GETASSE30 Elevation Model |

### GETASSE30 Elevation Model

GETASSE30 stands for Global Earth Topography And Sea Surface Elevation at 30 arc second resolution. This elevation model is currently the default DEM of the Sentinel Toolbox and has kindly been provided to the development team by Marc Bouvet (marc.bouvet@esa.int) of ESA/ESRIN. This documentation is also by courtesy of Marc Bouvet.

Because of its size, the Sentinel Toolbox installer will not automatically install the GETASSE30 DEM files. But at first use the Toolbox will offer to download the DEM for you. Alternatively the GETASSE30 dataset can be manually downloaded from the STEP home page at
http://step.esa.int/auxdata/dem/GETASSE30
. The size is about 300 MB zipped and 2.3 GB unzipped. Due to the file size, we recommend this only if your internet connection is fast enough. If you like to install by yourself, please follow the instructions given here:

1. Download `GETASSE30.zip` from the STEP home page.
2. Create directory `${USER_HOME}/.snap/auxdata/dem/GETASSE30`
3. The simplest way is unzip the `GETASSE30.zip` and copy the files into

   `${USER_HOME}/.snap/auxdata/dem/GETASSE30/`.

   The DEM is now installed.

If you like to share the DEM along multiple Sentinel Toolbox installations in your network or if you simply don't want to install in the auxdata folder, then:

4. Copy the unzipped GETASSE30 dataset to a location accessible by your Sentinel Toolbox installation(s). This directory we call `${SHARED_GETASSE30}` from now on.
5. Create a file named "dem.properties" in `${USER_HOME}/.snap/auxdata/dem/GETASSE30/` and open the new file in a text editor.
   Add the following line

   `dem.installDir = ${SHARED_GETASSE30}`

   to it and don't forget to replace `${SHARED_GETASSE30}` by your actual, shared DEM installation path. The properties file format requires to esacpe the backspace and colon characters using a backslash. Hence, on windows systems you'll have to escape the drive colon and file separator, e.g.

   `dem.installDir = P\:\\Data\\DEMs\\GETASSE30`.

   The DEM is now installed.

#### The GETASSE30 Data Set

GETASSE30 is a composite of four other DEM datasets. It is using the SRTM30 dataset, ACE dataset, Mean Sea Surface (MSS) data and the EGM96 ellipsoid as sources. The resulting GETASSE30 dataset represents the Earth Topography And Sea Surface Elevation with respect to the WGS84 ellipsoid at 30 arc second resolution. The dataset has no missing values, but at the junction of the arctic and antarctic continents there are some tens of strange negative values (down to -700 m) inherent to the ACE dataset. All latitude/longitude values refer to the center of a pixel, not to one of its corners. The GETASSE30 dataset is organised as multiple tiles. Each tile is stored in associated file which can be imported into the Sentinel Toolbox using the GETASSE30 Reader. (This reader is also internally used by the Orthorectification algorithm.)

Example of a GETASSE30 tile

#### The GETASSE30 Data

1. Mean sea surface height over sea and height over land, both referenced to the WGS84 ellipsoid.

   Resolution: | 30 arc second latitude and longitude
   Unit: | meter
   File name example: | 45S045W.GETASSE30 where the first number is the latitude of the most South West pixel and the second number its longitude
   Data format: | binary, 1800*1800 signed 16-bit integer values, big endian order

The GETASSE30 elevation data

There are 5 types of values:

1. Over land, between 60N and -60S, where SRTM30 DEM data are available, the output value is the sum of SRTM30 elevation and the EGM96 geoid height.
2. Over land, between 60N and -60S, where SRTM30 DEM data are not available, the output value is the sum of ACE elevation and the EGM96 geoid height.
3. Over land, above 60N and -60S, where ACE data are available, the output value is the sum of ACE elevation and the EGM96 geoid height.
4. Over sea, where neither ACE DEM nor SRTM30 data are not available, if MSS data are available, then the output value is the MSS.
5. Over sea, where neither ACE DEM nor SRTM30 data are not available, if MSS data are not available, then the output value is the EGM96 value.

- Flag
  - Pixels described as in 2) and 3) are flagged with the value 0
  - Pixels described as in 3) are flagged with the value 1
  - Pixels described as in 4) are flagged with the value 2
  - Pixels described as in 1) are flagged with the value 3

  Resolution: | 30 arc second latitude and longitude
  Unit: | none
  File name example: | 45S045W.GETASSE30_flag where the first number is the latitude of the most South West pixel and the second number its longitude
  Data format: | binary, 1800*1800 unsigned 8-bit integer values, big endian order

  The flag associated to the GETASSE30 data

#### Input Data used in GETASSE30 Generation

- ACE DEM at 30 arc second resolution referenced to the EGM96 geoid
  See ACE Report at
  http://www.cse.dmu.ac.uk/geomatics/products_ace_overview.html
- SRTM30 data are reference to the EGM96 geoid
  See
  http://www2.jpl.nasa.gov/srtm/
- Mean Sea Surface (MSS) height at 2 minutes resolution referenced to the WGS84 ellipsoid.
  From the RA2 auxilliary file RA2_MSS_AXVIEC20031208_145545_20020101_000000_20200101_000000
  See RA2 product handbook at
  https://earth.esa.int