Preliminary Documentation for
San Francisco North, CA, 30-meter DEM

This data set contains the USGS 30 meter DEM data for the San Francisco North, CA, 7.5-degree map quadrangle, which extends from 37.75 to 37.875 north latitude and from 122.375 to 122.5 west longitude.

The values represent spot elevations on a rectangular grid in the UTM projection with 30-meter horizontal spacing in both east-west and north-south directions . The USGS documentation describes the generation and structure of DEMs in detail. When inported into ArcInfo (using, for example, DEMLATTICE) or LAS (DEM2LAS), the grid points are placed at the center of each 30m by 30m grid cell or pixel.

Note that the rows and columns of the 30-meter elevation grid are parallel to lines of constant northing and easting in the UTM projection. They are not, in general, parallel to the lines of constant latitude and longitude which define the edges of the 7.5-minute quadrangle. As a result, when the maximum and minimum UTM easting and northing values for the elevation grid points are used to define a rectangle in the UTM projection, this rectangle contains narrow wedges along its edges which are outside the quadrangle boundaries. Elevation values are not given for grid points falling within these wedges. When data for adjacent quadrangles are seamed together, however, using standard image processing or GIS software such as LAS or ArcInfo, these wedge-shaped areas will be correctly filled from the adjacent quadrangles so long as they are in the same UTM zone.

The file name extension (.dem or .sdts) indicates whether the file is in traditional USGS ASCII format or the new Spatial Data Transfer Standard (SDTS) format, respectively.

The elevation grid is in the UTM map projection, zone 10.

The bounding longitudes and latitudes for which elevation values are supplied are:
Longitude: -122.5 to -122.375
Latitude: 37.75 to 37.875
As noted above, the rectangle in UTM coordinates which contains all the elevation grid points is somewhat larger in area than the box specified by the bounding longitudes and latitudes.


Last change: 2001 Jun 1, R. A. White / raw@essc.psu.edu