The Nile River Basin from SRTM data

Someone asked the other day whether the cross-platform, free and open-source GIS Whitebox GAT can handle watershed delineation from massive, regional-scale DEMs. They had a particular interest in the Nile River basin. Heck, if you’re going to go big, why not go huge, right? So I decided to give it a try. First off, I used the Retrieve SRTM Data tool to download the approximately 800 SRTM 3-arcsecond (~90 m) tiles that make up the Nile River basin. This required some experimentation because my first attempt at doing so hit the boundary of the basin and I had to give it a second try. The tool downloaded each of the tiles and mosaicked them into a single large DEM. The final DEM was 45,601 rows by 25,201 columns (a little over 1.1 billion grid cells) and was 4.28 GB in size. I then used the new Breach Depressions (Fast) tool to hydrologically pre-process the DEM by removing artifact topographic depressions and flat areas (i.e. cells with no downslope neighbours). I used a D8 flow algorithm to calculate flow directions, perform flow accumulation, trace the flowpaths issuing from Lake Victoria (the White Nile) and Lake Tana (the Blue Nile), and lastly, to delineate the watershed. The result was this map:

The Nile River Basin (click to enlarge)

The Nile River Basin (click to enlarge)

To fully appreciate this amazing map, you need to enlarge it. Just to put a bit of perspective on the scale of this analysis, take a look at this one:

Nile River World Map (click to enlarge)

Nile River World Map (click to enlarge)

All of Europe has an area of approximately 10,180,000 km2 and the Nile River basin has an area of 3,400,000 km2. That is truly vast.

For my initial attempt, the one in which I truncated the watershed, I used my 13 inch Macbook Pro (2.8 GHz dual-core i7, 16 GB RAM, SSD). When I expanded the area, I also moved to my workstation (3.0 GHz 8-core Xeon, 64 GB RAM, SSD) just to speed up the process a little. I even extracted a long-profile for the White and Blue Nile, although I should have converted the distance units to metres:

Long profile for the Nile River, extracted from SRTM 3-arcsecond data (click to enlarge)

Long profile for the Nile River, extracted from SRTM 3-arcsecond data (click to enlarge).

It was the longest river that I have ever plotted a long profile for; of course, it is the longest river so I guess you can’t get much larger than that! I probably should have extracted the river network using a dispersive flow algorithm like Tarboton’s excellent D-infinity, since there are places where the river bifurcates (i.e. the river course splits), even before the delta. Nonetheless, I’m quite pleased with the result. In fact, I was quite surprised at how well the river course, extracted from the 90 m resolution SRTM DEM data, matched a mapped Nile River shapefile that I located:

Mapped vs. extracted river (click to enlarge)

Mapped vs. extracted river (click to enlarge)

Leave your comments below and, as always, best wishes and happy geoprocessing.

SRTM DEM data downloader in Whitebox GAT

I just finished developing a tool for Whitebox GAT that will automatically download Shuttle Radar Topography Mission (SRTM) digital elevation models (DEMs) from the USGS SRTM FTP site. SRTM-3 data are among the best global elevation data, with a grid resolution of approximately 90 m. In many areas SRTM data provide the only topographic data set available. Within the United States, the SRTM-1 dataset provides an improved 30 m resolution. Not only does this Whitebox tool retrieve the SRTM tiles contained within the bounding box of a specified area of interest, but it will also import the tiles to Whitebox GAT, fill missing data holes (which are common with SRTM data in rugged terrain) and mosaic the tiles.

Whitebox's new Retrieve SRTM Data tool

Whitebox’s new Retrieve SRTM Data tool

There have been many times in the past when I have needed to download numerous SRTM files, import the files, fill the missing data holes, and finally mosaic the individual tiles. It can be a laborious workflow indeed. This tool will save me a great deal of time, and so I’m rather excited about it. It’s as though the data magically appear in Whitebox!

SRTM data in Whitebox GAT

SRTM data in Whitebox GAT

Of course, with Whitebox GAT’s extensive Terrain Analysis and Hydrological Analysis toolboxes, there’s plenty of interesting things that you can do once those data do magically appear.

90 m DEM of the entire British Isles

90 m DEM of the entire British Isles. (Note that the image is of a coarser resolution than the actual DEM.)

As an example, I created the SRTM-3 90 m DEM of the entire British Isles shown above in 4 minutes, 56.27 seconds, including the time to download 91 individual SRTM tiles, fill their missing data gaps, and mosaic the tiles. My son was even watching Netflix during my downloading, so I can only imagine how much that slowed things down! I only wish that other data providers could follow a similar data sharing model as the USGS and use an anonymous FTP server to distribute their data. If that were the case, we could have many other data sets automatically ingested directly into Whitebox. I’ll release this new SRTM retrieval tool in the next public release of Whitebox GAT (v 3.2.1), which will likely be sometime later this summer. If you are as keen to try it out as I am, email me for a preview copy. If you have any other comments or feedback, please leave them in the comments section below. As always, best wishes and happy geoprocessing.

John Lindsay

*****EDIT*****

Version 3.2.1 of Whitebox has now been released, with the SRTM downloader tool embedded in the Data Layers menu. Please let me know if you have any issues in using the tool.

****EDIT****

I’ve included an image of the DEM of Ireland for John below:

SRTM DEM of Ireland

SRTM DEM of Ireland (Click to enlarge)