Hillshade Surface Representing the Floor of Lake Mead and the surrounding area: UTM Projection 30m cellsize TIFF format

Frequently-anticipated questions:

• What does this data set describe?
  1. How should this data set be cited?
  2. What geographic area does the data set cover?
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
  5. What is the general form of this data set?
  6. How does the data set represent geographic features?
  7. How does the data set describe geographic features?
• Who produced the data set?
  1. Who are the originators of the data set?
  2. Who also contributed to the data set?
  3. To whom should users address questions about the data?
• Why was the data set created?
• How was the data set created?
  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
• How reliable are the data; what problems remain in the data set?
  1. How well have the observations been checked?
  2. How accurate are the geographic locations?
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
  5. How consistent are the relationships among the data, including topology?
• How can someone get a copy of the data set?
  1. Are there legal restrictions on access or use of the data?
  2. Who distributes the data?
  3. What's the catalog number I need to order this data set?
  4. What legal disclaimers am I supposed to read?
  5. How can I download or order the data?
• Who wrote the metadata?

Title:

Hillshade Surface Representing the Floor of Lake Mead and the surrounding area: UTM Projection 30m cellsize TIFF format

Abstract:

Lake Mead is a large interstate reservoir located in the Mojave Desert of southeastern Nevada and northwestern Arizona. It was impounded in 1935 by the construction of Hoover Dam and is one of a series of multi-purpose reservoirs on the Colorado River. The lake extends 183 km from the mouth of the Grand Canyon to Black Canyon, the site of Hoover Dam, and provides water for residential, commercial, industrial, recreational, and other non-agricultural users in communities across the southwestern United States. Extensive research has been conducted on Lake Mead, but a majority of the studies have involved determining levels of anthropogenic contaminants such as synthetic organic compounds, heavy metals and dissolved ions, furans/dioxins, and nutrient loading in lake water, sediment, and biota (Preissler, et al., 1998; Bevans et al, 1996; Bevans et al., 1998; Covay and Leiker, 1998; LaBounty and Horn, 1997; Paulson, 1981). By contrast, little work has focused on the sediments in the lake and the processes of deposition (Gould, 1951). To address these questions, sidescan-sonar imagery and high-resolution seismic-reflection profiles were collected throughout Lake Mead by the USGS in cooperation with researchers from University of Nevada Las Vegas (UNLV). These data allow a detailed mapping of the surficial geology and the distribution and thickness of sediment that has accumulated in the lake since the completion of Hoover Dam. Results indicate that the accumulation of post-impoundment sediment is primarily restricted to former river and stream beds that are now submerged below the lake while the margins of the lake appear to be devoid of post-impoundment sediment. The sediment cover along the original Colorado River bed is continuous and is typically greater than 10 m thick through much of its length. Sediment thickness in some areas exceeds 35 m while the smaller tributary valleys typically are filled with less than 4 m of sediment. Away from the river beds that are now covered with post-impoundment sediment, pre-impoundment alluvial deposits and rock outcrops are still exposed on the lake floor.

Supplemental_Information:

Questions regarding the swath bathymetry portion of this dataset should be directed to Steve Belew, while questions regarding the remainder of the dataset and the integration should be directed to VeeAnn Cross.

1. How should this data set be cited?

   Belew, Stephen D. , and Cross, VeeAnn A. , 2003, Hillshade Surface Representing the Floor of Lake Mead and the surrounding area: UTM Projection 30m cellsize TIFF format:.

   Online Links:

   This is part of the following larger work.

   Twichell, David C. , Cross, VeeAnn A. , and Belew, Stephen D. , 2003, Mapping the floor of Lake Mead (Nevada and Arizona): Preliminary discussion and GIS data release: Open-File Report 03-320, U.S. Geological Survey, Woods Hole Field Center, Woods Hole, MA.

2. What geographic area does the data set cover?

   West_Bounding_Coordinate: -114.996607

   East_Bounding_Coordinate: -114.003223

   North_Bounding_Coordinate: 36.747107

   South_Bounding_Coordinate: 35.758065

3. What does it look like?

4. Does the data set describe conditions during a particular time period?

   Beginning_Date: 19300000

   Ending_Date: Nov-2001

   Currentness_Reference: ground condition

5. What is the general form of this data set?

   Geospatial_Data_Presentation_Form: remote-sensing image

6. How does the data set represent geographic features?
   1. How are geographic features stored in the data set?

      This is a Raster data set. It contains the following raster data types:

      • Dimensions 3586 x 2885 x 1, type Pixel

   2. What coordinate system is used to represent geographic features?

      Grid_Coordinate_System_Name: Universal Transverse Mercator

      Universal_Transverse_Mercator:

      UTM_Zone_Number: 11

      Transverse_Mercator:

      Scale_Factor_at_Central_Meridian: 0.999600

      Longitude_of_Central_Meridian: -117.000000

      Latitude_of_Projection_Origin: 0.000000

      False_Easting: 500000.000000

      False_Northing: 0.000000

      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 30.000000
      Ordinates (y-coordinates) are specified to the nearest 30.000000
      Planar coordinates are specified in meters

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.000000.
      The flattening of the ellipsoid used is 1/298.257222.
      

7. How does the data set describe geographic features?

   preshshd

   ObjectID

   Internal feature number. (Source: ESRI)

   Sequential unique whole numbers that are automatically generated.

   Value

   Count

1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
   • Stephen D. Belew
   • VeeAnn A. Cross

2. Who also contributed to the data set?

3. To whom should users address questions about the data?

   Steve Belew
   Bureau of Reclamation
   GIS Specialist
   P.O. Box 61470
   Boulder City, NV 89006-1470
   sbelew@usbr.gov
   

   (702) 293-8150 (voice)
   

This GIS (ESRI) grid represents the integration swath bathymetry, on land DEM, and underwater surface derived from countour maps generated pre-impoundment.

1. From what previous works were the data drawn?

2. How were the data generated, processed, and modified?

   (process 1 of 12)

   Three sets of data were acquired. The DEM from the area surrounding Lake Mead was acquired from NED (National Elevation Dataset) in 1998. This data was available as 30m/pixel. The swath bathymetry data was collected in Lake Mead during the fall of 2001. Bureau of Reclamation had this data at a very high resolution. The area covered by this surveying is effectively limited to the areas of post-impoundment sediment. The underwater contours available from the DRG's (digital raster graphics) were digitized. These contours were then used to create an underwater surface using the ESRI topogrid command available in Arc/Info. This surface was created with a 10 m/pixel resolution.

   (process 2 of 12)

   The lake bounds shapefile was used to clip the DEM dataset such that all values falling within the lake bounds were turned to NODATA values.

   (process 3 of 12)

   The lake bounds shapefile was used to clip the topogrid dataset such that all values falling outside the lake bounds were turned to NODATA values.

   (process 4 of 12)

   The high-resolution bathymetry data was resampled to a 10 m cellsize using bilinear interpolation.

   (process 5 of 12)

   The swath bathymetry grid was merged with the topogrid surface such that where swath bathymetry data was available, the topogrid surface was overwritten. This provided the present day lakefloor of Lake Mead.

   (process 6 of 12)

   The on land DEM was resampled to a 10 m cellsize grid (super sampling).

   (process 7 of 12)

   This supersampled on land DEM was then combined with the lake floor surface.

   (process 8 of 12)

   The resulting surface then underwent several repetitions of a 4x4 focal mean filtering to remove and null data values and to smooth the transition from one dataset to the next.

   (process 9 of 12)

   The surface was then resampled to a 30m cellsize.

   (process 10 of 12)

   This grid surface was then used to generate a hillshade relief image in ArcMap using the default settings.

   (process 11 of 12)

   The ArcInfo command gridimage was used to convert the hillshade grid to a TIFF image.

   (process 12 of 12)

   This TIFF image was then brought into Corel PhotoPaint, version 11, and converted to an 8-bit paletted image and saved with PackBit compression.

1. How well have the observations been checked?

   The nature of combining three different datasets tends to reduce the overall accuracy. Therefore a nominal accuracy of 50-100m is assigned to this dataset.

2. How accurate are the geographic locations?

3. How accurate are the heights or depths?

4. Where are the gaps in the data? What is missing?

   This dataset is complete.

5. How consistent are the relationships among the observations, including topology?

   This surface represents the integration of three datasets. One is the swath bathymetry data collected in Lake Mead in November of 2001. The second dataset is the on land DEM acquired from the NED (National Elevation Dataset) project. And the final dataset is the surface generated from pre-impoundment elevation contours as digitized from DRG's of Lake Mead (digital raster graphics). Due to the nature of combining these three different datasets, discrepancies are evident at the bounds of each of the datasets.

Are there legal restrictions on access or use of the data?

Access_Constraints: NONE

Use_Constraints:

The U.S. Geological Survey and Bureau of Reclamation must be referenced as the originator of the dataset in any future products or research derived from these data.

1. Who distributes the data set? (Distributor 1 of 1)

   VeeAnn A. Cross
   U.S. Geological Survey
   Marine Geologist
   384 Woods Hole Rd.
   Woods Hole, MA 02543-1598
   

   (508) 548-8700 x2251 (voice)
   (508) 457-2310 (FAX)
   vatnipp@usgs.gov
   

2. What's the catalog number I need to order this data set?

   Downloadable Data

3. What legal disclaimers am I supposed to read?

   These data were prepared by an agency of the United States Government. Neither the United States government nor any agency thereof, nor any of their employees, make any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. Any views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Although all data published in this report have been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data and related materials and/or the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in the use of this data, software, or related materials.

4. How can I download or order the data?
   • Availability in digital form:

     Data format:	Size: 9.540

   • Cost to order the data:

Dates:

Last modified: 06-Oct-2003

Metadata author:

VeeAnn A. Cross
U.S. Geological Survey
Marine Geologist
384 Woods Hole Rd.
Woods Hole, MA 02543-1598

(508) 548-8700 x2251 (voice)
(508) 457-2310 (FAX)
vatnipp@usgs.gov

Metadata standard:

FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)

Metadata extensions used:

• <http://www.esri.com/metadata/esriprof80.html>