# Merging DEMs (Digital Elevation Model)

To build the best topography from heterogeineous inputs, two DEMs can be merged. In this example, the first one is an extraction from the [GEBCO](https://www.gebco.net/) topography product (extraction from https://download.gebco.net/), and the second one is a DEM provided by SHOM (the French Naval Hydrographic and Oceanographic Service).

DEMs properties: 
- GEBCO resolution is 15 arcseconds (~300m at 45°N) 
    - gebco_2026_n50.797_s42.527_w-9.356_e0.413.nc
- SHOM resolution of 100m
    - MNT_ATL100m_HOMONIM_WGS84_NM_ZNEG.grd

*TIPS: Creating topography is a long an iterative process so we recommand to create dedicated directory (for example 1-merging_DEMs/)*

## Step 1: change the resolution of the “base” bathymetry
We want to produce a final bathymetry product with a resolution close to that of the SHOM DEM, which is 100m. We will use [topo-create](../../grid-topography-processing/topo-create.md) to change the resolution of GEBCO, which is the ‘base’ bathymetry. We set the resolution to 5 arcseconds which corresponds to ~110m at 45°N.

```bash
rep=/ecola/share/tmp/training/tuto_bathy/data/
topo-create --mapping "[-9:5arcsec:0;43:5arcsec:50]" \
            -b $rep/gebco_2026_n50.797_s42.527_w-9.356_e0.413.nc \
            -o GEBCO_Gascogne_5arcsec.nc
```

Options used here (see [topo-create](../../grid-topography-processing/topo-create.md) for more):

| Name | Description |
|------|-------------|
| --mapping | frame and resolution wanted, "[lon_min:dlon:lon_max;lat_min:dlat:lat_max]" |
| -b | working bathymetry |
| -o | output file |


## Step 2: merge the two DEMs

We will use [topo-merge](../../grid-topography-processing/topo-merge.md) to integrate the SHOM DEM into the working bathymetry generated in the previous step.

```bash
rep=/ecola/share/tmp/training/tuto_bathy/data/
topo-merge $rep/MNT_ATL100m_HOMONIM_WGS84_NM_ZNEG.grd \
           -b GEBCO_Gascogne_5arcsec.nc \
           -f netcdf \
           -o GEBCO5arcs_MNTAtlSHOM
```

Options used here (see [topo-merge](../../grid-topography-processing/topo-merge.md) for more):

| Name | Description |
|------|-------------|
|-b | base bathymetry |
|-f | output format |
|-o | output file |