ALOS Global Digital Surface Model (ALOS World 3D - 30m)
The AW3D30 product is created by the stacking process with multiple data observed from 2006 to 2011. Therefore, it is not possible to uniquely identify the observation date.
For AW3D30 v2.1 and later versions, the observation date of the data used is indicated in the included "LST file". The last column of each line means the date in UTC.
To know exactly path, row and date of the data covering your interested region, the AUIG2 system may useful.
Note for AUIG2;
* Internet Explorer is recommended
* "Start Searching" button is available for guest user without login
The AW3D30 product is created with the following definitions.
Digital Surface Model (DSM), elevation from height above sea level (in meter) converted from the ellipsoidal height based on ITRF97 coordinate system and GRS80 ellipsoid, using EGM96 geoid model.
The target accuracy of the elevation is 5 m (RMS).
Please see the reference.
It is meter.
Average value of elevation (in meter) over the range of pixel are rounded off to the integer and stored in each pixel of the AW3D30 product.
The latitude and longitude in the file name means the coordinates of the lower left corner of the lower left pixel of the image.
Please note that SRTM and ASTER GDEM have different definition.
The following figure shows the outline of the difference between AW3D30 and ASTER GDEM grid.
The value of the elevation of each pixel is a representative value at the center position of that pixel.
Zero means that there is no PRISM data to calculate the elevation of that pixel.
Other auxiliary DEM data was adopted to the point.
You can know what DEM was used referring to "MSK" file.
We determined land/sea extents using external coastline vector data and PRISM image.
SWBD and GSHHG were used for version 1.0, 1.1 and 2.1.
JGI and OpenStreetMap coastline data for version 2.2.
DEM (digital elevation model) is the most general term for topographic digital data including DTM and DSM.
DTM (digital terrain model) is a word especially refers to data of bare earth, that is, the heights data not including objects on the ground.
In contrast, DSM (digital surface model) refers to data of the heights including objects on the ground such as buildings and vegetation.
We have no DTM product. EORC creates only DSM as AW3D30.
Ice shelves are originally land ice that has been pushed out to sea and floats seamlessly from land to sea.
Therefore, the place where the ice shelves are located is not land but sea. But they are treated the same as land without a sea mask. Because they are connected to the land and can be several tens of meters above the sea surface.
The position of the ice shelf edge is not constant. Therefore, please note that the location of the ice shelf edge may differ between the sea mask based on OpenStreetMap and the DSM based on satellite data.
The cause of these anomalous values is not one, however most of them are "low correlation".
The DSM is created from images captured by optical sensor onboard satellite. The images are processed using a method like aerial photogrammetry.
If the surface is covered by low contrast materials, such as water, snow and ice, the correlation between images becomes low. This induces mismatch of images, and as a result, spikes or hollows appears at that place.