Delft 3D FM Interpolation methodsDelft 3D FM Interpolation methodshttps://oss.deltares.nl/c/message_boards/find_thread?p_l_id=1806746&threadId=9943682020-10-24T10:10:44Z2020-10-24T10:10:44ZDelft 3D FM Interpolation methodsAndrea Nerihttps://oss.deltares.nl/c/message_boards/find_message?p_l_id=1806746&messageId=9943672016-08-11T17:17:31Z2016-08-11T17:04:25ZGood Evening,<br /><br />I am trying to understand how the interpolation of the computational grid works and the differences between different grid interpolation methods.<br /><br />I have a DEM file which has a resolution of 30 meters. Therefore the xyz file that I import in the software as "bed level" has the same resolution (30m).<br />My computation grid is a combination of a rectangular grid and a triangular grid, as shown in the attached picture (islands_scale.png), and the length of the edge of one triangle ranges from 400m to 600m.<br /><br />The software allows to use these Interpolations methods:<br /><br />1) <em>Triangulation Interpolation</em><br />2) <em>Averaging</em><br /><br /><em>Averaging</em> can be assessed in these ways:<br /><br />1) Simple average<br />2) Closest point<br />3) Maximum value<br />4) Minimum value<br />5) Inverse weighted distance average<br />6) MinAbs<br />7) KdTree<br /><br />and gives the possibility to choose the minimum number of points which the average is based on and also the size of the search volume: 1.0 is the "actual cell" and 1.5 is 50% more than the "actual cell" in all direction.<br /><br />The first strange behaviour I notice is that regardless the <em>Averaging</em> method I use (I didn't try MinAbs and KdTree), I obtain the same interpolated grid, meaning that all the nodes have the same elevation value. [I don't notice changes even if (using the same <em>Averaging</em> method) I increase the minimum number of sample points to be taken into account to compute the average. I notice changes (between the same <em>Averaging</em> method) only between a search cell size value equal to 1.0 and a search cell size value equal to 1.5.]<br /><br />So my first question is: <strong>Why the values of the elevation for each node are not changing according to the <em>Averaging</em> method?</strong><br /><br />If I use the <em>Triangulation Interpolation</em>, then I obtain an interpolated grid with different elevations compared to the grids obtained with the <em>Averaging</em> methods. I read on the user manual that the <em>Triangulation Interpolation</em> "performs a Delauney triangulation on the sample point set before projecting onto the grid".<br /><br />Second question: <strong>How does exaclty the <em>Triangulation Interpolation</em> method work in this case?</strong> (I know what a triangulation inerpolation is but I can't understand how it is applied on a xyz file which has equally distributed points and how this triangulation is used to caculate the elevation of the nodes of the computational grid)<br /><br />Third question: <strong>What is the "actual cell" that works as search volume needed to compute the average of sample points?</strong><br /><br /><br />Please if I was not clear contact me at Andrea.Neri@deltares.nl or andreaneri.91@gmail.com<br /><br /><br />Best Regards,<br />AndreaAndrea Neri2016-08-11T17:04:25Z