<< Click to Display Table of Contents >> Navigation:  Help Content > Import Data  > DEM Extraction > General Tools - Digital Elevation Model Extraction - SRTM-3 Version 2

Purpose

 

This tool downloads, mosaics and optionally resamples the 3 arc second SRTM Digital Elevation Model tiles (SRTM-3 version 2.1) , which are needed to cover the specified area.

 

Technical Note

 

Each tile covers an area of 1° in Latitude and 1° in Longitude.

 

The tiles are first searched for in the  "SRTM-3 DEM directory" specified in the Directories Preferences.

 

If no tiles covering the specified area are found, they are downloaded and stored in a folder called  SRTM_DEM_DIR in SARscape's work directory.

In case a tile is not found on the internet, or the connection fails before or during the data downloading, the names of the required SRTM tiles are written to the Process.log file in order to enable the user to retrieve the files manually and store them in the SRTM_DEM_DIR folder in the work directory.

The URLs, which are used to download the SRTM Digital Elevation Model tiles are subject to changes. In this case the new URL can be entered and saved as default for successive SARscape sessions.

 

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 90 m, which corresponds to the approximate DEM posting at mid-latitudes. Alternatively, a grid size of 0.0008333333 decimal degrees can be specified, which preserves the original 3 arc-second DEM posting,

 

Note: The SRTM-3 version 2.1 tiles can be affected by "holes" of no data (NaNs). To fill (i.e. interpolate) small holes the Relax Interpolation method described below can be used. For larger holes it might be preferable to use the SRTM-3 Version 4.1 product.

 

This interpolation algorithm, which is based on the solution of the heat transfer (Poisson) equation, adapts a soft surface to the valid height values surrounding a NaN area.

This method is optimally suited to interpolate small areas where abrupt morphological changes (i.e. steep slopes) are not present.

 

NASA/JPL is acknowledged for the provision and free distribution of these data.

 

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.

 

Cartographic Parameters

The reference parameters for some projection systems (e.g. Stereographic, Polar Stereographic, Gnomonic, Mercator, Miller, Albers, etc.) can be set.

 

Parameters - Principal Parameters

 

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).

 

The Westernmost cartographic co-ordinate.

 

The Easternmost cartographic co-ordinate.

 

The Northernmost cartographic co-ordinate.

 

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.

 

 

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.

 

Close        

The window will be closed.

 

Help

Specific help document section.

 

 

Specific Function(s)

 

None.

 

References

 

Farr, T. G., et al. (2007), The Shuttle Radar Topography Mission, Rev. Geophys., Vol. 45, RG2004.