Sample Digital Elevation Model Dialog Box

The Sample Digital Elevation Model dialog box is used to read Digital Elevation Models (DEMs) in the format described in the DEM data users guide (http://nationalmap.gov/standards/demstds.html). Formerly, the 1:250K DEMs at http://edc2.usgs.gov/geodata/index.php were in this format. Now that URL is redirected to a different site that has DEMs in a different format. If the DEMs coordinates are in arc seconds, they are converted to Universal Transverse Mercator (UTM) coordinates. The dialog box is displayed by selecting File|Import|Sample DEM Data... and then selecting the Digital elevation models to import.

It is possible to read more than one DEM at a time. To do so, just select multiple DEM files when you select File|Import|Sample DEM Data.. However, if the DEMs come from zones that use different central meridians to calculate the UTM coordinates, the first file in the file list will determine which central meridian will be used.

When determining the central meridian for calculating the UTM coordinates, all UTM zones are treated as being exactly 6 degrees wide. This is not correct for a few zones. The area where this is not true is from 0 to 42 degrees east north of 72 degrees north and from 0 to 12 degrees east between 56 and 64 degrees north. Because the author is unaware of any available DEMs in this area in the format accepted by this program, no effort has been made to address this issue.

Some DEMs generated by ArcInfo use a special value to indicate missing data. If you have such a DEM and you wish to ignore a value that indicates missing data, check the check box labeled Ignore elevations coded as and indicate the value that indicates missing data in the edit box following the check box. The default value in the edit box is likely to be the correct value to use.

The DEM data is typically very numerous and much more closely spaced than the discretization in a groundwater model. Sampling the data, reduces the amount of data stored by ModelMuse to manageable levels. The data are imported into ModelMuse at cell centers for MODFLOW and at either element centers or nodes for SUTA or PHAST. You may choose to sample the data in one of several ways

•The lowest value of any DEM point in a cell or element

•The highest value of any DEM point in a cell or element

•The average of all the values of any DEM points in a cell or element

•The value of the DEM point closest to the node, element center, or cell center.

DEM data in the SDTS format, can be converted to the USGS native format using the free program sdts2dem (http://www.cs.arizona.edu/topovista/sdts2dem/). (Thanks to Sandra Warner for pointing out sdts2dem to me and to Gregg Townsend for maintaining it.)

If you have a DEM in a format that ModelMuse can not read and you have access to a GIS program, here is a way you can import the data (from Jon Traum).

1.Export you grid to a 2D points shapefile (File|Export|Shapefile|Export Grid Data to Shapefile).

2.In the GIS, map the DEM elevations onto the shapefile

3.Import the shapefile back into ModelMuse (File|Import|Shapefile).

If you change your model grid, you will need to reimport the data.