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Hi,

Does anybody know exactly what the 'LONGITUDE/LATITUDE OF ORIGIN POINT' variable is actually used for in TELEMAC?

I typically run models my models in either spherical coordinates and WGS84 UTM projection. In both cases, the model should be able to determine the latitude/longitude of the grid points without any further specification. Are these variables required under some other specific case/particular projection type, etc.?

Regards,
Toby

Hi Toby,

Just select a point approximately in the middle of your domain (doesn't have to be precise as far as I'm concerned). Hope this helps.

Cheers,
Nayef

Hello Toby,

When using WGS84 UTM projection, LONGITUDE/LATITUDE OF ORIGIN POINT is useless.
If using spherical coordinates, TELEMAC needs to convert to Mercator to solve equations. What suggests Nayef is often recommended,

Chi-Tuan

Hi Nayef and Chi-Tuan

Appreciate the feedback. I still wonder why the model would need to know the latitude and longitude origin when the grid is already in spherical coordinates. It doesn't seem to make sense to me unless I'm overlooking something - quite confusing.

I have run quite a few large regional models in spherical coordinates and with spherical bottom grids and haven't seemed to run into any issues but I want to make sure I'm not doing anything wrong here. Is it possible my grid is being wrongly projected?

Regards,

Hello Toby,

TELEMAC (2D and 3D) only solves equations in plane projections, not in longitude/latitude. If you use SPHERICAL COORDINATES = YES, if not already done, the coordinates will be converted in Mercator projection and the keyword LATITUDE OF ORIGIN POINT has to be filled.

Chi-Tuan

Hi Chi-Tuan,

The projection of Lat Long to Mercator was clear from your previous post. I guess the confusion from my side was a general lack of knowledge around how mercator projections are made.

After doing a bit of research I believe the natural origin just sets the 0 position of the Mercator projection that telemac uses, but doesn’t actually affect how the projection is made (which I was worried about).. Based on this it seems that setting the point of origin somewhere in the middle of the grid would just mean telemac is working with Mercator coordinates of a smaller value than if it were for instance set somewhere far away.

I tested this with an existing model I had today and the results were of no significant difference. If I’m using spherical coordinates I always view the results in spherical coordinates so I never see the projected coordinate system.

Regards,
Toby

Hello Toby,

Just a practical remark, if you set:
SPHERICAL COORDINATES = YES
SPATIAL PROJECTION TYPE = 3 (longitude/latitude)
and use default choice for RESULT FILE IN LONGITUDE-LATITUDE = YES,

your geometry file is written in longitude/latitude and so is your result file.

But TELEMAC solves the equations with a projection for sure.

Chi-Tuan