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Hi all,

I was using relative coordinate for all of my data in Telemac 2D, because if I use the real coordinate, then the mesh looks weird. I am creating variable mesh using density polygon. I will have 0.25, 1 and 4 m mesh sizes within the outline. If I use relative coordinate, the mesh looks perfect. But I want to use real coordinate now. Is it a common issue? Is it possible to work with only relative coordinate as well in telemac? I wanted to know both's pros and cons....

Hi ssiddi7,

I am not sure I understand the word "relative coordinate" but I can share with you just an experience, (and I am not expert so...).

When I am in Metric coordinate like UTM WGS84 (my bathymetry data and boundaries), I easily use the density mesh as in the user manual,
but when I use geographic data (bathymetry or...) it is different. I can't use density and so forth... instead, the density of my mesh depends on the density of the points on/of my boundary limits. In the area I want fine mesh I increase the number of points on my boundary lines, and on the contrary I decrease. I have been helped by this video and channel associated

hope this help

Josias

Thank you for your reply. I attached two files, one is the real coordinate of the density polygon where I want to create a finer mesh and another one is the relative coordinates of the same polygon. Relative coordinate basically worked but the real coordinate didn't create a good mesh. The relative coordinate is a coordinate value where I subtract a particular number from all X and another number from all Y coordinates and created a set of relative coordinates for a good mesh. I attached both files if you can have a look and find any possible reasons why the real coordinates are not working.

As you said, the density of the mesh depends on the density of the points of boundary limits of the polygon, I have more points (72) on my boundary lines when I am trying to use real coordinates than when I used relative coordinates (65 points).

Hi,

Your files are missing....

I suspect it has to do with coordinate precision, you probably solve it by using double precision slf-file.

Hope that helps,
Patrick

Hi PMV,

I am sorry I couldn't attach the file, not sure why it's not attached after trying many times.
Anyway, you can see the data here below,
Real coordinate of the polygon that I am using in density to create a finer mesh.

595009.782 4755249.783
595009.1 4755249.722
595008.418 4755249.779
595007.755 4755249.951
595007.131 4755250.233

Relative coordinate, which worked better, it's not the same polygon as above, but I just wanted to show what the values look like. Let me know if you see any differences which are basically prohibiting the real coordinate to have a better mesh on it.

2509.78256 149.783516
2509.100541 149.722767
2508.418154 149.779227
2507.755394 149.951242
2507.131681 150.233772

Using double precision in the selafin (.slf) should solve your problem. Your coordinates have more than 7 digits which are stored in the normal selfin format, therefore rounding it to meters 4755249.783 becomes 4755249. Double precision will increase the digits stored.

Hope that helps,
Patrick

Hi PMV,

Thanks for your answer. Can you tell me how can I create a double-precision selafin format? I got what you are saying but I am not sure how can I change the format and which file I need to modify. Ignore my ignorance, maybe it's a very simple thing, but I just started learning the software, therefore needs more explanations.

Thanks,

Hi
This topic deals with meshing and not Telemac-2D. It will be moved in the appropriate section
Did you read the forum rules and particularly the part on attachment (type of file, size ...)?


It could be great to also read BK manual and use the X/Y origin option at the Selafin file creation. You could keep all the original information and just shift the mesh at the writing step as this is only the simple precision in selafin file which leads to rounding problems...

Regards

Got it, thanks.

Hello,

To answer to the only question concerning TELEMAC-2D, if you do not need that TELEMAC-2D knows the exact location of your domain (e.g. if you do not have to know latitude/longitude for features like varying Coriolis coefficient, astronomical tidal force, tidal boundary conditions) e.g. if you model a flume, you can work with local coordinates (what you call relative coordinates).

For the single tidal boundary conditions, it is possible to work with local coordinates if changing 2 integers in the geometry file, you can have a look at the TELEMAC-2D user manual:
"Since release 8.2, it is possible to use x and y origin coordinates stored in the geometry file to decrease the number of digits of coordinates when modelling tide, e.g. when using UTM or Lambert projections. The two numbers are stored in the I_ORIGIN and J_ORIGIN variables reachable with the help of GET_MESH_ORIG subroutine. Caution: these two numbers are integers, not floats as the preliminary structure available in the SERAFIN format expected this type for these 2 variables. The tidal computations automatically take into account this offset for UTM + Lambert projections when generating the HARMONIC CONSTANTS FILE for JMJ database or when interpolating harmonic constants to compute boundary or initial conditions for solutions coming from OSU (e.g. TPXO). In particular for the operations to locate the nodes correctly (a simple translation) but TELEMAC-2D still continues to compute other steps in a local coordinate system."

Otherwise, as suggested by the other users, you should use file format with SERAFIND (with a D as double precision) or MED.

Hope this helps,

Chi-Tuan