[Qgis-user] Maps over multiple projected coordinate systems

[Andre Joost]

Am 26.07.2015 um 17:13 schrieb James Keener:
> If I want to use a PCS to do distance calculations over a large area.
> For instance, lets say I want to calculate a 650km buffer around
> Pittsburgh, Pa.  The buffer would go outside the state plane for
> Pittsburgh and outside the UTM zone. I decided to punt and just put
> everything in UTM Zone 17N and do the calculation that way, but I'm not
> sure if that is the correct thing to do.

I would suggest to use one of the contiguos US CRS, see 
http://gis.stackexchange.com/questions/141580/which-projection-is-best-for-mapping-the-contiguous-united-states


>
> http://jimkeener.com/posts/projections if anyone wants to see what I'm
> talking about or would be kind enough to give me feedback on the post
> (especially to point out if I'm wrong anywhere in it).

Actually, GCS and PCS are both 3D, becaue you always have a height above 
ellipoid as well. And you can desribe a place on Earth also with a XYZ 
coordinate system originating in the Earth's Center like GCS.

PCS can be valid for the whole globe too, think of Miller or Robinson 
projection: 
http://gis.stackexchange.com/questions/70411/qgis-display-world-country-shape-files-centered-on-pacific-ocean-using-robinson
or even more exotic: 
http://gis.stackexchange.com/questions/1775/discontinuous-or-interrupted-map-projections

>
> In QGIS can't figure out how to create a buffer not in the native units
> of the projection the layer is part of.

Since the buffer is drawn on a new layer, you can set the CRS of that 
layer independent from your source layer. You just have to reproject the 
buffer center into the new CRS.

HTH,
André Joost