Learn to get hold of Strahler or Shreve order on vector layer

Pricey reader, on this article, I wish to dive into one thrilling hydrological matter. I’ll begin with the image:

Disclaimer: this text is written each for geographers who face the issue of rating river vector layer utilizing geoinformation techniques (GIS) and for individuals who have generally seen “good rivers” on a map however have no idea precisely how they’re made. We’ll collectively sequentially discover how spatial knowledge are represented in GIS purposes, the best way the construction of river networks may be analysed, and what visualisation strategies can be utilized.

Now the query is: Which map appears prettier? — For me, the one on the underside (b).

Really, the second visualisation is extra right from a typical sense perspective as properly. The extra tributaries flowing into the riverbed, the broader and fuller the river might be. For instance, one of many largest rivers on this planet, the Nile, at its supply (excessive within the mountains), barely resembles a strong river at its mouth: with every kilometer of the best way to the ocean, the river absorbs increasingly more tributaries and turns into increasingly more full-flowing.

The map proven above (Determine 1b) was ready on the idea of structural info on the river community. On this publish I wish to talk about through which methods this extra details about rivers may be obtained and what instruments can be utilized for this goal.

What’s the river

Let’s begin by explaining how details about rivers is represented. In cartography and geo-sciences, rivers are represented as a linear vector layer: every river part is represented as a line with some traits. For instance, the size of the part, its geographic coordinates (geometry of the thing), floor kind, common depth, circulation velocity, and many others. (Animation 1).

So, usually, when you see a river on a map, you see a set of those easy geometric primitives (particular person rows in attrbute desk) assembled into one massive system. Totally different colors can be utilized to visualise saved traits (Determine 2).

Both programming languages or specialised purposes equivalent to ArcGIS (proprietary software program) or QGIS (open-source) are used for visualization.

River construction

Such info within the attributes desk on rivers may be collected in several methods: from distant sensing knowledge, expeditions, gauges, and hydrological stations. Nonetheless, details about the river construction is often assigned by the specialist on the final second, when she or he sees on the map what the entire system appears like. For instance, a researcher can themselves add a brand new column to the vector layer description through which they assign a rank to every river phase (Determine 3).

Now we are able to see that the image resembles the map from the start of the article (Determine 1b). However the query arises: what precept can be utilized to assign such values? — the reply is: lots. There are a number of usually accepted techniques for rating watercourses in hydrology — see the Stream order wiki web page or paper Stream orders. Beneath are a number of approaches that I’ve used myself in the course of the work (Determine 4).

For what purpose

Now it’s time to reply the query of the aim of such rating techniques. We are able to distinguish two causes:

• Visualisation — utilizing rank as an attribute of the dimensions of a linear object on the map, it’s doable to create good maps (Determine 1);
• Additional evaluation.

Data of river community construction may be mixed with different traits for additional evaluation, e.g. to establish the next patterns (Determine 5).

assign stream orders utilizing present instruments

It’s tough to rank massive techniques manually, so specialised instruments have been created for stream ordering. There are two basically other ways:

• Stream ordering utilizing raster knowledge (digital elevation mannequin);
• Stream ordering on vector layers.

Above, now we have described how we are able to assign ranks to vector layers. Nonetheless, spatial knowledge are sometimes represented in one other format — as rasters (matrices) (Determine 6). Digital elevation fashions (matrices the place every pixel has a selected dimension, equivalent to 90 by 90 metres, for instance, and an elevation worth above the ocean stage floor that’s saved in every cell of that matrix) are significantly generally used.

The raster layer (digital elevation mannequin — DEM) is used to calculate the circulation route matrix and circulation accumulation. The Stream Order (Spatial Analyst) instrument in ArcGIS, for instance, works in keeping with this precept. On this publish I can’t describe intimately how such an algorithm works, as there are fairly good visualisations and descriptions within the official documentation (please examine Stream Path operate web page if you wish to know extra). Beneath I’ve listed among the instruments you should utilize to get an Strahler order utilizing raster knowledge:

Nonetheless, this all requires quite a lot of raster knowledge manipulation. What must you do if you have already got a vector layer? (This could occur if in case you have, for instance, a vector layer of the river community loaded from OpenStreetMap.) I’ll let you know subsequent!

get hold of Shreve, Strahler and Topological order on vector layer utilizing QGIS

Throughout our work 4 years in the past, my colleagues and I got here up with an algorithm that enables us to calculate Shreve, Strahler, and Topological order based mostly solely on the vector layer and the ultimate level (the purpose the place the river system ends and flows right into a lake / sea / ocean). The primary model of the algorithm is described in my first article on medium: “The Algorithm for Rating the Segments of the River Community for Geographic Info Evaluation Primarily based on Graphs” (wiping away a tear of nostalgia). Just lately, I lastly obtained round to writing some clearer documentation for it and making ready a plugin replace.

In QGIS for stream ordering, the Strains Rating plugin can be utilized. To make use of, it is going to require loading a vector layer, reprojecting it into the specified metric projection, and assigning a ultimate level (you’ll be able to simply click on on the map) — the next end result might be obtained (Determine 7).

Now you, pricey reader, have dived just a little deeper into the subject of stream ordering in hydrology and realized the way to use completely different instruments to get it from uncooked knowledge (raster or vector). As soon as there may be details about the river’s construction, you’ll be able to put together stunning and clear visualisations, or proceed the evaluation by combining the obtained info with different traits.

Helpful hyperlinks:

The speak on stream ordering in hydrology was introduced by Mikhail Sarafanov