TruFlite - 7.5 Minute DEMs of California, Description

USGS 7.5 Minute Digital Elevation Files of California - Description

Introduction

This site provides access to a set of digital elevation model (DEM) files of California. The files are located at a California State University Northridge Department of Geography FTP server.

For a comprehensive description of the USGS DEMs, see http://edc.usgs.gov/products/elevation/dem.html at the USGS Global Land Information System server.

The 7.5 minute files (a.k.a. 30 meter DEMs) are the highest resolution DEMs currently available for a major percentage of the U.S.: see http://edc.usgs.gov.

The resolution is exactly 30 meters on the ground, which is approx. 1 arc second on the meridian. The data points are gridded on a Universal Transverse Mercator basis (UTM, easting and northing) rather than a geographical basis (longitude and latitude).

This has a major advantage: towards the high north the resolution on a parallel does not increase due to the meridians converging. No need to downsample the Alaska area; homogenous resolution all over the area.

However, a major drawback: contiguous DEMs can't be easily tiled, because the data point rows and columns do not have a straightforward continuation on the neighbor DEM. See http://edc.usgs.gov/products/elevation/dem.html for details.

The Clickable Image Maps

From the Overview map of California, locate the one degree quadrangle you are interested in. By clicking it, you will get a zoomed-in map subdivided into 7.5 minute quadrangles. Each can contain a link to the gzipped DEM file behind.

This is the legend for the one degree maps:

map legend

The DEM Files

The files have been compressed with the GNU "gzip" utility. The gzip program is available via anonymous FTP at many sites:

gzip for UNIX: http://www.google.com/search?q=gzip+UNIX

gzip for MAC: http://www.google.com/search?q=gzip+MAC

gzip for MSDOS: http://www.google.com/search?q=gzip+MSDOS