HOW TO BUILT A TOPOBATHYMETRIC DEM

1. What is a topobathymetric DEM?

A Digital Elevation Model (DEM) is a representation of the terrain surface elevation. A common and widely used format of DEM is the raster ASCII.  This format allows an easy transfer between different GIS systems. 

The ONA Toolbox needs a Digital Elevation Model (DEM) embracing both emerged land and submerged beach to simulate first the bi-dimensional storm wave propagation, second, the land flooding due to sea-level rise and storm waves, and third, estimate the potential beach erosion area.

For low-lying coastal areas, it is important that the DEM embraces enough area inland to properly estimate flooding processes.

The Raster ASCII format

Raster ASCII files have two different sections. The first section contains the information defining the raster properties: number of rows and columns, the origin coordinates of the raster and the cell size. The second section is the cell data representing the elevation model.

The basic structure of the ASCII raster for the ONA Toolbox must have the raster information at the beginning of the file followed by the cell data. The spatial location of the raster is specified by the location of the lower left cell corner.

The raster information keyword is followed by cell value information specified in space-delimited row-major order, with each row separated by a carriage return.  The cell value data must follow the next requirements:

• Cell values should be delimited by spaces.

• No carriage returns are necessary at the end of each row in the raster. The number of columns in the header determines when a new row begins

• Row 1 of the data is at the top of the raster, row 2 is just under row 1, and so on.

2. How to CREATE a DEM as input for ONA-Toolbox

While high-resolution LIDAR topography DEM are extended and easy world-wide available and free downloadable, high-resolution bathymetries are not so extended. ONA Toolbox need as input a single DEM file containing both types of data.

There are many ways to combine DEM rasters with bathymetric raster.

Here there is an example of how you can prepare the input raster ASCII for the ONA toolbox using the QGIS platform:

1. Get a high-resolution digital elevation model. For the Spanish territory, you can download for free the high resolution DEM by areas at the download center of IGN webpage (National Geographic Institute,  https://centrodedescargas.cnig.es/CentroDescargas/index.jsp): Digital Elevation Models 🡪 Digital Terrain Model - MDT05 🡪 By list 🡪 Select by municipality or by map sheet. 

2. Import the DEM into QGIS in the corresponding coordinate system (Layer Properties Information SRC (ETRS 1989 UTM Zone 31N).

3. Import the high-resolution bathymetry. Common formats of bathymetries are: the xyz ASCII files containing the x, y and z coordinates, GIS shapefiles (*.shp), or even raster ASCII. 

4. Convert bathymetry layer to raster type:  Raster Conversion Rasterise (vector to raster).

5. Both rasters can be combined by: Raster Miscellaneous Combine. Note that the resulting raster will have the coarsest resolution of the rasters being combined. 

6. It is recommended to use the Terrain Profile tool or similar to check that the final combination of the rasters does not present discontinuities or unrealistic shapes. 

7. To extract the final combined raster or a section: i) save the combined layer, ii) toggle layer edit, iii) add a polygon and draw the area of the raster corresponding to the study area, and iV) extract the raster zone by Toolbox GDAL Raster extraction Cut raster by mask layer. Save the resulting layer as raster ASCII file.

Note that the current version of ONA Toolbox works in ETRS89 UTM 31 N projection that is the corresponding to the Balearic Islands zone. This is necessary to obtain the background maps of the soil use and characterization catalogue and characterization from the Balearic Islands Territorial information Service (SITIBSA) and for its internal transformation to WGS84 projection. In a future, ONA will allow consider other projections.