the 1:24,000 scale) for coverage development. An estimated 85 percent of the archived geologic and hydrogeologic coverages are accurate to within .003 RMS deviation. Quality assurance is maintained by comparing a proof plot of each coverage to the original base map. Any line that deviates from the original position by more than .012 inch (about 1 to 1.5 line widths) is redigitized, replotted, and corrected until it is acceptable.
Geology coverages are digitized using either a digitizing tablet and/or an optical scanner in conjunction with raster-to-vector (R-to-V) conversion software. Point themes are typically generated using a digitizing tablet. The NJGS uses CalComp 9100 and 9500 digitizers with a reported accuracy of (+-)0.005 in. (+-0.127 mm). A set of at least four points (tics) is used for registering map sheets on a digitizing tablet at the beginning of each editing session.
Line and polygon themes are commonly built by optically scanning a map as a raster image, and then tracing linear arrays of image cells (pixels) with R-to-V conversion software. The NJGS uses a CalComp ScanPlus II roll-feed, E-format, two-camera scanner for maps larger than legal-sized documents. Most maps are scanned using 400 dots-per-inch (dpi) image resolution. The scanned image accuracy is reported as (+-).25 percent. The NJGS has obtained the best imaging results from scanning either translucent or clear mylar separates with drafted neatlines of black rapidigraph ink. Acceptable results have been obtained from using soft-lead pencils (at least a No. 2, or HB pencil lead) on white paper or mylar.
R-to-V coverage development usually requires more time preparing media for reproduction than normally spent when using a digitizing tablet, but an estimated 50 percent of the time developing a coverage is recouped using the R-to-V approach if the coverage is physically large or detailed. The R-to-V approach allows a uniform coverage to be developed without having to worry about errors stemming from repeatedly registering maps on a digitizing tablet at the start of consecutive digitizing sessions. This concern frequently arises when digitizing large maps drafted at intermediate (1:100,000) and small (1:250,000 or less) scales. The R-to-V method works best with maps having continuous lines requiring no ornamentation. Separate mylar sheets should be prepared for each set of points, lines, and polygons to be individually generated from a pre-existing map.
The NJGS uses CADCore, Version 2.0 R-to-V software and Arc/Info GRID. Original maps or mylar separates are scanned and saved using a TIFF-5.0 image format. The TIFF image is imported to CADCore where it is converted into a CADCore image format (*.hrf) used for image display, processing, and line tracing. The raster image is centerline or outline traced with vector-line segments measured in inches. The vectors are saved as a CADCore drawing file (*.drw) and exported as an input file for use with Arc/Info generate command. The output files are generated as lines in Arc/Info and built into an arc coverage having inch units. GRID can perform a centerline trace without intervention from the digital compiler. The TIFF file is converted to a GRID and then processed by the GRID module into an arc coverage. The map is then transformed from inch units into NAD27 or NAD83 coordinates using the standard reference tics or other sets of links in the map-transformation process. Coverages are subsequently edited using Arc/Info ARCEDIT. Quality assurance is maintained by comparing a proof plot of each coverage to the original base map. Any line that deviates from the original position by more than .012 inch (about 1 to 1.5 line widths) is digitized, replotted, and corrected until it is acceptable.
The Field data Management System (FMS v. 2.1) is computer software designed for managing, analyzing, and plotting structural geology data. The FMS is composed of two sets of files: one for managing structural geology data on the DOS-PC platform, and another for displaying and generating structural geology themes within the Arc/Info GIS environment. The latter set of programs was developed for use on Sun SPARCstations running Arc/Info (v. 7.0) and has not been tested on other platforms.
The FMS is used by the NJGS for managing and analyzing outcrop-based and remotely-sensed structural data and for integrating these data into full-color, quadrangle-scale geologic maps with oriented and annotated structural symbols. The FMS uses geological data that are organized into ASCII data files, usually through keyboard entry. Structural data can then be sorted based on location, stratigraphic unit, and structural variables and graphically plotted in either the map or profile view using a variety of PC-analysis tools. Most of these tools use standard or circular histograms for analyzing the frequency of structural bearing. Statistics are also available for the structural inclination and the quantity of structural data within a sorted data set. Other DOS-PC utilities include data import and export filters that format structural data for use with other commercial geologic analysis software. The FMS does not build GIS coverages of structural symbols on its own. It provides tools for automatically plotting oriented structural geology symbols on maps. It also can be used for generating GIS themes for the structural bearing or apparent inclination of structures such as fracture traces, in conjunction with Arc/Info.
NJGS geology coverages are geo-referenced sets of points, lines, and polygons stored as electronic data in computers running GIS software. Coverages are developed for both map-based and cross-sectional views. Point coverages are built for sets of geologic features unable to be displayed as a line or polygon (field stations, wells). Line coverages represent features too narrow to be displayed as a polygon (structural contacts, contours). Polygon coverages denote an area (geologic unit, aquifer).
Cross sections represent a special case for GIS coverage development because they depict subsurface geologic information based on the vertical (z) dimension relative to the map (x and y) dimensions. Cross-section coverages are currently unable to be geo-referenced in Arc/Info because they are built using the standard GIS programs and contain x and y coordinates. They are built at the scale in which they are drafted, digitized or scanned.
The Survey has developed a standardized set of coverage items (database fields) and item variables for assigning attributes to features in bedrock line and polygon coverages. The line attribute list is called GEOITEM. It contains coverage items and variables, and variable descriptions. GEOABB is the polygon attribute list. It holds coverage items and variables as well as geologic unit names for bedrock and surficial geology. Cross section coverages use the same item fields and attributes as the map-based bedrock coverages. A line attribute list is under development for surficial geology.
Metadata is defined by the Federal Geographic Data Committee (FGDC) as data that describe the content, quality, condition, and other characteristics of data, or in other words "data about data." Metadata are required as an integral part of a complete GIS coverage in order to convey details explicating its origin and use. These details include important information such as a citation, the physical limitations, and scope.
The NJGS produces and archives geologic, hydrogeologic, geophysical, and geographic data as digital data files for electronic distribution to the public. One method of electronic-data transfer uses the Internet mail protocol from the World Wide Web (WWW) home page for the NJGS (http://www.state.nj.us/dep/njgs/). Because the Web reaches a global market, the NJGS developed a metadata file format based on the content standards for digital geospatial metadata proposed by the Federal Geographic Data Committee (FGDC). These content standards were evaluated for completeness, applicability, and content for use with geologic data produced by the NJGS. A comparison was also made to the NJDEP data dictionary file, which currently serves as the NJDEP metadata standard. An ASCII-text format was chosen as a document template due to its broad user base and because of the need to develop metadata files utilizing different computer platforms (DOS, Windows, Apple, and UNIX). A prototype NJGS metadata format was reviewed by the NJGS staff and the NJDEP Office of Information Resources Management in 1995, and the abstracted version of the FGDC standard was adopted for use by the NJGS in 1996.
This standard is applied to all electronic files intended for distribution over the Web and all GIS coverages to be archived by the NJGS. The NJGS currently archives Arc/Info coverages and related dBASE relational data files as part of its Digital Geodata Archive. Compressed data files containing less than 1.4 megabytes of information are also being made available as Digital Geodata Series (DGS) publications. The NJGS DGS products are designed for use by ESRI's ARCVIEW software. The NJGS metadata documents therefore focus on Arc/Info coverages and dBASE files. However, metadata are also being generated for other products that include ASCII-text document files and computer-software programs such as the NJGS FMS.
Digital data are also distributed through the NJDEP publication and sales office and by written request. A reproduction fee is normally charged for the data.
NJGS uses a modern GIS lab to capture, manage, and display geologic data. Machine drafted lines and oriented map symbols are now a part of the standard cartographic process. Data capture is accomplished by digitizing or scanning separates of hard-copy maps. The FMS software is used to manage, analyze, display, and plot structural geology data. Point, line, polygon, and cross sectional coverages are built on the GIS. Bedrock line and polygon coverages employ a standard set of coverage items and variables to assign attributes to features. Electronic data intended for distribution over the Internet or archive on the GIS have metadata based on FGDC standards. Data are distributed through the World-Wide Web, a publication sales office, and by written request.