Gap in polar stereographic map

Leland

I have a dataset in Mercator projection that spans the globe horizontally from -180 to +180 degrees longitude. However, it's more convenient for me to store and use the data in the range -170 to +190, since -170 degrees makes a better place to cut the map without slicing through features I'm interested in.

(I indicate the origin of -170 degrees through the xllcorner value in the .hdr file. The cells size in meters is set so that the width of the whole file at the equator comes out to 6378137.0, to match the WGS84 ellipsoid, so it should wrap around the globe perfectly and match on the east and west edges.)

All this works fine in Global Mapper as long as I stay with a cylindrical projection like Mercator. But when I reproject to Polar Stereographic, it looks to me like it only handles the range -170 to +180 longitude, leaving a 10-degree missing pie slice in the map. A similar effect occurs with Double Stereographic and other projections. For instance, Gnomonic projection with a 180-degree central longitude shows a missing vertical strip.

I can also envision the need in other situations to use a longitude range of, say, -190 to +170 (so some longitudes are less than -180) or 0 to 360, so that any longitude from -360 to +360 is probably possible.

I've attached an example as a low-resolution image file, which demonstrates the problem. Just load the file and change the projection to Polar Stereographic in the Configuration window.

-- Leland

global_mapper

Leland,

What you are seeing is because when the data coordinates go beyond +180 they are no longer valid coordinates in the projection system that you have defined (Mercator in this case), so they can't be reprojected as they are undefined. What you need to do is define your projection system as Mercator with a central meridian of 10E (this will shift the entire thing 10 degrees east) rather than changing the corner coordinates but keeping the projection defined as centered at the normal 0 (prime meridian). This way all of your coordinates are still valid for the selected projection and can then be reprojected to other projections.

Let me know if I can be of further assistance.

Thanks,

Mike
Global Mapper Support
support@globalmapper.com

Leland

Ah, ok, that makes sense. After fixing the corner coordinates, I had to adjust the central meridian in both the projection used to interpret the raster file (so it would georeference it correctly) and in the projection used to display it (in the Configuration window), in order to view it the way I want in Mercator projection. Then it also reprojects to polar stereographic correctly. Excellent!

I guess I should stop posting my questions under "Bug Report" now, as they're starting to turn out to be my own errors :-)

Thanks again for all your prompt and helpful responses!

-- Leland