Average rainfall 2001-2016, global tropics

Map: Average rainfall 2001-2016, global tropics

Create regional DEM

Introduction

This post covers how to define a geographic region for hydrological modeling using DEM.

Defining a regional DEM

Defining a region for use in hydrological modeling is an iterative process as the aim is to include complete river basins. And these basins are usually not known beforehand - or if they are it is not certain that they will exactly fit your DEM. Once defined, you must create one or more default regions in the projection system you want to use for the hydrological modeling, then define a user project and region (tract) for that default region. The principal steps for identifying and extracting a regional DEM for hydrological modeling include:

  1. Identify DEM region that encloses the complete basin(s) of interest
  2. Adjust region(s) to the selected projection system
  3. Define default region(s) and system in the Framework
  4. Define user project(s) for each of the defined default regions
  5. Link DEM tiles to the default region(s)
  6. Mosaic regional DEM(s)

Identify DEM region

As the objective of creating regional DEMs is to prepare data for hydrological modelling, you need to make sure that the complete river basins of interest are included in your region(s). Global datasets of river basins, like The World Bank Major River Basins Of The World or the CEO water mandate Interactive Database of the Wolrd’s River Basins are coarse and with large gaps. The best option is to identity the river basins directly from the elevation data itself.

As identifying the boundaries of the river basins is the overall aim, locating basin boundaries for selecting the region to work with becomes a catch-22. That is overcome by first running the basin analysis using a coarser version the DEM at hand. The suggested steps for identifying the geographical region(s) thus becomes:

  • Create a coarser spatial resolution DEM,
  • Mosaic the region of interest, including margins,
  • Run the complete basin delineation as outlined in this blog section,
  • Manually identify the geographical region(s).

Create a coarser spatial resolution DEM

Use averaging for reducing the reduction of your DEM data. A reduction of 10 % in cell size leads to a reduction of 99% of the DEM layer itself. You can change the spatial resolution of any dataset registered in Karttur’s GeoImagine Framework with the GDAL linked commands TranslateTiles and TranslateRegion. If the only thing you want to do with the tiles of changed resolution is to mosaic them, the process MosaicTiles can also change the spatial resolution on the fly, and thus you can skip this step.

Here is an example that changes the Copernicus DEM 90 m version to 1 km virtual tiles for the northern hemisphere projected to EASE-grid North (EPSG:6931).

Mosaic the region of interest

In my example I want to delineate all river basins emptying into Arctic waters. With Arctic waters also including the North Atlantic, Hudson Bay, the Baltic Sea etc. I thus need to create a mosaic for large parts of the northern hemisphere - the region I have called northlandease2n. Below I have included the json parameterisations for solid (GeoTiff) mosaics both from prepared virtual 1 km tiles, and from the original tiles in 90 m spatial resolution.

Adjust region to projection system

To align different DEM regions you need to set the corners of your DEM such that they fit the pixels of your original DEM tiles. Karttur’s GeoImagine Framework contains a support module, regionfit that aligns a set of 4 corner coordinates to the selected projection system. The script can fit two different spatial resolutions for the same region. If for instance you want to apply a filtering or dual scale analysis the, regionfit will make sure both resolutions are fitted both to each other and to the default projection system. The script will also generate the json codes for creating the regions.

The script outputs two new json parameter files, one for creating the default regions, and one for creating user project linking to the default region.

Before you can create the regional DEM, the system tiles covering the new region must be identified and registered. The Framework process LinkDefaultRegionTiles does that.

Mosaic a regional DEM

Create the regional DEM layer from tiles with the process MosaicTiles.