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Digital Mapping Techniques '02 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 02-370

A Geologic Gazetteer of the Lake Tahoe Region

By Jordan Hastings and Ada Otter

Department of Geography
University of California Santa Barbara
Santa Barbara, CA 93106
Telephone: (805) 893-3663
Fax: (805) 893-8617
e-mail: jordan@geog.ucsb.edu

ABSTRACT

The Lake Tahoe Region, straddling California and Nevada, presents a wealth of cultural, ecological, scientific, and scenic values; it has been inhabited for at least 8,000 years. The Lake Tahoe Region is within a relatively young, large, and very deep graben. The region's ecosystem is actively shaped by its geology, which includes strong tectonism and a history of recent landslides/tsunamis around the lake. Archaeological, historical-survey, and recent scientific mapping activities document on numerous maps the region's evolution. Over 3,400 names for topographic and geologic features appear on these maps.

In this paper, we describe the development of a geologically sophisticated gazetteer service, the Tahoe Regional Gazetteer (TARGA), which interrelates feature names with geologic maps. In conjunction, TARGA has built an inventory of 69 data sets, including 16 geologic maps, for the LTR, accumulated into a standardized repository. All three of TARGA's component subsystems--inventory, repository, and gazetteer--are Web accessible and Web mapped, providing convenient answers to such questions as: What geological maps exist for cultural and/or physical feature(s) "X"? In addition to its online capabilities, TARGA has accumulated a valuable database for future research on the geology of the LTR, and for geologic-map data-management systems in general.

INTRODUCTION

The Lake Tahoe Region 1 (LTR), straddling the states of California and Nevada (Figure 1), was designated a "national concern" under Presidential Order #13057 (Clinton, 1997), citing its "extraordinary natural, recreational, and ecological resources." This order also mandated the development of a comprehensive management plan for the LTR. In turn, this plan has spurred collection, digitization (where needed), and synthesis of a large number of geospatial information resources for the region within a geographic information system (GIS), specifically the Tahoe Environmental GIS (TEGIS). TEGIS was developed collaboratively by the U.S. Geological Survey (USGS) and the Tahoe Regional Planning Authority (TRPA) and is made available on the Lake Tahoe Data Clearinghouse Web site: http://tahoe.usgs.gov/.

Map of Lake Tahoe region     Figure 1. The Lake Tahoe region: 2° box surrounding 1° TARGA boundary, with TEGIS inset.

A fundamental component of TEGIS, given Lake Tahoe's setting in a large, deep, and still tectonically active graben, is regional geology. Numerous geologic maps for the region exist (Cartier and others, 1994; Hess and Johnson, 1997; Bedford and others, 2002). These maps have been produced by different mappers at different times, in a variety of scales (1:24K, 1:62.5K, 1:250K, 1:500K [Nevada], and 1:750K [California]), projections (UTM10, UTM11, State Plane, Albers), and datums (NAD27, NAD83), as shown in Figure 2. The larger scale (<1:100K) and the smaller scale (≥1:250K) maps independently tile the region, thus overlapping each other and providing a welter of technical GIS challenges, in addition to scientific ones. A variety of USGS topographic base maps at 1:24K, 1:100K, and 1:250K also exist for the region.

TEGIS geologic mapping by USGS and TRPA     Figure 2. TEGIS geologic mapping by USGS and TRPA; 10 geologic maps cover 16 1:24K topographic quadrangles (from Cartier and others, 1994).

Humans have occupied the environs of Lake Tahoe for at least 8,000 years (Forbes, 1982; Moratto, 1984), and in this period a large number of its cultural and physical features have been named (and renamed) in several languages. These names form the traditional basis for place identification, navigation, and wayfinding in the region. Some names are essentially point features (mountain peaks, springs, trail heads, forks, etc.), others are largely linear (creeks, roads, trails, etc.), and the majority are clearly areal (lakes and ponds, settlements, mine sites, wilderness areas, etc.). Most of the modern names appear on topographic maps, which the USGS Geographic Names Information System (GNIS, http://geonames.usgs.gov/) excerpts to point locations (often feature centroids, but sometimes simply georeferenced map-label positions). Other sources of georeferenced names include GIS/map data from commercial providers, the various Federal land management agencies, the states of California and Nevada (particularly their state geological surveys), local municipalities, historical documents, and, ultimately, anecdotal/common usage. Obviously, people can find their way to and among named features, whether or not these have been mapped, so undoubtedly more names exist than are documented in GNIS.

In this article, we describe a fusion of geological and topographic maps with a digital gazetteer 2 (Hill, 2001) for the LTR: the Tahoe Regional Gazetteer (TARGA). In the first instance, TARGA's purpose is largely geological, designed to answer such questions as:

Currently, the "X" and "Y" are restricted to cultural and physical geographic features; future extension to geological provinces and type sections is planned.

DEVELOPMENT

The TARGA system incorporates three major component subsystems: an Inventory, a Repository, and a Gazetteer. The development of each of these subsystems is briefly described below.

Inventory Subsystem

TARGA first developed an Inventory of the spatial datasets available for the LTR, including geologic maps, topographic maps (in both raster and vector forms), digital terrain models, and satellite imagery. For simplicity, this inventory was implemented in Microsoft (MS) Excel 2000. Altogether, 34 original data sets were identified, researched from various sources, and described in Excel by 18 attributes,3 including name, source, type, scale and projection, and minimum bounding rectangle (MBR), as shown in Figure 3a. Visual Basic for Applications (VBA) scripts were written to assist with proofing the attributes as they were entered, and also to produce a descriptive Web page for each data set, complete with a thumbnail image of it (manually prepared), as shown in Figure 3b.

TARGA Inventory workbook

Figure 3a. TARGA Inventory workbook.

TARGA Inventory Web page for one map product

Figure 3b. TARGA Inventory Web page for one map product.

Repository Subsystem

Source materials for the 34 original inventoried data sets were subsequently acquired in digital form, preprocessed in various ways, and stored in a companion repository for easy GIS access. Preprocessing tasks included format conversion, georeferencing, deprojection and/or reprojection, and clipping to the TARGA regional boundary. The result was a derived collection of standardized, conformable data sets in NAD27 geographic coordinates.4 A total of 69 data sets were produced from the original 34 and incorporated in the TARGA repository, as summarized in Appendix A.

Gazetteer Subsystem

Each of the spatial data sets inventoried and processed covers an area of tens to hundreds of square kilometers. Within each area, numerous named sub-areas provide important context and/or foci for geologic map work. GNIS lists over 3,400 named places and features in the LTR, each identified by a point location derived from topographic maps at scales of 1:24K and smaller (i.e., less detailed). A commercial vendor, Geographic Data Technology (GDT), has documented about 1,000 of the larger features, complete with their polygonal geometry, predominantly taken from the same maps in digital form, (digital line graphs--DLG's). Typically, the GNIS locations fall within the GDT polygon boundaries for like-named features; however, there are exceptions. The same name may be given to multiple features; for example, there are three Frog Lakes and six Mud Lakes identified in the LTR. Also, a single feature may have several names; Lake Tahoe itself was formerly known as Lake Bigler. Redundant locations may exist for large features, particularly those that appear in portions on multiple maps. All such details were carefully resolved for each feature and recorded in an MS Access 2000 database, with the following principal fields: feature name, feature type, location(s), and MBR (computed to at least include all valid locations). The simple database schema appears in Appendix B.

APPLICATION

TARGA is intended to assist in finding maps, particularly geological maps, and other geospatial data sets relating to named features in the LTR. A prototypical query is: Show me the maps on which geographic feature "X" appears. Answering this query involves integrating the three component subsystems described above: the inventory, the repository, and the gazetteer. The answer to the query is ideally shown in a GIS; tabular results also may be desired.

Specifically, the query process is: (1) the selected feature names/types are matched within the gazetteer, which returns their locations and/or MBR's; (2) the inventory is searched to determine data sets that contain the places (by point-in-rectangle and/or rectangle-on-rectangle overlay calculation, based on the data sets' MBR's); and (3) the qualifying data sets from the repository are shown in map view. In addition, a simple list of the qualifying data sets is made available outside the GIS. To increase the specificity of the system, and to reduce false retrievals, feature matches may be limited within a region of interest, indicated either by MBR (directly) or by name (indirectly, again using the gazetteer). For convenience, the system is Web accessible, which in turn mandates that the query interface be kept simple. A sample query/retrieval dialog from TARGA is shown in Figure 4.

TARGA Gazetteer interface

Figure 4. TARGA Gazetteer interface, showing maps on which Lake Tahoe appears.

Access to descriptive data about the data sets retrieved--metadata--also is important. Basic metadata for each of the datasets is provided by supporting Web pages, one per data set, automatically generated by VBA scripts from the inventory subsystem, as previously mentioned. A sample page is shown in Figure 3b. These metadata pages are explicitly linked in the list of retrieved data sets; they are also implicitly linked to the Web-mapping display (accessible by right-click). The entire collection of metadata pages is centrally available via the inventory subsystem.

TARGA is hosted on an MS Windows 2000 Server system, running MS Internet Information Server 5. Accordingly, the query interface is implemented in .asp scripts.

Basic GIS/Web-mapping support is provided by Environmental Systems Research Institute (ESRI) ArcView IMS software, running on the same system.

FUTURE WORK

TARGA has achieved its primary design objective: stimulating the collection and documentation of an integrated suite of geospatial data sets and maps for the Lake Tahoe region. Of particular note is standardization of the 16 geological maps, including one soils map, for the region in geographic (NAD27) coordinates.

Future work with TARGA will be to migrate its gazetteer database from Excel to Access, and also to modernize its Web mapping, using ESRI ArcIMS v4 in place of ArcView IMS. In conjunction with these changes, more sophisticated spatial operations, involving general polygon-on-polygon operations, as well as the present simple MBR tests, will be supported. Also, nongeographic reprojection of raster data sets together with vector data sets will be possible.

ACKNOWLEDGMENTS

The authors acknowledge the kind support of the California Geological Survey, Geographic Data Technology, the Nevada Bureau of Mines and Geology, and the U.S. Geological Survey, for both dataset access and supporting discussions during the course of this project. Collaborative discussions with the Alexandria Digital Library Gazetteer Project at the University of California Santa Barbara were also helpful.

REFERENCES

Bedford, D.R., Ludington, S., Nutt, C., Stone, P.A., Miller, D.M., Miller, R.J., Wagner, D.L., and Saucedo, G.J., 2002, Geologic database for digital geology of California, Nevada, and Utah--an application of the North American Data Model: USGS OFR 02-XXX.

Cartier, K.D., Peltz, L.A., and Smith, J.L., 1994, Development and documentation of spatial data bases for the Lake Tahoe Basin, California and Nevada: U.S. Geological Survey Water Resources Investigation Report 93-4182, 65 p.

Clinton, W.J., 1997, Federal actions in the Lake Tahoe Region: Office of the President, Executive Order #13057, dated July 26, 1997; http://www.usda.gov/news/tahoe/ninteen.htm.

Forbes, J.D., 1982, Native Americans of California and Nevada: Happy Valley, California, Naturegraph Publishers, 240 p.

Hess, R.H., and Johnson, G.L., 1997, County digital geologic mapping project: Nevada Bureau of Mines and Geology Open File Report 97-1, CD-ROM.

Hill, L.L., 2000, Core elements of digital gazetteers: placenames, categories, and footprints, in Borbinha, J., and Baker, T., eds., Research and advanced technology for digital libraries: Proceedings of the 4th European Conference, ECDL 2000 Lisbon, Portugal, September 18-20, 2000, Springer: Berlin, p. 280-290; http://www.alexandria.ucsb.edu/~lhill/paper_drafts/ECDL2000_paperdraft7.pdf. Moratto, M.J., 1984, California archaeology. Orlando, Florida, Academic Press, 757 p.

Appendix A. TARGA Repository Contents

DSID
#Auto 		    Hier_ID
*DSID 		    Name     Format
@Form 		    Projection
@Proj 		    SourceScale
100     Sample Data                 UTM10-27      
101           Carson City Topo Collared     IMG     UTM11-27     1:100,000
102     101     Carson CityTopo Decollared     IMG     UTM11-27     1:100,000
103     102     Carson City West Topo Clipped     GRD     Geographic     1:100,000
104           Smith Valley Topo Collared     IMG     UTM11-27     1:100,000
105     104     Smith Valley Topo Decollared     IMG     UTM11-27     1:100,000
106     105     Smith Valley West Topo Clipped     GRD     Geographic     1:100,000
107           Placerville Topo Collared     IMG     UTM10-27     1:100,000
108     107     Placerville Topo Decollared     IMG     UTM10-27     1:100,000
109     108     Placerville East Topo Clipped     GRD     Geographic     1:100,000
110           Truckee Topo Collared     IMG     UTM10-27     1:100,000
111     110     Truckee Topo Decollared     IMG     UTM10-27     1:100,000
112     111     Truckee East Topo Clipped     GRD     Geographic     1:100,000
113           Soils (LTDC)     SHP     UTM10-27     1:24,000
114           Vegetation     SHP     UTM10-27     1:24,000
115     117     DEM Image 10-m     IMG     UTM10-27      
116     117     DEM with Bathymetry 10-m     COV     UTM10-27      
117           DEM with Bathymetry 10-m     EXP     UTM10-27      
118           Buildings     SHP     UTM10-27     1:24,000
119           Docks     SHP     UTM10-27     1:24,000
120           Hydrology     SHP     UTM10-27     1:24,000
121           Hypsography     SHP     UTM10-27     1:24,000
122           Roads     SHP     UTM10-27     1:24,000
123           Lake Tahoe (LTDC) Topo     IMG     UTM10-27     1:24,000
124           Landcover     GRD     UTM10-27     1:24,000
125           GDT Water Polygons     SHP     Geographic      
126           GDT Major Water Features (Rivers)     SHP     Geographic      
127           GDT Parks     SHP     Geographic      
128           GDT Recreation Areas     SHP     Geographic      
129           GDT Area Landmarks     SHP     Geographic      
130           CA Geology     COV     Albers CEA-27     1:250,000
131     130     CA Geology Clipped     SHP     Albers CEA-27     1:250,000
132     131     CA Geology Clipped     SHP     Geographic     1:250,000
133           NV Geology- Wahoe     SHP     UTM11-27     1:250,000
134     133     NV Geology- Wahoe Reprojected     SHP     UTM10-27     1:250,000
135     134     NV Geology- Wahoe Clipped     SHP     UTM10-27     1:250,000
136     135     NV Geology- Wahoe Clipped     SHP     Geographic     1:250,000
137           NV Geology- Lyon,Douglas,Carson     SHP     UTM11-27     1:250,000
138     137     NV Geology- Lyon, Douglas, Carson Reprojected     SHP     UTM10-27     1:250,000
139     138     NV Geology- Lyon,Douglas,Carson Clipped     SHP     UTM10-27     1:250,000
140     139     NV Geology- Lyon, Douglas, Carson Clipped     SHP     Geographic     1:250,000
141           Geology Lake Tahoe Basin (TEGIS)     COV     UTM11-27     1:24,000
1:62,500
1:125,000
142     141     Geology Lake Tahoe Basin (TEGIS)     SHP     UTM11-27     1:24,000
1:62,500
1:125,000
143           Geology Lake Tahoe Basin (TRPA)     COV     UTM11-27     1:24,000
1:62,500
1:125,000
144     143     Geology Lake Tahoe Basin (TRPA)     SHP     UTM11-27     1:24,000
1:62,500
1:125,000
145           Soils (TRPA)     COV     UTM11-27     1:24,000
147           LTDC Mapbounds     SHP     Geographic     n/a
148           TRPA, TEGIS Mapbounds     SHP     Geographic     n/a
149           1 Degree Mapbounds     Geographic     SHP     n/a
150           2 Degree Mapbounds     SHP     Geographic     n/a
151           3 Degree Mapbounds     SHP     Geographic     n/a
152           Hydrologic Units Map Great Basin     COV     Albers CEA-1866     1:250,000
153     152     Hydrologic Units Map Great Basin Clipped     SHP     Albers CEA-1866     1:250,000
154     153     Hydrologic Units Map Great Basin Clipped w/ 5 Mapbounds     SHP     Geographic     1:250,000
155           Hydrologic Units Map CA 250K     COV     Albers CEA-1866     1:250,000
156     155     Hydrologic Units Map CA Clipped     SHP     Albers CEA-1866     1:250,000    
157     156     Hydrologic Units Map CA Clipped w/ 9 Mapbounds     SHP     Geographic     1:250,000
158           Streams- US     COV     Albers CEA-1866     1:2,000,000
159     158     Streams- CA,NV Clipped     SHP     Albers CEA-1866     1:2,000,000
160     101     Carson City Topo Collared     GRD     UTM11-27     1:100,000
161     101     Carson City Topo Collared Unprojected     GRD     Geographic     1:100,000
162     104     Smith Valley Topo Collared     GRD     UTM11-27     1:100,000
163     104     Smith Valley Topo Collared Unprojected     GRD     Geographic     1:100,000
164     107     Placerville Topo Collared     GRD     UTM10-27     1:100,000
165     107     Placerville Topo Collared Unprojected     GRD     Geographic     1:100,000
166     110     Truckee Topo Collared     GRD     UTM10-27     1:100,000
167     110     Truckee Topo Collared Unprojected     GRD     Geographic     1:100,000
168     103; 106; 109,112     Tahoe Region Topo     GRD     Geographic     1:100,000
169     168     Tahoe Region Topo     IMG     Geographic     1:100,000

NOTE: Only selected attributes are shown; the full list of attributes for each data set is:
UniqueID #, Hierarchical/Parent ID#, Source Scale, File Size, Code (Web availability), Name, Category, Format, Projection,
Mapbound extent (Lat1, Lon1, Lat2, Lon2), Author, Author Date, Editor, Editor Date, Publisher, Publisher Date, Description,
Comments, Thumbnail Image, and Filepath.

Appedix B. TARGA Gazetteer Database Schema

TARGA Gazetteer Database Schema

SOURCE (ID, Name, . . .)

TYPE (ID, Parent-ID, Code, Name, . . . )

FEATURE (ID, Parent-ID, SOURCE-ID, TYPE-ID, Name, . . .)

GEOMETRY (FEATURE-ID, Location-Lat, Location-Long, MBR-Lat1, MBR-Lon1, MBR-Lon2, . . .)

Notes:

  • Primary Keys are underlined, viz. ID
  • Foreign Keys are italicized, viz. Parent-ID, TYPE-ID
  • Both FEATURE and TYPE are heirarchically-structured, viz. Parent-ID, indicated by reflexive (circular) relationship in upper left-hand corner

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