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Elevation

The terrain surface is the foundation of every Hydrata model. It defines the ground the water flows over. In the interface this input is labelled Terrain.

There are two ways to get one: upload a GeoTIFF of your own, or import free global terrain for your area.

Requirements

  • Format: GeoTIFF (.tif or .tiff)
  • CRS: any projected coordinate system. Hydrata reprojects into the UTM zone for your area. A file with no CRS of its own is accepted, but you have to say which one it was authored in.
  • Resolution: 1 m to 50 m is typical. The DEM resolution sets how much detail is available; the mesh you build over it is controlled separately by the scenario's base mesh size.
  • Coverage: must cover the whole domain boundary. Gaps are detected and reported.

Uploading

  1. Open Inputs in the project sidebar
  2. Click the upload icon and select your file
  3. Confirm the coordinate system. Hydrata detects the CRS and shows what it found. When the file carries none, it asks you to select the CRS it was authored in, or enter an EPSG code, and verifies that code as the file uploads.
  4. Answer the vertical datum question (below)
  5. Processing takes a few minutes depending on file size

After upload, Hydrata creates a colour-coded elevation layer, a hillshade layer for context, and empty Boundary and Inflow layers to draw into.

Importing global terrain

If you have no DEM of your own, import one. Hydrata downloads Copernicus GLO-30 terrain for an area you draw on the map: click one corner, then the opposite corner.

Before it starts, you get the area in square kilometres, the estimated number of cells at 30 m, and an estimated time until the terrain is ready. Import areas are capped at 40,000 km², about 200 km by 200 km, so a larger study area is imported as several tiles.

GLO-30 heights are referenced to EGM2008, so an imported terrain needs no datum decision from you.

Vertical datum

Horizontal CRS and vertical datum are different questions, and getting the vertical one wrong shifts your whole model up or down by tens of metres without looking broken.

At upload, Hydrata asks what the heights are referenced to: an orthometric datum, or ellipsoidal (WGS84, as a GPS or many CAD exports produce). If you do not know, say so and check it afterwards.

After upload, Hydrata measures your terrain against Copernicus GLO-30, which is EGM2008, and shows one of:

  • Datum: EGM2008, when the measurement agrees with an orthometric surface.
  • Check vertical datum, with its evidence: the best guess, and the offset in metres your terrain reads against GLO-30.
  • Datum tag disagrees with measurement, when the file's own tag and the measurement point at different datums. This one needs a human look; Hydrata will not pick for you.

From there you can Convert to EGM2008, Keep as-is, or mark it already correct. Converting does not edit your upload: it builds an EGM2008 copy as a background job, and the new terrain appears in the Tasks panel when it is ready. You can also ask for the conversion at upload time.

Once a converted copy supersedes the original, the original's elevation and hillshade layers stop drawing on the map, so you cannot leave the wrong surface visible by accident. Clearing that relationship brings them back.

The plausibility screen

Some DEMs contain values that are not terrain: sentinel values that survived an export, or the flat near-zero padding a CAD or TIN export writes outside the surveyed area. Left alone, those become a cliff in your mesh.

Hydrata screens the terrain against a reference surface on the same grid, in two tiers:

  • Reported, never masked: pixels more than 100 m from the terrain's own median offset. A dam face, a deep pit or major cut and fill legitimately sits far from any reference, so magnitude alone is not treated as evidence of junk. You are told; nothing is changed.
  • Masked to nodata: values that deviate by more than 1000 m, which is sentinel-scale rather than topographic; and placeholder plateaus, meaning near-zero heights where the reference sits far above. That second pattern is the export-padding signature, and real coastlines do not match it because the reference is near zero there too.

The screen runs at upload, and again when terrains are merged. Because every rule is centred on the terrain's own median offset, a uniform vertical shift is invisible to it. That is the datum check's job, not this one's.

Combining terrains

A Combined surface merges several terrains into one, which is how you put a fine survey over a coarse regional DEM, or fill a coverage gap that would otherwise fail the build check.

Order matters: inside its own footprint, the terrain above replaces the one below. Hydrata warns when a coarser DEM sits above a finer one, because that throws away the better data. That is exactly what you want for a design surface placed over a survey, and almost never what you want otherwise, so the warning lets you move it down or derive anyway.

When a terrain has coverage gaps, Hydrata says so and offers a combined surface with a base terrain as the fix.

Sources of elevation data

Source Coverage Resolution License
Copernicus GLO-30 (built in) Global 30 m Copernicus terms
Geoscience Australia ELVIS Australia 1 m - 5 m CC BY 4.0
USGS 3DEP USA 1 m - 10 m Public domain
OpenTopography Global Varies Varies

How elevation is used

ANUGA uses the terrain raster to set the elevation quantity on the computational mesh. During mesh generation, elevation values are interpolated from the raster to mesh vertices or centroids.

If structures are present, their footprints are burned into the elevation by adding a height offset, 5 m by default, before mesh generation. See Structures.