DTED_2

Purpose

This tool downloads, mosaics and optionally resamples the DTED Digital Terrain Elevation Data tiles, which are needed to cover the specified area

Technical Note

DTED - Digital Terrain Elevation Data is a standard mapping format designed by the NGA. Each file or cell contains a matrix of vertical elevation values spaced at regular horizontal intervals measured in geographic latitude and longitude units.

The DTED digital dataset is characterized by 3 levels, level 0, 1 and 2.

The latitude spacing for each level is:

Level 0: 30 arc secs, which are approximately 900 m.

Level 1: 3 arc secs, which are approximately 90 m.

Level 2: 1 arc secs, which are approximately 30 m.

The longitude spacing depends on the world zones:

Level 0:

•latitude range 0°-50°: 30 arc secs

•latitude range 50°-70°: 60 arc secs

•latitude range 70°-75°: 90 arc secs

•latitude range 75°-80°: 120 arc secs

•latitude range 80°-90°: 180 arc secs

Level 1:

•latitude range 0°-50°: 3 arc secs

•latitude range 50°-70°: 6 arc secs

•latitude range 70°-75°: 9 arc secs

•latitude range 75°-80°: 12 arc secs

•latitude range 80°-90°: 18 arc secs

Level 2: 1 arc secs, which are approximately 30 m.

•latitude range 0°-50°: 1 arc secs

•latitude range 50°-70°: 2 arc secs

•latitude range 70°-75°: 3 arc secs

•latitude range 75°-80°: 4 arc secs

•latitude range 80°-90°: 6 arc secs

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The DTED tiles can be downloaded from the USGS Earth Explorer (select the "SRTM 1 Arc Second Global" product from the Digital Elevation->SRTM category and download the geotiff format).

Place the unzipped tiles in the "SRTM-1 DEM directory" specified in the Directories Preferences.

If no directory is specified in the preferences, the files will be searched for in sub-folder "SRTM1_DEM_DIR" in SARscape's working directory.

Note: Registration is required to download products through the USGS Earth Explorer. If several tiles need to be downloaded, we suggest you follow the USGS bulk order instructions.

Note: Although this is a "void-filled" product, some tiles may still contain voids.

The area of interest, which has to be covered by the extracted Digital Elevation Model, can be specified by entering either the area cartographic co-ordinates or a reference image or a list of images.

The default grid size is set to 30 m, which corresponds to the approximate DEM posting at mid-latitudes. Alternatively, a grid size of 0.0002777777 decimal degrees can be specified, which preserves the original 1 arc-second DEM posting,

The user should also be aware that above latitude 50N and below latitude 50S, the SRTM-1 products are distributed with a 2 arc-second longitude posting (and a 1 arc-second latitude posting).

Note: Subtract Geoid (Other Parameters)

By setting this flag the output product will be referred to the ellipsoid. The EGM96 geoid is subtracted. Set this flag if the extracted Digital Elevation Model has to be used for further processing in SARscape.

Input Files

OPTIONAL Reference Slant Range Image

Input file name(s) of the image(s) to be used as geographic reference for the Digital Elevation Model extraction. This file is mandatory if the cartographic co-ordinates are not specified. In case a geocoded image is used, the "Cartographic System" specified below must be the same of the reference image. The geographical extent of the output Digital Elevation Model corresponds to the common coverage of the Reference file list.

DEM/Cartographic System

Output Projection

The parameters relevant to the Cartographic System of the output Digital Elevation Model must be entered:

State

Definition of the country or general projection systems.

Hemisphere

Definition of the hemisphere. This is chosen according to the selected State and Projection.

Projection

Definition of the projection system of the selected State. In case that a general projection system is selected in State, the Projection is automatically set.

Zone

Definition of the zone. This is chosen according to the selected State and Projection.

Ellipsoid

Definition of the ellipsoid. This is chosen according to the selected State and Projection.

Datum Shift Parameters

Definition of the datum shift parameters. These are chosen according to the selected State and Projection.

Reference Height

A reference ellipsoidal height can be provided. Default Reference Height is 0.

Parameters - Principal Parameters

Generate Slope

By setting this flag, three slope files are created: slope east-west, slope north-south (between -90 and 90 degrees), and a third output representing the slope magnitude (sqrt(ew^2 +ns^2)), with values between 0 and 90 degrees.

By setting the following parameters it is possible to specify a spatial region for the Digital Elevation Model extraction (co-ordinates are referred to the output cartographic reference system).

West

The Westernmost cartographic co-ordinate.

East

The Easternmost cartographic co-ordinate.

North

The Northernmost cartographic co-ordinate.

North

The Southernmost cartographic co-ordinate.

X Grid Size, Y Grid Size

The grid size of the output data must be defined. The default measurement unit is meters.

If a Geographic projection was specified and grid size values higher than 0.2 are entered the measurement unit will be assumed to be meters. if values lower than 0.2 are entered the measurement unit will be assumed to be degrees.

A different grid size can be specified for the X and Y dimensions.

Replace Dummy with sea level value

By setting this flag all dummy values (i.e. NaN) will be replaced by the value zero, if the "subtract geoid" flag is not active, or by the ellipsoidal equivalent if the "subtract geoid" flag is active.

Source DEM Type

The user must select the Source DEM type : DTED-0 , DTED-1 and DTED-2 forlevel 0, 1 and 2 respectively.

Parameters - Global

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - DEM FTP Address

It brings to the FTP section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Parameters - Other Parameters

It brings to the general section of the Preferences parameters. Any modified value will be used and stored for further processing sessions.

Output Files

Output DEM file

Output file name of the extracted and mosaiced Digital Elevation Model. This file is mandatory.

_dem

Extracted Digital Elevation Model, with user-defined projection and grid size, and associated header files (.sml, .hdr).

_dem_wgs84_dem

Extracted Digital Elevation Model, with original projection and grid size, and associated header files (.sml, .hdr).

_wgs84_dem_geoid_geo

Geoid height with original projection and grid size, and associated header files (.sml, .hdr). This file is generated only when the "Subtract Geoid" flag is checked.

_W2E_slope

Easting direction slope image and associated header files (.sml, .hdr).

_S2N_slope

Northing direction slope image and associated header files (.sml, .hdr).

_slope

Combined slope image and associated header files (.sml, .hdr).

These last three files are generated only if the relevant flag is set.

Details specific to the measurement units and nomenclature of the output products can be found in the Data Format section.

General Functions

Exec

The processing step is executed.

Store Batch

The processing step is stored in the batch list. The Batch Browser button allows to load the batch processing list.

The window will be closed.

Help

Specific help document section.

Specific Function(s)

None.

References

None.

General Tools - Digital Elevation Model Extraction - "DTED"

General Tools - Digital Elevation Model Extraction - "DTED"