Drawing topics lines?

Eric G

Good night.

I would like to know how to draw a parallel at a specified latitude which would take the correct curve when re-projected.

In my case it would be particularly useful for representing in an easy way the tropics: for now, when drawing a line at these latitudes with too few points, when re-projecting, these lines are strait between each point. My current solution is to draw a line with a lot of points so that it takes automatically a relative curved shape in an other projection, but this of course isn't practical.

Also, I don't know how to draw a line at a specified latitude or longitude.

Thanks for your answers.

EDIT: Oups! Can you change the tittle? It should be "tropics" instead of "topics", of course. Thanks and sorry.

global_mapper

When you reproject vector data in Global Mapper it does just do the reprojection at each point, so for very long segments the view in the current projection will always be a straight line between vertices, which is not what you want in this case.

What I would suggest is just using the lat/lon grid options on the General tab of the Configuration dialog to display a lat/lon grid, which should give you the curved display that you are after (internally it just automatically uses a lot of points).

Another option would be to export the native latitude line (you can create with the Digitizer Tool and edit the vertices with the Feature Info Tool) to a raster file, then load that in and reproject it, which will reproject every rendered pixel in the raster representation.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

Eric G

Thanks Mike for your answer but it doesn't give me a satisfactory solution.

If I understand well your first proposition, I would have to display a huge amount of grid lines in order to have one close enough to the tropics lines (23° 26', not a “practical” latitude!) which for a world-size map will be hard to cope with.

I think the easiest for me will be to draw these two lines once for all at their correct position and with enough vertices and save them in shp or dat format so I can import them easily in whatever map I will create later.

Have a good day.

global_mapper

I agree that to get those particular lines of latitude that your best bet is just to draw a line with a bunch of vertices at that latitude. The easiest thing might be to create a simple text file with the vertices listed like the following, then just load that with the File->Open Generic ASCII Data File menu command.

-180,23 26
-179,23 26
...
179,23 26
180,23 26

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

Eric G

Excellent! I'm a non-professional cartographer and make maps only as a hobby and so I don't know much about all those file formats. I didn't know a simple and easily created text file would let me draw a line. It's exactly what I was looking for.

Many thanks for this tip.