ArcHydro : 2- Preparing a corrected DTM for hydrology – Part 1

A  Digital Terrain Model is a representation of the elevations of a territory. Each cell (pixel) of this DTM contains a height value. According to the generation means used for the surface and the allocated size ofr the cells, the height assigned to each one is more or less close to the exact reality.

If you want to use the DTM for a 3D view of the territory (ith ArcScene, for example), you can use it as it is and without any special precaution. By cons, if you want to model the water flow on the surface of this territory, the first thing to do, and the most important is to correct and adapt it to this objective.

What you will be doing during this step will define the quality of the results obtained regarding the watersheds and the different hydrological calculations available.

A usual correction is to search the cells surrounded by higher cells. These basins will pose a problem when determining the flow directions since the algorithm cannot emerge from the basin. Then, we’ll proceed, in the pre-treatment of the DTM, to the loading of these basins till finding an adjacent cell which is lower than the loading height . This cell will be the outlet cell when calculating the flow.

However, it is extremely unusual to find references for another type of correction, equally important,: the domes.
Increasingly the acquisition means of the DTMs are very acute. The acquired heights do not always correspond to ground level, but often to the top layer of the vegetation. A forest with trees with an average height will be defined with this elevation delta.

In this, quite theoretical, example, the basic DTM isrepresented by the blue line, whilethe levelitshouldhave to match realityisindicated by the red line.

If we keepthesedomes, theyartificially alter the  water flow, especiallyin flat or weakslope areas .

So, we will start by clipping the domes , using ArcHydro Tools and the Spatial Analyst raster calculator .

How to remove the domes

ArcHydro has a tool that fills the basins. We will use it to remove the domes, which are ultimately reversed basins. If we select the previous image and invert it, we have basins that we can fill .

To this end, it’s enough to display the DTM in ArcMap , check that the symbology is  “stretched”  in the legend window in order to have the DTM Mini and Maxi values.
We will work with a DTM of the Finistère region ( data srtm ) and hydrological layers of SANDRE. If you want to follow through the process, you can Download these files in our site, by clicking here .

We are going to use the maximum value and add 25%. In our example the DTM maximum value is 200m, therefore we will use the value 250.
We are going to invert the DTM by subtracting 250 from each DTM cell. For this we use the Spatial Analyst raster calculator.

The result will be found in a new DTM_A raster.

Youcanverifythat, now the valuesrange between 257 (250 – (-7)) and 50 (250 – 200), and that low lying areas havebecome high areas and vice versa.

If now we apply the archydro basins filling command, we will fill the reversed domes.
Click on Terrain pre-processing -> DTM Manipulation -> Fill Sinks

The Fill sinks configuration window appears ;

Leave all the default options, except the name of the resulting raster. Click -the DTM_A_Fill.

This raster contains the original DTM WITHOUT domes that have been filled .
But, it’s always reversed . Now we will return it to the place, with the raster calculator

The result is our new DTM, without domes , and in place .

Up to this point we have just dealt with the domes, but not with real basins which are in our DTM. We could use right now the Fill Sinks command to fill these true basins, but we have to perform two operations to the DTM. It is easier to let this filling operation for the end of the pre-treatment.

In the next article we will perform this second part.



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