I - Accessing and Using Digital Elevation Models (DEMs) in the Department


All GMES DEMs are available in the following locations:


In the "\\slush\gis\dems" directory, you will find the following files:


North America contains all of the 30 m DEMs (30 m refers to a cellular resolution of 30 meters) for every 7.5 minute quadrangle in the United States. A map is included to aid in the locating the needed quadrangle. Central America contains DEMs for volcanoes in Guatemala, El Salvador, and Nicaragua. South America contains a volcano located in central Ecuador. The following Volcanoes are included for each country:

Guatemala El Salvador Nicaragua Ecuador

Agua San Miguel Casita Tungurahua

San Miguel San Salvador Cerro Negro

Fuego San Vicente Concepcion

Acatenango Tecapa Las Pilas

Pacaya Usulutan   Masaya

San Pedro Mombacho

Santiaguito Momotombo

Santa Maria San Cristobal

Toliman      Telica

Atitlan


All of the DEMs are stored as a ".dem" file that may be imported using the "demlattice" command in ArcInfo. The DEMs for Central America are available in both NAD27 and NAD83 datums. In addition to all of the DEMs, transportation, hydrography (water), and hypsography (contour maps) coverages are available. For the Tungurahua DEM in Ecuador, the DEM is available in the S.A. 1959 Datum (PRP_E) together with the hydrography and transportation coverages.


DEMs must be downloaded to the ArcInfo Workspace (usually C:\\Workspace or D:\\Workspace) before any modifications can be made. Once the DEM has been downloaded, it must be converted to a grid file in ArcInfo so that it may be viewed in ArcView or ArcGIS. To do this, load ArcInfo and use the following command (Agua is used here as an example DEM name):

demlattice agua.dem agua

This converts the ".dem" file to a grid file. The DEM may now be loaded into ArcView, ArcGIS, or the GRID module of ArcInfo. If you need to splice more than one DEM to make one seamless DEM, the mosaic command in ArcInfo must be used. At the Arc prompt, type "grid" without the quotations. At the grid prompt, type the following command:

newgrid = mosaic(oldgrid, oldgrid, oldgrid,??., oldgrid)


Throughout this document, oldgrid refers to the file you wish to change and newgrid is the name of the new file. For example, if you wish to change the name of the agua grid to water, the following command must be used:


copy oldgrid newgrid

or

copy agua water