forums

Use these forums to ask questions and discuss Tableau.

spanning prime meridian

Posted February 13th, 2008 by krees
in Maps & Links

Hi,

I have a map of the pacific rim. It spans the prime meridian, so my left longitude value is -110 and the right side is 160. When I put in these values, Tableau flips my background image (so it's like a mirror image). What I need it to do is shift the x axis values so they match my background image.

Is this possible?

Thanks

Add new comment

Comments

On February 14th, 2008 Austin Dahl said:

Yes this is possible.

I think you may be confusing the 180 degrees longitude, which runs down the middle of the Pacific ocean, with the prime meridian, which runs through Greenwich in the UK. But that's really besides the point, you want the middle part of the map to be the Pacific, rather than the Atlantic.

First off, create a calculation based on your longitude to shift the values to be all between 0 and 360 instead of being between -180 and 180. If your existing longitude field is called Long, the formula would be
IIF([Long] > 0, [Long], 360 + [Long])
Edit your background image to use this new calculation instead of your existing longitude field.

Secondly, adjust the left and right boundaries of the background image to go from 0 and 360 -- essentially applying the above formula to your map boundaries. For a map of the Pacific, I would guess the the left would be near 100 and the right would be near 260.

I'm attaching a screen shot and an example packaged workbook that shows population for five cities on the Pacific rim

I hope this helps,
Austin

AttachmentSize
Pacific.twbx69.97 KB
Across the Pacific.png463.96 KB
reply
On February 15th, 2008 krees said:

Hi Austin,

Yep, you're correct about 180 degrees.

Ugh, that solution is not optimal, but it will work. I think this is a bug with Tableau, no? I wonder if there's a place to note bugs.

Thanks for the reply.

reply
On February 26th, 2008 krees said:

What projection is being used for calculating the location on the map? My locations are slightly off when I plot them on the map you provided.

I guess the projection shouldn't matter. If your locations are showing up correctly then mine should as well. Hmm. Any ideas?

In the attached image, the dots in the gulf of Alaska should actually be on mainland Alaska.

AttachmentSize
deadmap.png1.66 MB
reply
On June 10th, 2008 Austin Dahl said:

For background maps, you end up with a Mercator projection, since there really isn't any transformation on the points -- it's a scatter plot of the long and lat.

I do see that your points are slightly off, which I blame on my suggested boundaries being slightly off. Errors are exaggerated in Alaska due to the disproportional stretching in a Mercator projection.

With respect to reporting bugs, support@tableausoftware.com is usually the best place to report them. In this particular case, I know that a bug has already been entered.

Austin

reply
On June 12th, 2008 Austin Dahl said:

I misspoke, the backround maps essentially use a Platte Carre projection.

reply