Help Desk

Questions concerning Lake Tahoe? .....

Need help with the data? .....

You are in the right place !

About Lake Tahoe Data Clearinghouse

Question:
Who maintains this website?

Answer:
Lake Tahoe Data Clearinghouse website is maintained by the Western Geographic Science Center in Menlo Park, California.The site is maintained in conjunction with the USGS Water Resources Discipline and the rest of the Lake Tahoe Presidential Commitment Partners.

If you have questions about the Lake Tahoe Data Clearinghouse, please send email to T. Mathiasmeier at tmathias@usgs.gov, or use the Feedback form (a link at the bottom of every page).

Question:
Can I link to the Lake Tahoe Data Clearinghouse?

Answer:
Of course, we welcome links to the Clearinghouse. Feel free to use the following code and image:

Please visit Lake Tahoe Data Clearinghouse for geo-spatial and GIS data on the Lake Tahoe Basin.

Click here to view the source code in a text file.

About USGS data:

Question:
How can I view the data? I tried downloading the 30-m DEMs, but what program do I need to read the maps?

Answer:
The DEMs, DLGs, DRGs, and DOQs can be viewed with most GIS software packages (such as ARC/INFO, ArcView, MicroStation, SmallWorld, Intergraph GeoMedia, Idrisi, etc.).

The USGS provides two free downloadable software tools (Dem3d and Dlgv32 viewers) for viewing digital data. For more information and downloads of the viewers, please visit the DEM and DLG Viewer page.

Question:
Where do the data come from?

Answer:
The data available on this site are provided by agencies and organizations partnering in the Lake Tahoe Initiative. For example, the USGS has provided an array of GIS data products to the public free of charge because of the Presidential Commitments.

Question:
What is a projection?

Answer:

A map projection is an attempt to portray the curved 3-dimensional surface of the Earth on the flat 2-dimensional surface of a map. One method to perform this task is to physically project the surface of the earth onto a flat surface. Map projections are usually performed using mathematical formulas. There is no perfect way to project a sphere onto a plane, so some geographic distortion will occur when choosing any projection. At least one of the following geographic properties is lost when the projection is made.

• Shape -- "Shape" refers to the physical dimensionality of an object, so that at any point the scale is the same in every direction. If the shape of an area is preserved, its size will be distorted. If the angle at which the intersection occurs is other than a right angle (90-degrees), then angles are not preserved. Map projections that preserve angle or shape are called "conformal" or "orthomorphic".
• Area -- "Area" refers to the property of maps to accurately represent the relative size of regions. No projection can preserve both shape and area. Map projections that preserve area are called "equal-area" or "equivalent".
• Distance -- "Distance" refers to the physical distance between points on the map. The preservation of distances is the same as the preservation of scale. Distances measured between any two points on a map are a constant fraction of the distances if they were measured on the ground. Map projections that preserve distance are called "equidistant".
• Direction -- "Direction" refers to a map projection's ability to represent the shortest distance between any two points as a straight line. This path is also called a great circle. This projection is limited in the fact that it can only correctly show directions or angles to all other points on the map with respect to a central point. Map projections that preserve direction are called "azimuthal".

The trick is to pick the projection that causes the least distortion or emphasizes the desired properties for your area of interest. Each map projection has specific properties that make it useful for specific purposes.

For more information on projections, please see:

• http://mcmcweb.er.usgs.gov/drg/mercproj/index.html
• http://mac.usgs.gov/mac/isb/pubs/factsheets/fs07701.html
• http://mac.usgs.gov/mac/isb/pubs/MapProjections/projections.html

Question:
The data are in what projection?

Answer:
All digital data for Lake Tahoe Data Clearinghouse are in UTM NAD27 zone 10.

The original data are a mix of two projections. Universal Transverse Mercator (UTM) zone 11 runs between 114W and 120W longitude, and UTM zone 10 runs between 120W and 126W longitude. Because the project needed a standard projection, UTM zone 10 was chosen. The maps east of 120 degrees longitude are usually mapped in UTM zone 11 and those west of 120 degrees are mapped in UTM zone 10. There is no possible way to combine these different projections in a GIS. Therefore, maps in zone 11 have been reprojected into zone 10. An effort has been made to provide a copy of reprojected files in their original zone of origin. The maps reprojected into zone 10 are not a standard USGS products and only available through this web site.

Question:
How do I change projections?

Answer:
Many full-featured GIS packages will provide the capability to reproject data. However, each software package performs this operation differently and the commands are often complex. Because of the difficulty in reprojecting data, refer to your GIS's manual or use the UTM NAD27 zone 10 data provided on this website.

Questions from users

Question:
How can I view the DEM elevation information in ArcView?

Answer:
The DEMs are raster data which can only be queried in ArcView with the use of the Spatial Analyst extension.The hypsography DLGs contain the base information from which the DEMs were created. The DLGs are vector data easily downloaded from this site (http://tahoe.usgs.gov/DLG.html) and read into ArcView.

Question:
Is there similar data on other areas?

Answer:
Many clearinghouse websites exist. The gateway to many clearinghouses on the web is Geospatial Data Clearinghouse.

Similar websites include:

• Crater Lake Data Clearinghouse: http://craterlake.wr.usgs.gov/
• Florida Ecosystem Project: http://sflwww.er.usgs.gov/
• Mojave Desert Ecosystem Program: http://www.mojavedata.gov/
• San Francisco Bay area: http://sfbay.wr.usgs.gov/

Question:
How do I view data on the Lake Tahoe Data Clearinghouse presented in MrSID format?

Answer:
MrSID is a compression technology created by LizardTech that can compress very large files with very little data loss. The USGS is working with LizardTech to find new, convenient ways of bringing our data to the American public through a Cooperative Research and Development Agreement (CRADA). Images compressed with MrSID can be viewed with a number of software packages. Lizard Tech makes available a number of different viewers for free. With a reasonably modern computer, the MrSID images can be used in a standalone viewer, or viewed within a program such as ArcView, PhotoShop, or MicroStation.

Bathymetry

Question:
Can I see objects on the bottom of Lake Tahoe?

Answer:
The bathymetry project was not trying to uncover archaeological objects. If an object was large enough it would show up in the bathymetric map; however, it might be hard to distinguish it from a natural feature. The USGS is concerned about the protection of cultural artifacts and has not made any effort to highlight them.

Question:
Is there a poster version of the bathymetry available?

Answer:
Yes! The Carson City, Nevada office of the USGS created a poster which includes bathymetry called "Selected Hydrologic Features of the Lake Tahoe Basin, California and Nevada". Click here for a view of the poster.

Two posters are contained in "Bathymetry and Selected Prespective Views of Lake Tahoe, California and Nevada" by Gardner, Dartnell, Mayer, and Clarke. This report, "USGS Water Resources Investigation Report 99-4043", is available for $4 plus $5 Shipping and Handling from the following:

USGS Information Service
Box 25286
Federal Center Denver
Denver, CO 80225
(303)202-4200

One poster shows a large view of the multi-colored bathymetry (preview) and the second poster shows some of the perspective views shown in this web site (preview).

For additional information or ordering assistance, call 1-888-ASK-USGS (1-888-275-8747), or contact any USGS Earth Science Information Center (ESIC).