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 rater 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 is structured by keyword and their corresponding values, followed by cell value data in space-delimited format and each row separated by a carriage return, as follows:

nrows XXXX

ncols XXXX

xllcorner XXXX

yllcorner XXXX

cellsize XXXX

NODATA_value XXXX

row 1

row 2

...

row n

Table at the right panel describes the keywords in the raster information section.

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 built 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 ­čí¬ Digital Terrain Model - MDT05 ­čí¬ By list ­čí¬ Select by municipality or by map sheet.

  2. Import the DEM into QGIS in the corresponding coordinate system (Layer ­čí¬ Properties ­čí¬ Information ­čí¬ 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 ­čí¬ Conversion ­čí¬ Rasterise (vector to raster).

  5. Both rater can be combined by: Raster­čí¬ Miscellaneous ­čí¬Combine. Note that the resulting raster will have the resolution of the coarser resolution of the rasters to combine.

  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 ­čí¬ GDAL ­čí¬ Raster extraction­čí¬ 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.

3. Topobathymetric DEM examples

Here you can download some examples of DEMs to be used as input for the ONA-Toolbox.

NOTE: Their intended use is as test examples for the software, not for official purposes. These DEMs have been developed by means of the Digital Terrain Model - DTM05 of Spain (CC-BY scne.es) and the Web Map service of SITIBSA (CC-BY SITIBSA).